QGIS User Guide - QGIS Documentation

QGIS User Guide
Release testing
QGIS Project
Mar 21, 2018
Contents
1
Preamble
1
2
Foreword
3
3
Conventions
3.1 GUI Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Text or Keyboard Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Platform-specific instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
5
6
6
4
Features
4.1 View data . . . . . . . . . . . . . . . . . .
4.2 Explore data and compose maps . . . . . .
4.3 Create, edit, manage and export data . . . .
4.4 Analyze data . . . . . . . . . . . . . . . .
4.5 Publish maps on the Internet . . . . . . . .
4.6 Extend QGIS functionality through plugins
4.7 Python Console . . . . . . . . . . . . . . .
4.8 Known Issues . . . . . . . . . . . . . . . .
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5
What’s new in QGIS testing
6
Getting Started
6.1 Installation . . . . . . . . . . . . . . . . . . .
6.2 Launching QGIS . . . . . . . . . . . . . . . .
6.3 Sample Session: Load raster and vector layers
6.4 Projects . . . . . . . . . . . . . . . . . . . . .
6.5 Output . . . . . . . . . . . . . . . . . . . . .
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QGIS GUI
7.1 Menu Bar . . . . . .
7.2 Panels and Toolbars
7.3 Map View . . . . .
7.4 Status Bar . . . . . .
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21
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30
32
32
General Tools
8.1 Context help . . . . . . . . . .
8.2 Panels . . . . . . . . . . . . . .
8.3 Rendering . . . . . . . . . . .
8.4 Save and Share Layer Properties
8.5 Color Selector . . . . . . . . .
8.6 Blending Modes . . . . . . . .
8.7 Zooming and Panning . . . . .
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i
8.8
8.9
8.10
8.11
8.12
8.13
8.14
8.15
8.16
8.17
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67
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81
10 Working with Projections
10.1 Overview of Projection Support . . . . . . . .
10.2 Global Projection Specification . . . . . . . .
10.3 Define On The Fly (OTF) CRS Transformation
10.4 Coordinate Reference System Selector . . . .
10.5 Custom Coordinate Reference System . . . . .
10.6 Default datum transformations . . . . . . . . .
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83
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9
Measuring . . . . . . . . .
Selecting features . . . . . .
Data defined override setup
Identify Features . . . . . .
Annotation Tools . . . . . .
Spatial Bookmarks . . . . .
Nesting Projects . . . . . .
Decorations . . . . . . . . .
Authentication . . . . . . .
Variables . . . . . . . . . .
QGIS Configuration
9.1 Options . . . . . .
9.2 Project Properties .
9.3 Customization . .
9.4 Keyboard shortcuts
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11 Managing Data Source
89
11.1 Opening Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
11.2 Creating Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
11.3 Exploring Data Formats and Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
12 Working with Vector Data
12.1 The Symbol Library . . . . . . .
12.2 The Vector Properties Dialog . .
12.3 Expressions . . . . . . . . . . . .
12.4 Working with the Attribute Table
12.5 Editing . . . . . . . . . . . . . .
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123
123
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189
205
219
13 Working with Raster Data
241
13.1 Raster Properties Dialog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
13.2 Raster Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
14 Laying out the maps
14.1 Overview of the Print Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2 Layout Items . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3 Creating an Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
255
255
265
294
15 Working with OGC Data
301
15.1 QGIS as OGC Data Client . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
15.2 QGIS as OGC Data Server . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310
16 Working with GPS Data
339
16.1 GPS Plugin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
16.2 Live GPS tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
17 Authentication System
349
17.1 Authentication System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
17.2 User Authentication Workflows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356
17.3 Security Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367
ii
18 GRASS GIS Integration
18.1 Demo dataset . . . . . . . . . . . . . . . . . . . . . . . . . .
18.2 Loading GRASS raster and vector layers . . . . . . . . . . .
18.3 Importing data into a GRASS LOCATION via drag and drop
18.4 Managing GRASS data in QGIS Browser . . . . . . . . . . .
18.5 GRASS Options . . . . . . . . . . . . . . . . . . . . . . . .
18.6 Starting the GRASS plugin . . . . . . . . . . . . . . . . . .
18.7 Opening GRASS mapset . . . . . . . . . . . . . . . . . . . .
18.8 GRASS LOCATION and MAPSET . . . . . . . . . . . . . .
18.9 Importing data into a GRASS LOCATION . . . . . . . . . .
18.10 The GRASS vector data model . . . . . . . . . . . . . . . .
18.11 Creating a new GRASS vector layer . . . . . . . . . . . . . .
18.12 Digitizing and editing a GRASS vector layer . . . . . . . . .
18.13 The GRASS region tool . . . . . . . . . . . . . . . . . . . .
18.14 The GRASS Toolbox . . . . . . . . . . . . . . . . . . . . . .
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371
371
371
372
372
372
372
373
373
373
376
376
377
379
379
19 QGIS processing framework
19.1 Introduction . . . . . . . . . . . . . . . . . . . . .
19.2 Vector menu . . . . . . . . . . . . . . . . . . . . .
19.3 The toolbox . . . . . . . . . . . . . . . . . . . . . .
19.4 The history manager . . . . . . . . . . . . . . . . .
19.5 The graphical modeler . . . . . . . . . . . . . . . .
19.6 The batch processing interface . . . . . . . . . . . .
19.7 Using processing algorithms from the console . . .
19.8 Writing new Processing algorithms as python scripts
19.9 Configuring external applications . . . . . . . . . .
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389
389
392
395
403
404
410
412
418
421
20 Plugins
20.1 QGIS Python console . .
20.2 QGIS Plugins . . . . . . .
20.3 Using QGIS Core Plugins
20.4 Coordinate Capture Plugin
20.5 DB Manager Plugin . . .
20.6 eVis Plugin . . . . . . . .
20.7 GDAL Tools Plugin . . .
20.8 Geometry Checker Plugin
20.9 Georeferencer Plugin . . .
20.10 MetaSearch Catalog Client
20.11 Offline Editing Plugin . .
20.12 Topology Checker Plugin
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427
427
430
436
436
437
439
448
452
455
459
464
465
21 Help and Support
21.1 Mailing lists
21.2 IRC . . . . .
21.3 BugTracker .
21.4 Blog . . . .
21.5 Plugins . . .
21.6 Wiki . . . .
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469
469
470
470
470
471
471
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22 Contributors
473
22.1 Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473
22.2 Translators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474
23 Appendix
475
23.1 GNU General Public License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475
23.2 GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478
24 Literature and Web References
485
iii
iv
CHAPTER
1
Preamble
This document is the original user guide of the described software QGIS. The software and hardware described
in this document are in most cases registered trademarks and are therefore subject to legal requirements. QGIS is
subject to the GNU General Public License. Find more information on the QGIS homepage, http://www.qgis.org.
The details, data, and results in this document have been written and verified to the best of the knowledge and
responsibility of the authors and editors. Nevertheless, mistakes concerning the content are possible.
Therefore, data are not liable to any duties or guarantees. The authors, editors and publishers do not take any
responsibility or liability for failures and their consequences. You are always welcome to report possible mistakes.
This document has been typeset with reStructuredText. It is available as reST source code via github and online
as HTML and PDF via http://www.qgis.org/en/docs/. Translated versions of this document can be downloaded in
several formats via the documentation area of the QGIS project as well. For more information about contributing
to this document and about translating it, please visit http://qgis.org/en/site/getinvolved/index.html.
Links in this Document
This document contains internal and external links. Clicking on an internal link moves within the document, while
clicking on an external link opens an internet address. In PDF form, internal and external links are shown in blue
and are handled by the system browser. In HTML form, the browser displays and handles both identically.
User, Installation and Coding Guide Authors and Editors:
The list of the persons who contribute on writing, reviewing and translating the following documentation is available at Contributors.
Copyright (c) 2004 - 2017 QGIS Development Team
Internet: http://www.qgis.org
License of this document
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in Appendix GNU
Free Documentation License.
1
QGIS User Guide, Release testing
2
Chapter 1. Preamble
CHAPTER
2
Foreword
Welcome to the wonderful world of Geographical Information Systems (GIS)!
QGIS is an Open Source Geographic Information System. The project was born in May of 2002 and was established as a project on SourceForge in June of the same year. We’ve worked hard to make GIS software (which
is traditionally expensive proprietary software) a viable prospect for anyone with basic access to a personal computer. QGIS currently runs on most Unix platforms, Windows, and macOS. QGIS is developed using the Qt toolkit
(https://www.qt.io) and C++. This means that QGIS feels snappy and has a pleasing, easy-to-use graphical user
interface (GUI).
QGIS aims to be a user-friendly GIS, providing common functions and features. The initial goal of the project
was to provide a GIS data viewer. QGIS has reached the point in its evolution where it is being used by many for
their daily GIS data-viewing needs. QGIS supports a number of raster and vector data formats, with new format
support easily added using the plugin architecture.
QGIS is released under the GNU General Public License (GPL). Developing QGIS under this license means that
you can inspect and modify the source code, and guarantees that you, our happy user, will always have access to
a GIS program that is free of cost and can be freely modified. You should have received a full copy of the license
with your copy of QGIS, and you also can find it in Appendix GNU General Public License.
Tip: Up-to-date Documentation
The latest version of this document can always be found in the documentation area of the QGIS website at http:
//www.qgis.org/en/docs/.
3
QGIS User Guide, Release testing
4
Chapter 2. Foreword
CHAPTER
3
Conventions
This section describes the uniform styles that will be used throughout this manual.
3.1 GUI Conventions
The GUI convention styles are intended to mimic the appearance of the GUI. In general, a style will reflect the
non-hover appearance, so a user can visually scan the GUI to find something that looks like the instruction in the
manual.
• Menu Options: Layer → Add a Raster Layer or Settings → Toolbars → Digitizing
• Tool:
Add a Raster Layer
• Button : [Save as Default]
• Dialog Box Title: Layer Properties
• Tab: General
• Checkbox:
Render
• Radio Button:
Postgis SRID
EPSG ID
• Select a number:
• Select a string:
• Browse for a file:
• Select a color:
• Slider:
• Input Text:
A shadow indicates a clickable GUI component.
5
QGIS User Guide, Release testing
3.2 Text or Keyboard Conventions
This manual also includes styles related to text, keyboard commands and coding to indicate different entities, such
as classes or methods. These styles do not correspond to the actual appearance of any text or coding within QGIS.
• Hyperlinks: http://qgis.org
• Keystroke Combinations: Press Ctrl+B, meaning press and hold the Ctrl key and then press the B key.
• Name of a File: lakes.shp
• Name of a Class: NewLayer
• Method: classFactory
• Server: myhost.de
• User Text: qgis --help
Lines of code are indicated by a fixed-width font:
PROJCS["NAD_1927_Albers",
GEOGCS["GCS_North_American_1927",
3.3 Platform-specific instructions
GUI sequences and small amounts of text may be formatted inline: Click
File
QGIS → Quit to close
QGIS. This indicates that on Linux, Unix and Windows platforms, you should click the File menu first, then Quit,
while on macOS platforms, you should click the QGIS menu first, then Quit.
Larger amounts of text may be formatted as a list:
•
Do this
•
Do that
•
Or do that
or as paragraphs:
Do this and this and this. Then do this and this and this, and this and this and this, and this and this and this.
Do that. Then do that and that and that, and that and that and that, and that and that and that, and that and that.
Screenshots that appear throughout the user guide have been created on different platforms; the platform is indicated by the platform-specific icon at the end of the figure caption.
6
Chapter 3. Conventions
CHAPTER
4
Features
QGIS offers many common GIS functionalities provided by core features and plugins. A short summary of six
general categories of features and plugins is presented below, followed by first insights into the integrated Python
console.
4.1 View data
You can view and overlay vector and raster data in different formats and projections without conversion to an
internal or common format. Supported formats include:
• Spatially-enabled tables and views using PostGIS, SpatiaLite and MS SQL Spatial, Oracle Spatial, vector
formats supported by the installed OGR library, including ESRI shapefiles, MapInfo, SDTS, GML and
many more. See section Working with Vector Data.
• Raster and imagery formats supported by the installed GDAL (Geospatial Data Abstraction Library) library,
such as GeoTIFF, ERDAS IMG, ArcInfo ASCII GRID, JPEG, PNG and many more. See section Working
with Raster Data.
• GRASS raster and vector data from GRASS databases (location/mapset). See section GRASS GIS Integration.
• Online spatial data served as OGC Web Services, including WMS, WMTS, WCS, WFS, and WFS-T. See
section Working with OGC Data.
4.2 Explore data and compose maps
You can compose maps and interactively explore spatial data with a friendly GUI. The many helpful tools available
in the GUI include:
• QGIS browser
• On-the-fly reprojection
• DB Manager
• Print layout
• Overview panel
7
QGIS User Guide, Release testing
• Spatial bookmarks
• Annotation tools
• Identify/select features
• Edit/view/search attributes
• Data-defined feature labelling
• Data-defined vector and raster symbology tools
• Atlas map composition with graticule layers
• North arrow scale bar and copyright label for maps
• Support for saving and restoring projects
4.3 Create, edit, manage and export data
You can create, edit, manage and export vector and raster layers in several formats. QGIS offers the following:
• Digitizing tools for OGR-supported formats and GRASS vector layers
• Ability to create and edit shapefiles and GRASS vector layers
• Georeferencer plugin to geocode images
• GPS tools to import and export GPX format, and convert other GPS formats to GPX or down/upload directly
to a GPS unit (On Linux, usb: has been added to list of GPS devices.)
• Support for visualizing and editing OpenStreetMap data
• Ability to create spatial database tables from shapefiles with DB Manager plugin
• Improved handling of spatial database tables
• Tools for managing vector attribute tables
• Option to save screenshots as georeferenced images
• DXF-Export tool with enhanced capabilities to export styles and plugins to perform CAD-like functions
4.4 Analyze data
You can perform spatial data analysis on spatial databases and other OGR- supported formats. QGIS currently
offers vector analysis, sampling, geoprocessing, geometry and database management tools. You can also use the
integrated GRASS tools, which include the complete GRASS functionality of more than 400 modules. (See section GRASS GIS Integration.) Or, you can work with the Processing Plugin, which provides a powerful geospatial
analysis framework to call native and third-party algorithms from QGIS, such as GDAL, SAGA, GRASS and
more. (See section Introduction.)
4.5 Publish maps on the Internet
QGIS can be used as a WMS, WMTS, WMS-C or WFS and WFS-T client, and as a WMS, WCS or WFS server.
(See section Working with OGC Data.) Additionally, you can publish your data on the Internet using a webserver
with UMN MapServer or GeoServer installed.
8
Chapter 4. Features
QGIS User Guide, Release testing
4.6 Extend QGIS functionality through plugins
QGIS can be adapted to your special needs with the extensible plugin architecture and libraries that can be used
to create plugins. You can even create new applications with C++ or Python!
4.6.1 Core Plugins
Core plugins include:
1. Coordinate Capture (Capture mouse coordinates in different CRSs)
2. DB Manager (Exchange, edit and view layers and tables from/to databases; execute SQL queries)
3. eVIS (Visualize events)
4. GDALTools (Integrate GDAL Tools into QGIS)
5. Georeferencer GDAL (Add projection information to rasters using GDAL)
6. GPS Tools (Load and import GPS data)
7. GRASS (Integrate GRASS GIS)
8. Metasearch Catalogue Client
9. Offline Editing (Allow offline editing and synchronizing with databases)
10. Processing
11. Spatial Query Plugin
12. Topology Checker (Find topological errors in vector layers)
4.6.2 External Python Plugins
QGIS offers a growing number of external Python plugins that are provided by the community. These plugins
reside in the official Plugins Repository and can be easily installed using the Python Plugin Installer. See Section
The Plugins Dialog.
4.7 Python Console
For scripting, it is possible to take advantage of an integrated Python console, which can be opened from menu:
Plugins → Python Console. The console opens as a non-modal utility window. For interaction with the QGIS environment, there is the qgis.utils.iface variable, which is an instance of QgsInterface. This interface
allows access to the map canvas, menus, toolbars and other parts of the QGIS application. You can create a script,
then drag and drop it into the QGIS window and it will be executed automatically.
For further information about working with the Python console and programming QGIS plugins and applications,
please refer to PyQGIS-Developer-Cookbook.
4.8 Known Issues
4.8.1 Number of open files limitation
If you are opening a large QGIS project and you are sure that all layers are valid, but some layers are flagged as
bad, you are probably faced with this issue. Linux (and other OSs, likewise) has a limit of opened files by process.
Resource limits are per-process and inherited. The ulimit command, which is a shell built-in, changes the limits
only for the current shell process; the new limit will be inherited by any child processes.
4.6. Extend QGIS functionality through plugins
9
QGIS User Guide, Release testing
You can see all current ulimit info by typing:
$ ulimit -aS
You can see the current allowed number of opened files per process with the following command on a console:
$ ulimit -Sn
To change the limits for an existing session, you may be able to use something like:
$ ulimit -Sn #number_of_allowed_open_files
$ ulimit -Sn
$ qgis
To fix it forever
On most Linux systems, resource limits are set on login by the pam_limits module according to the settings
contained in /etc/security/limits.conf or /etc/security/limits.d/*.conf. You should be
able to edit those files if you have root privilege (also via sudo), but you will need to log in again before any
changes take effect.
More info:
http://www.cyberciti.biz/faq/linux-increase-the-maximum-number-of-open-files/
open-files-in-linux?lang=en
10
http://linuxaria.com/article/
Chapter 4. Features
CHAPTER
5
What’s new in QGIS testing
This release contains new features and extends the programmatic interface over previous versions. We recommend
that you use this version over previous releases.
This release includes hundreds of bug fixes and many new features and enhancements over QGIS 2.18 that
will be described in this manual. You may also review the visual changelogs at http://qgis.org/en/site/forusers/
visualchangelogs.html.
11
QGIS User Guide, Release testing
12
Chapter 5. What’s new in QGIS testing
CHAPTER
6
Getting Started
This chapter gives a quick overview of installing QGIS, some sample data from the QGIS web page, and running
a first and simple session visualizing raster and vector layers.
6.1 Installation
Installation of QGIS is very simple. Standard installer packages are available for MS Windows and macOS. For
many flavors of GNU/Linux, binary packages (rpm and deb) or software repositories are provided to add to your
installation manager. Get the latest information on binary packages at the QGIS website at http://download.qgis.
org.
6.1.1 Installation from source
If you need to build QGIS from source, please refer to the installation instructions. They are distributed with the
QGIS source code in a file called INSTALL. You can also find them online at http://htmlpreview.github.io/?https:
//raw.github.com/qgis/QGIS/master/doc/INSTALL.html. If you want to build a particular release, you should
replace master by the release branch (commonly in the release-X_Y form) in the above-mentioned link
because instructions may differ.
6.1.2 Installation on external media
QGIS allows you to define a --configpath option that overrides the default path for user configuration (e.g.,
~/.qgis2 under Linux) and forces QSettings to use this directory, too. This allows you to, for instance, carry a
QGIS installation on a flash drive together with all plugins and settings. See section System Settings for additional
information.
6.1.3 Sample Data
The user guide contains examples based on the QGIS sample dataset.
The Windows installer has an option to download the QGIS sample dataset. If checked, the data will be downloaded to your My Documents folder and placed in a folder called GIS Database. You may use Windows
Explorer to move this folder to any convenient location. If you did not select the checkbox to install the sample
dataset during the initial QGIS installation, you may do one of the following:
13
QGIS User Guide, Release testing
• Use GIS data that you already have
• Download sample data from http://qgis.org/downloads/data/qgis_sample_data.zip
• Uninstall QGIS and reinstall with the data download option checked (only recommended if the above solutions are unsuccessful)
For GNU/Linux and macOS, there are not yet dataset installation packages available as rpm, deb or dmg. To
use the sample dataset, download the file qgis_sample_data as a ZIP archive from http://qgis.org/downloads/
data/ and unzip the archive on your system.
The Alaska dataset includes all GIS data that are used for examples and screenshots in the user guide; it also
includes a small GRASS database. The projection for the QGIS sample dataset is Alaska Albers Equal Area with
units feet. The EPSG code is 2964.
PROJCS["Albers Equal Area",
GEOGCS["NAD27",
DATUM["North_American_Datum_1927",
SPHEROID["Clarke 1866",6378206.4,294.978698213898,
AUTHORITY["EPSG","7008"]],
TOWGS84[-3,142,183,0,0,0,0],
AUTHORITY["EPSG","6267"]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG","8901"]],
UNIT["degree",0.0174532925199433,
AUTHORITY["EPSG","9108"]],
AUTHORITY["EPSG","4267"]],
PROJECTION["Albers_Conic_Equal_Area"],
PARAMETER["standard_parallel_1",55],
PARAMETER["standard_parallel_2",65],
PARAMETER["latitude_of_center",50],
PARAMETER["longitude_of_center",-154],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["us_survey_feet",0.3048006096012192]]
If you intend to use QGIS as a graphical front end for GRASS, you can find a selection of sample locations (e.g.,
Spearfish or South Dakota) at the official GRASS GIS website, http://grass.osgeo.org/download/sample-data/.
6.2 Launching QGIS
6.2.1 Starting and Stopping QGIS
Starting QGIS is done as you usually do for any other application on your platform. It means that you can launch
QGIS by:
• typing qgis at a command prompt, assuming that QGIS is added to your PATH or you’re in its installation
folder
• using
the Applications menu if using a precompiled binary,
the Start menu or
the Dock
• double clicking the icon in your Applications folder or desktop shortcut
• double clicking an existing QGIS project (.qgs) file. Note that this will also open the project
To stop QGIS, click:
•
•
the menu option Project → Exit QGIS or use the shortcut Ctrl+Q
QGIS → Quit QGIS, or use the shortcut Cmd+Q
• or use the red cross at the right top corner of the main interface of the application.
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6.2.2 Command Line Options
In previous section you learned how to start QGIS. You will see that QGIS also provides further command line
options.
QGIS supports a number of options when started from the command line. To get a list of the options, enter qgis
--help on the command line. The usage statement for QGIS is:
qgis --help
Returns:
QGIS - 2.16.1-Nødebo 'Nødebo' (8545b3b)
QGIS is a user friendly Open Source Geographic Information System.
Usage: /usr/bin/qgis.bin [OPTION] [FILE]
OPTION:
[--snapshot filename]
emit snapshot of loaded datasets to given file
[--width width]
width of snapshot to emit
[--height height]
height of snapshot to emit
[--lang language]
use language for interface text
[--project projectfile]
load the given QGIS project
[--extent xmin,ymin,xmax,ymax]
set initial map extent
[--nologo] hide splash screen
[--noversioncheck] don't check for new version of QGIS at startup
[--noplugins]
don't restore plugins on startup
[--nocustomization] don't apply GUI customization
[--customizationfile]
use the given ini file as GUI customization
[--optionspath path]
use the given QSettings path
[--configpath path] use the given path for all user configuration
[--authdbdirectory path] use the given directory for authentication database
[--code path]
run the given python file on load
[--defaultui]
start by resetting user ui settings to default
[--dxf-export filename.dxf] emit dxf output of loaded datasets to given file
[--dxf-extent xmin,ymin,xmax,ymax] set extent to export to dxf
[--dxf-symbology-mode none|symbollayer|feature]
symbology mode for dxf output
[--dxf-scale-denom scale]
scale for dxf output
[--dxf-encoding encoding]
encoding to use for dxf output
[--dxf-preset visiblity-preset]
layer visibility preset to use for dxf output
[--help]
this text
[--]
treat all following arguments as FILEs
FILE:
Files specified on the command line can include rasters,
vectors, and QGIS project files (.qgs):
1. Rasters - supported formats include GeoTiff, DEM
and others supported by GDAL
2. Vectors - supported formats include ESRI Shapefiles
and others supported by OGR and PostgreSQL layers using
the PostGIS extension
Tip: Example Using command line arguments
You can start QGIS by specifying one or more data files on the command line. For example, assuming you are
in the qgis_sample_data directory, you could start QGIS with a vector layer and a raster file set to load on
startup using the following command: qgis ./raster/landcover.img ./gml/lakes.gml
Command line option --snapshot
This option allows you to create a snapshot in PNG format from the current view. This comes in handy when you
have a lot of projects and want to generate snapshots from your data.
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Currently, it generates a PNG file with 800x600 pixels. This can be adjusted using the --width and --height
command line arguments. A filename can be added after --snapshot.
Command line option --lang
Based on your locale, QGIS selects the correct localization. If you would like to change your language, you can
specify a language code. For example, qgis --lang it starts QGIS in Italian localization.
Command line option --project
Starting QGIS with an existing project file is also possible. Just add the command line option --project
followed by your project name and QGIS will open with all layers in the given file loaded.
Command line option --extent
To start with a specific map extent use this option. You need to add the bounding box of your extent in the
following order separated by a comma:
--extent xmin,ymin,xmax,ymax
Command line option --nologo
This command line argument hides the splash screen when you start QGIS.
command line option --noversioncheck
Don’t check for new version of QGIS at startup.
Command line option --noplugins
If you have trouble at start-up with plugins, you can avoid loading them at start-up with this option. They will still
be available from the Plugins Manager afterwards. Command line option --customizationfile
Using this command line argument, you can define a GUI customization file, that will be used at startup.
Command line option --nocustomization
Using this command line argument, existing GUI customization will not be applied at startup.
Command line option --optionspath
You can have multiple configurations and decide which one to use when starting QGIS with this option. See
Options to confirm where the operating system saves the settings files. Presently, there is no way to specify a file
to write settings to; therefore, you can create a copy of the original settings file and rename it. The option specifies
path to directory with settings. For example, to use /path/to/config/QGIS/QGIS2.ini settings file, use
option:
--optionspath /path/to/config/
Command line option --configpath
This option is similar to the one above, but furthermore overrides the default path for user configuration (~/.
qgis2) and forces QSettings to use this directory, too. This allows users to, for instance, carry a QGIS installation
on a flash drive together with all plugins and settings.
Command line option --authdbdirectory
Again, this option is similar to the one above but define the path to the directory where the authentication database
will be stored.
Command line option --code
This option can be used to run a given python file directly after QGIS has started.
For example, when you have a python file named load_alaska.py with following content:
from qgis.utils import iface
raster_file = "/home/gisadmin/Documents/qgis_sample_data/raster/landcover.img"
layer_name = "Alaska"
iface.addRasterLayer(raster_file, layer_name)
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Assuming you are in the directory where the file load_alaska.py is located, you can start QGIS, load the
raster file landcover.img and give the layer the name ‘Alaska’ using the following command: qgis --code
load_alaska.py
Command line options --dxf-*
These options can be used to export QGIS project into a DXF file. Several options are available:
• –dxf-export: the DXF filename into which to export the layers;
• –dxf-extent: the extent of the final DXF file;
• –dxf-symbology-mode: several values can be used here: none (no symbology), symbollayer (Symbol layer
symbology), feature (feature symbology);
• –dxf-scale-deno: the scale denominator of the symbology;
• –dxf-encoding: the file encoding;
• –dxf-preset: choose a visibility preset. These presets are defined in the layer tree, see Layers Panel.
6.3 Sample Session: Load raster and vector layers
Now that you have QGIS installed and a sample dataset available, we would like to demonstrate a short and simple
QGIS sample session. We will visualize a raster and a vector layer. We will use:
• the landcover raster layer i.e., qgis_sample_data/raster/landcover.img
• and the lakes vector layer i.e., qgis_sample_data/gml/lakes.gml.
1. Start QGIS as seen in Starting and Stopping QGIS
2. Click on the
Add Raster Layer
icon.
3. Browse to the folder qgis_sample_data/raster/, select the ERDAS IMG file landcover.img
and click [Open].
4. If the file is not listed, check if the Files of type
combo box at the bottom of the dialog is set on the
right type, in this case Erdas Imagine Images (*.img *.IMG).
5. Now click on the
6.
Add Vector Layer
icon.
File should be selected as Source Type in the new Add vector layer dialog. Now click [Browse] to select
the vector layer.
7. Browse to the folder qgis_sample_data/gml/, select Geography Markup Language [GML]
[OGR] (*.gml *.GML) from the Filter
combo box, then select the GML file lakes.gml and
click [Open]. In the Add vector layer dialog, click [OK]. The Coordinate Reference System Selector dialog
opens with NAD27 / Alaska Alberts selected, click [OK].
8. Zoom in a bit to your favourite area with some lakes.
9. Double click the lakes layer in the map legend to open the Properties dialog.
10. Click on the Style tab and select a blue as fill color.
11. Click on the Labels tab and select Show labels for this layer in the drop-down menu to enable labeling.
Then from the Label with list, choose the NAMES field as the field containing labels.
12. To improve readability of labels, you can add a white buffer around them by clicking Buffer in the list on
the left, checking
Draw text buffer and choosing 3 as buffer size.
13. Click [Apply]. Check if the result looks good, and finally click [OK].
You can see how easy it is to visualize raster and vector layers in QGIS. Let’s move on to the sections that follow
to learn more about the available functionality, features and settings, and how to use them.
6.3. Sample Session: Load raster and vector layers
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6.4 Projects
The state of your QGIS session is considered a project. QGIS works on one project at a time. Settings are
considered as being either per-project or as a default for new projects (see section Options). QGIS can save the
Save or Project →
Save
state of your workspace into a project file using the menu options Project →
As. . . . If the loaded project file on disk was meanwhile changed, by default, QGIS will ask you if you want to
overwrite the changes into the project file. This behavior is set by checking
source changes when required under Settings → Options → General menu .
Load saved projects into a QGIS session using Project →
→ Open Recent →.
Prompt to save project and data
Open. . . , Project → New from template or Project
At startup, a list of screenshot with the name and path of each of the most recent projects (up to ten) is shown
instead of a white and empty map canvas. This is a handy and quicker way to remember what a project was about
and double-click a row opens the selected project. If you’re willing to create a new project, just add new layers
and the list disappears.
New. Either of these menu options will
If you wish to clear your session and start fresh, choose Project →
prompt you to save the existing project if changes have been made since it was opened or last saved.
The kinds of information saved in a project file include:
• Layers added
• Which layers can be queried
• Layer properties, including symbolization and styles
• Projection for the map view
• Last viewed extent
• Print layouts
• Print layout elements with settings
• Print layout atlas settings
• Digitizing settings
• Table Relations
• Project Macros
• Project default styles
• Plugins settings
• QGIS Server settings from the OWS settings tab in the Project properties
• Queries stored in the DB Manager
The project file is saved in XML format, so it is possible to edit the file outside QGIS if you know what you are
doing. The file format has been updated several times compared with earlier QGIS versions. Project files from
older QGIS versions may not work properly any more. To be made aware of this, in the General tab under Settings
→ Options you should tick
Warn when opening a project file saved with an older version of QGIS.
Whenever you save a project in QGIS a backup of the project file is made with the extension .qgs~.
The default extension for QGIS projects is .qgs but a project may be zipped in a .qgz file too. Actually, the
.qgs file is just embedded in an archive, so you still have the possibility to unzip it in order to manually edit
XML information in a text editor. These .qgz files can also be opened like default .qgs files. To zip a project,
the corresponding extension has just to be selected in Project →
Save action automatically zip your current project.
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Note: A zipped project may be particularly useful with the Auxiliary storage mechanism in order to embed the
underlying database.
6.5 Output
There are several ways to generate output from your QGIS session. We have discussed one already in section
Projects, saving as a project file. Here is a sampling of other ways to produce output files:
Save as Image. . . opens a file dialog where you select the name, path and type
• Menu option Project →
of image (PNG, JPG and many other formats). A world file with extension PNGW or JPGW saved in the
same folder georeferences the image.
• Menu option Project → DXF Export. . . opens a dialog where you can define the ‘Symbology mode’, the
‘Symbology scale’ and vector layers you want to export to DXF. Through the ‘Symbology mode’ symbols
from the original QGIS Symbology can be exported with high fidelity.
New Print Layout opens a dialog where you can layout and print the current
• Menu option Project →
map canvas (see section Laying out the maps).
6.5. Output
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CHAPTER
7
QGIS GUI
When QGIS starts, you are presented with the GUI as shown in the figure (the numbers 1 through 5 in yellow
circles are discussed below).
Fig. 7.1: QGIS GUI with Alaska sample data
Note: Your window decorations (title bar, etc.) may appear different depending on your operating system and
window manager.
The QGIS GUI is divided into five components:
1. Menu Bar
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2. Toolbars
3. Panels
4. Map View
5. Status Bar
These five components of the QGIS interface are described in more detail in the following sections. Two more
sections present keyboard shortcuts and context help.
7.1 Menu Bar
The menu bar provides access to various QGIS features using a standard hierarchical menu. The top-level menus
and a summary of some of the menu options are listed below, together with the associated icons as they appear on
the toolbar, and keyboard shortcuts. The shortcuts presented in this section are the defaults; however, keyboard
shortcuts can also be configured manually using the Keyboard Shortcuts dialog, opened from Settings → menu.
Although most menu options have a corresponding tool and vice-versa, the menus are not organized exactly like
the toolbars. The toolbar containing the tool is listed after each menu option as a checkbox entry. Some menu
options only appear if the corresponding plugin is loaded. For more information about tools and toolbars, see
section Toolbars.
Note: QGIS is a cross-platform application meaning that though it provides you with the same tools, they may be
placed in different menus according to the operating system specification. The lists below show the most common
location and precise when there is a variation.
7.1.1 Project
Menu Option
Shortcut
Reference
Toolbar
New
Ctrl+N
see Projects
Project
Ctrl+O
see Projects
see Projects
see Projects
Project
Save
Ctrl+S
see Projects
Project
Save As. . .
Ctrl+Shift+S see Projects
Project
Open
New from template →
Open Recent →
Save as Image. . .
DXF Export. . .
DWG/DXF Import. . .
Snapping Options. . .
Project
Proper-
see Output
see Output
S
see Setting the Snapping Tolerance and Search Radius
Ctrl+Shift+P see Projects
ties. . .
New Print Layout
Ctrl+P
Layout Manager. . .
Layouts →
Exit QGIS
Under
22
see Laying out the maps
Project
see Laying out the maps
see Laying out the maps
Project
Ctrl+Q
macOS, the Exit QGIS command corresponds to QGIS → Quit QGIS (Cmd+Q).
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7.1.2 Edit
Menu Option
Shortcut
Reference
Toolbar
Undo
Ctrl+Z
see Advanced digitizing
Advanced Digitizing
Redo
Ctrl+Shift+Z see Advanced digitizing
Advanced Digitizing
Cut Features
Ctrl+X
see Digitizing an existing layer
Digitizing
Copy Features
Ctrl+C
see Digitizing an existing layer
Digitizing
Ctrl+V
see Digitizing an existing layer
see Working with the Attribute
Table
see Selecting features
Digitizing
see Digitizing an existing layer
Digitizing
Add Circular String
see Digitizing an existing layer
Digitizing
Add Circular String by Radius
see Digitizing an existing layer
Digitizing
Move Feature(s)
see Digitizing an existing layer
Digitizing
Delete Selected
see Digitizing an existing layer
Digitizing
see Editing attribute values
Digitizing
Rotate Feature(s)
see Advanced digitizing
Advanced Digitizing
Simplify Feature
see Advanced digitizing
Advanced Digitizing
Add Ring
see Advanced digitizing
Advanced Digitizing
Add Part
see Advanced digitizing
Advanced Digitizing
Fill Ring
see Advanced digitizing
Advanced Digitizing
Delete Ring
see Advanced digitizing
Advanced Digitizing
Delete Part
see Advanced digitizing
Advanced Digitizing
Reshape Features
see Advanced digitizing
Advanced Digitizing
Offset Curve
see Advanced digitizing
Advanced Digitizing
Split Features
see Advanced digitizing
Advanced Digitizing
Split Parts
see Advanced digitizing
Advanced Digitizing
Merge Selected Features
see Advanced digitizing
Merge Attr. of Selected Features
see Advanced digitizing
Node Tool
see Digitizing an existing layer
Paste Features
Paste features as →
Select →
Add Feature
Modify Attributes of Selected
Features
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Ctrl+.
Attributes
Advanced Digitizing
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Toggle editing
After activating
mode for a layer, you will enable the Add Feature icon in the Edit menu
depending on the layer type (point, line or polygon).
7.1.3 Edit (extra)
Menu Option
7.1. Menu Bar
Shortcut
Reference
Toolbar
Add Feature
see Digitizing an existing layer
Digitizing
Add Feature
see Digitizing an existing layer
Digitizing
Add Feature
see Digitizing an existing layer
Digitizing
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7.1.4 View
Menu Option
New Map View
Shortcut
Reference
Toolbar
Map Navigation
Ctrl+M
see Zooming and Panning
Pan Map
Map Navigation
Pan Map to Selection
Map Navigation
Zoom In
Ctrl+Alt++
see Zooming and Panning
Map Navigation
Zoom Out
Ctrl+Alt+-
see Zooming and Panning
Map Navigation
Ctrl+Shift+I
see Identify Features
see Measuring
Attributes
Attributes
see Statistical Summary Panel
Attributes
Identify Features
Measure →
Statistical Summary
Zoom Full
Map Navigation
Ctrl+Shift+F
Zoom To Layer
Zoom To Selection
Map Navigation
Map Navigation
Ctrl+J
Zoom Last
Map Navigation
Zoom Next
Map Navigation
Zoom To Native Resolution
Decorations →
Preview mode →
Map Navigation
see Decorations
Map Tips
see Display Properties
Attributes
New Bookmark. . .
Ctrl+B
see Spatial Bookmarks
Attributes
Show Bookmarks
Ctrl+Shift+B
see Spatial Bookmarks
Attributes
Refresh
F5
Map Navigation
Show All Layers
Ctrl+Shift+U
Manage Layers
Hide All Layers
Ctrl+Shift+H
Manage Layers
Show Selected Layers
Hide Selected Layers
Hide Deselected Layers
Panels →
Toolbars →
Toggle Full Screen Mode
Toggle Panel Visibility
Under
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see Panels and Toolbars
see Panels and Toolbars
F11
Ctrl+Tab
Linux KDE, Panels →, Toolbars → and Toggle Full Screen Mode are rather placed in Settings menu.
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7.1.5 Layer
Menu Option
Create Layer →
Shortcut
Reference
see Creating new vector layers
Add Layer →
see Exploring Data Formats and Fields
Embed Layers and Groups. . .
Add from Layer Definition
File. . .
see Nesting Projects
Copy style
see Save and Share Layer Properties
Paste style
see Save and Share Layer Properties
Toolbar
Manage Layers
Manage Layers
see Working with the Attribute Table
Attributes
Toggle Editing
see Digitizing an existing layer
Digitizing
Save Layer Edits
see Digitizing an existing layer
Digitizing
see Digitizing an existing layer
see Creating new layers from an existing
layer
Digitizing
Open Attribute Table
F6
Current Edits →
Save As. . .
Save As Layer Definition
File. . .
Remove Layer/Group
Duplicate Layer(s)
Set Scale Visibility of Layer(s)
Set CRS of Layer(s)
Set Project CRS from Layer
Properties. . .
Filter. . .
Labeling
Add to Overview
Ctrl+D
Ctrl+Shift+C
Ctrl+F
see The Vector Properties Dialog
see Query Builder
see Labels Properties
Manage Layers
Ctrl+Shift+O
Add All To Overview
Remove
Overview
All
From
7.1.6 Settings
Menu Option
Style Manager. . .
Shortcut
Reference
see The Style Manager
Custom Projections. . .
see Custom Coordinate Reference System
Keyboard Shortcuts. . .
see Keyboard shortcuts
Interface Customization. . .
Options. . .
Toolbar
see Customization
see Options
Under
Linux KDE, you’ll find more tools in Settings menu such as Panels →, Toolbars → and Toggle Full
Screen Mode.
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7.1.7 Plugins
Menu Option
Shortcut
Toolbar
see The Plugins Dialog
Manage and Install Plugins. . .
Install Plugins from ZIP. . .
Python Console
Reference
see The Plugins Dialog
Ctrl+Alt+P
When starting QGIS for the first time not all core plugins are loaded.
7.1.8 Vector
Menu Option
OpenStreetMap →
Shortcut
Reference
see Importing OpenStreetMap Vectors
Analysis Tools →
see Vector menu
Research Tools →
see Vector menu
Geoprocessing Tools →
see Vector menu
Geometry Tools →
see Vector menu
Data Management Tools →
see Vector menu
Toolbar
When starting QGIS for the first time not all core plugins are loaded. Many of the above-mentioned sub-menus
require the core plugin Processing to be activated.
7.1.9 Raster
Menu Option
Raster calculator. . .
Align Raster. . .
Shortcut
Reference
see Raster Calculator
see Raster Alignment
Toolbar
Analysis →
see GDAL Tools Plugin
Projection →
see GDAL Tools Plugin
Conversion →
see GDAL Tools Plugin
Miscellaneous →
see GDAL Tools Plugin
Extraction →
see GDAL Tools Plugin
When starting QGIS for the first time not all core plugins are loaded. Many of the above-mentioned sub-menus
require the core plugin Processing to be activated.
7.1.10 Database
Menu Option
Database →
Shortcut
Reference
see DB Manager Plugin
Toolbar
Database
When starting QGIS for the first time not all core plugins are loaded.
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7.1.11 Web
Menu Option
Metasearch
Shortcut
Reference
see MetaSearch Catalog Client
Toolbar
Web
When starting QGIS for the first time not all core plugins are loaded.
7.1.12 Processing
Menu Option
Shortcut
Reference
Toolbox
Ctrl+Alt+T
see The toolbox
Graphical Modeler. . .
Ctrl+Alt+M
see The graphical modeler
Ctrl+Alt+H
see The history manager
Ctrl+Alt+R
see Configuring external applications
History. . .
Results Viewer
Toolbar
When starting QGIS for the first time not all core plugins are loaded.
7.1.13 Help
Menu Option
Shortcut
Help Contents
What’s This?
API Documentation
Report an Issue
Need commercial support?
QGIS Home Page
Reference
Toolbar
F1
Help
Shift+F1
Help
Ctrl+H
Check QGIS Version
About
QGIS Sponsors
7.1.14 QGIS
This menu is only available under
macOS and contains some OS related commands.
Menu Option
Preferences
About QGIS
Hide QGIS
Show All
Hide Others
Quit QGIS
Shortcut
Reference
Cmd+Q
Preferences and About QGIS are the same commands as Settings → Options and Help → About. Quit QGIS
corresponds to Project → Exit QGIS under the other platforms.
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7.2 Panels and Toolbars
From the View menu (or
Settings), you can switch on and off QGIS widgets (Panels →) or toolbars (Toolbars
→). You can (de)activate any of them by right-clicking the menu bar or a toolbar and choose the item you want.
Each panel or toolbar can be moved and placed wherever you feel comfortable within QGIS interface. The list
can also be extended with the activation of Core or external plugins.
7.2.1 Toolbars
The toolbar provides access to most of the same functions as the menus, plus additional tools for interacting with
the map. Each toolbar item has pop-up help available. Hold your mouse over the item and a short description of
the tool’s purpose will be displayed.
Every toolbar can be moved around according to your needs. Additionally, they can be switched off using the
right mouse button context menu, or by holding the mouse over the toolbars.
Fig. 7.2: The Toolbars menu
Tip: Restoring toolbars
If you have accidentally hidden a toolbar, you can get it back by choosing menu option View → Toolbars →
(or
Settings → Toolbars →). If for some reason a toolbar (or any other widget) totally disappears from the
interface, you’ll find tips to get it back at restoring initial GUI.
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7.2.2 Panels
Besides toolbars, QGIS provides by default many panels to work with. Panels are special widgets that you can
interact with (selecting options, checking boxes, filling values. . . ) in order to perform a more complex task.
Fig. 7.3: The Panels menu
Below are listed default panels provided by QGIS:
• the Layers Panel
• the Browser Panel
• the Advanced Digitizing Panel
• the Spatial Bookmarks Panel
• the GPS Information Panel
• the Tile Scale Panel
• the Identify Panel
• the User Input Panel
• the Layer Order Panel
• the Layer Styling Panel
• the Statistical Summary Panel
• the QGIS Overview Panel
• the Log Messages Panel
• the Undo/Redo Panel
• the Processing Toolbox
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7.3 Map View
Also called Map canvas, this is the “business end” of QGIS — maps are displayed in this area. The map displayed
in this window will depend on the vector and raster layers you have chosen to load.
When you add a layer (see e.g. Opening Data), QGIS automatically looks for its Coordinate Reference System
(CRS) and zooms to its extent if you work in a blank QGIS project. The layer’s CRS is then applied to the project.
If there are already layers in the project, and in the case the new layer has the same CRS as the project, its features
falling in the current map canvas extent will be visualized. If the new layer is in a different CRS from the project’s,
you must Enable on-the-fly CRS transformation from the Project → Project Properties → CRS (see Define On
The Fly (OTF) CRS Transformation). The added layer should now be visible if data are available in the current
view extent.
The map view can be panned, shifting the focus of the map display to another region, and it can be zoomed in
and out. Various other operations can be performed on the map as described in the Toolbars description. The map
view and the legend are tightly bound to each other — the maps in view reflect changes you make in the legend
area.
Tip: Zooming the Map with the Mouse Wheel
You can use the mouse wheel to zoom in and out on the map. Place the mouse cursor inside the map area and roll
the wheel forward (away from you) to zoom in and backwards (towards you) to zoom out. The zoom is centered
on the mouse cursor position. You can customize the behavior of the mouse wheel zoom using the Map tools tab
under the Settings → Options menu.
Tip: Panning the Map with the Arrow Keys and Space Bar
You can use the arrow keys to pan the map. Place the mouse cursor inside the map area and click on the right
arrow key to pan east, left arrow key to pan west, up arrow key to pan north and down arrow key to pan south. You
can also pan the map using the space bar or the click on mouse wheel: just move the mouse while holding down
space bar or click on mouse wheel.
7.4 Status Bar
The status bar provides you with general information about the map view, and actions processed or available and
offers you tools to manage the map view.
On the left side of the status bar, you can get a summary of actions you’ve done (such as selecting features in
a layer, removing layer) or a long description of the tool you are hovering over (not available for all tools). On
startup, the bar status also informs you about availability of new or upgradeable plugins (if checked in Plugin
Manager settings).
In case of lengthy operations, such as gathering of statistics in raster layers or rendering several layers in map
view, a progress bar is displayed in the status bar to show the current progress of the action.
The
Coordinate option shows the current position of the mouse, following it while moving across the map
view. You can set the unit (and precision) to use in the project properties, General tab. Click on the small button
at the left of the textbox to toggle between the Coordinate option and the
Extents option that displays in map
units, the coordinates of the current lower leftmost and upper rightmost points of the map view, as you pan and
zoom in and out.
Next to the coordinate display you will find the Scale display. It shows the scale of the map view. If you zoom in
or out, QGIS shows you the current scale. There is a scale selector, which allows you to choose among predefined
and custom scales to assign to the map view.
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On the right side of the scale display you can define a current magnification level for your map view. This allows
to zoom in to a map without altering the map scale, making it easier to accurately tweak the positions of labels
and symbols. The magnification level is expressed as a percentage. If the Magnifier has a level of 100%, then
the current map is not magnified. Additionally, a default magnification value can be defined within Settings →
Options → Rendering → Rendering behaviour, which is very useful for high resolution screen to avoid too small
symbols.
To the right of the magnifier tool you can define a current clockwise rotation for your map view in degrees.
On the right side of the status bar, there is a small checkbox which can be used to temporarily prevent layers being
rendered to the map view (see section Rendering).
Current CRS:
icon with the EPSG code of the current project
To the right of the render functions, you find the
CRS. Clicking on this lets you Enable ‘on the fly’ CRS transformation properties for the current project and apply
another CRS to the map view.
Messages
button opens the Log Messages Panel which informs you on underlying process (QGIS
Finally, the
startup, plugins loading, processing tools. . . )
Tip: Calculating the Correct Scale of Your Map Canvas
When you start QGIS, the default CRS is WGS 84 (epsg 4326) and units are degrees. This means that QGIS
will interpret any coordinate in your layer as specified in degrees. To get correct scale values, you can either
manually change this setting, e.g. to meters, in the General tab under Project → Project Properties, or you can
Current CRS:
icon seen above. In the latter case, the units are set to what the project projection specifies
use the
(e.g., +units=us-ft).
Note that CRS choice on startup can be set in Settings → Options → CRS.
7.4. Status Bar
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CHAPTER
8
General Tools
8.1 Context help
When you need help on a specific topic, you can access context help via the [Help] button available in most
dialogs — please note that third-party plugins can point to dedicated web pages.
8.2 Panels
QGIS provides by default many panels to work with. Some of these panels are described below while others may
be found in different parts of the document. A complete list of default panels provided by QGIS is available at
Panels.
8.2.1 Layers Panel
The layers Panel lists all the layers in the project and helps you manage their visibility. A layer can be selected
and dragged up or down in the legend to change the Z-ordering. Z-ordering means that layers listed nearer the top
of the legend are drawn over layers listed lower down in the legend.
Note: The Z-ordering behavior can be overridden by the Layer Order panel.
At the top of the Layers panel, a toolbar allows you to:
•
Open the layer styling dock
: toggle the layer styling panel on and off.
•
Add new group
•
Manage Map Themes
•
Filter Legend by Map Content
: control visibility of layers and arrange them in different map themes.
: only the layers that are set visible and whose features intersect the current map
canvas have their style rendered in the layers panel. Otherwise, a generic NULL symbol is applied to the
layer. Based on the layer symbology, this is a convenient way to identify which kind of features from which
layers cover your area of interest.
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•
Filter Legend by Expression
: helps you apply an expression to remove from the selected layer tree styles that
have no feature satisfying the condition. This can be used for example to highlight features that are within
a given area/feature of another layer. From the drop-down list, you can edit and clear the expression set.
•
• and
Expand All
or
Collapse All
Remove Layer/Group
layers and groups in the layers panel.
currently selected.
Fig. 8.1: Layer Toolbar in Layers Panel
Note: Tools to manage the layers panel are also available for map and legend items of the print layout
Configuring map themes
The button
allows you to configure Map Themes in the legend. These themes are helpful to quickly switch
between different preconfigured layer combinations and can also be used in print layout. To create a map theme,
activate the layers you want to include and configure the desired layer styles. Then press the
button and choose
Add Theme. . . from the drop-down menu and enter a name for the new theme. The new theme is listed at the
bottom of the drop-down menu and can be restored by clicking on it.
The Replace Theme → option allows you to overwrite an existing theme with the currently enabled layers and
their styles while the Remove Current Theme button deletes the active theme.
All configured themes are also accessible in the print layout. This allows you to create a map layout based on
specific themes (see Main properties).
Overview of the context menu of the Layers panel
At the bottom of the toolbar, the main component of the Layers panel is the frame listing vector or raster layers
added to the project and, those layers can be organized in groups. Depending on the item selected in the panel, a
right-click shows a dedicated set of options presented below.
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Option
Vector Layer
Raster Layer
Group
Zoom to Layer/Group
Show in Overview
Zoom to Native Resolution (100%)
Stretch Using Current Extent
Remove
Duplicate
Set Layer Scale Visibility
Zoom to Visible Scale
Set Layer/Group CRS
Set Project CRS from Layer
Styles →
Copy Style
Paste Style
Open Attribute Table
Toggle Editing
Current Edits →
(in Edit mode)
Save As. . .
Save As Layer Definition File. . .
Filter. . .
Show Feature Count
Properties
Move to Top-level
Rename
Group Selected
Set Group WMS Data
Mutually Exclusive Group
Add Group
Table: Context menu from Layers Panel items
Toggle editing
is not available. See section Digitizing and editing a GRASS vector
For GRASS vector layers,
layer for information on editing GRASS vector layers.
Interact with Groups and layers
Layers in the legend window can be organized into groups. There are two ways to do this:
1. Press the
icon to add a new group. Type in a name for the group and press Enter. Now click on an
existing layer and drag it onto the group.
2. Select some layers, right click in the legend window and choose Group Selected. The selected layers will
automatically be placed in a new group.
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To bring a layer out of a group, you can drag it out, or right click on it and choose Move to top-level; the layer is
placed at the same level than the group it was inside. Groups can also be nested inside other groups.
The checkbox for a group will show or hide all the layers in the group with one click.
Enabling the Mutually Exclusive Group option you can make a group have only one layer visible at the same
time. Whenever a layer within the group is set visible the others will be toggled not visible.
It is possible to select more than one layer or group at the same time by holding down the Ctrl key while selecting
the layers with the left mouse button. You can then move all selected layers to a new group at the same time.
You may also delete more than one layer or group at once by selecting several items with the Ctrl key and
pressing Ctrl+D afterwards. This way, all selected layers or groups will be removed from the layers list.
Editing vector layer style
From the Layers panel, you have shortcuts to easily and quickly edit the layer rendering. Right-click on a vector
layer and select Styles → in the list in order to:
• see the currently applied styles to the layer. In case you defined many styles for the layer, you can switch
from one to another and have your layer rendering automatically updated in the map canvas.
• copy the current style, and when applicable, paste a copied style from another layer
• rename the current style, add a new one (which is actually a copy of the current one) or delete the current
style (when multiple styles available).
Note: The previous options are also available for raster layer.
Whether the features in the vector layer have all the same unique symbol or they are classified (in that case, the
layer is displayed in a tree structure with each class as sub-item), the following options are available at layer level
or class level:
• a Edit Symbol. . . button to open the The Symbol Selector dialog and update any property (symbol, size,
color. . . ) of the layer or feature symbol. Double-clicking on a feature does also open the Symbol Selector
dialog.
• a Color Selector widget with a Color Wheel from which you can click a color and have it automatically
update the symbol fill color. For convenience, Recent colors are available at the bottom of the color wheel.
• a
Show All Items and
Hide All Items to toggle on or off the visibility of all the classes of features.
This avoids (un)checking items one by one.
Tip: Quickly share a layer style
From the context menu, copy the style of a layer and paste it to a group or a selection of layers: the style is applied
to all the layers that are of the same type (vector vs raster) as the original layer and, in case of vector, have the
same geometry type (point, line or polygon).
8.2.2 Working with the Legend independent layer order
There is a panel that allows you to define an independent drawing order for the layers panel. You can activate
it in the menu Settings → Panels → Layer Order Panel. This feature allows you to, for instance, order your
layers in order of importance, but still display them in the correct order (see figure_layer_order; you can notice
that the airports features are displayed over the alaska polygon despite their layers placement in the Layers
panel). Unchecking the
behavior.
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Fig. 8.2: Define a legend independent layer order
8.2.3 Layer Styling Panel
This panel is somehow a shortcut to some of the features of the layer properties dialog. It indeed offers you a quick
and handy way to define the rendering and the behavior of a layer, and to visualize its effects without opening the
layer properties dialog.
Besides avoiding you dealing with the modal and blocking dialog of the layer properties, it also avoids you
cluttering the screen with features dialogs given that it embeds most of them (color selector, effects properties,
rule edit, label substitution. . . ): e.g., clicking color buttons inside the layer style panel causes the color selector
dialog to be opened inside the layer style panel itself rather than as a separate dialog.
From a drop-down list of current layers in the layer panel, select an item and:
• set its symbology, transparency, and histogram in case of raster layer. These options are the same available
in Raster Properties Dialog
• set its symbology, and labels. These options are the same available in The Vector Properties Dialog
• manage the associated style(s) as described in Managing Custom Styles
• follow the whole history of changes you applied to the layer style in the current project; you can therefore
cancel or restore to any state by selecting it in the list and hit [Apply] button.
Another powerful feature of this panel is the Live update checkbox. Tick it and your changes are automatically
rendered in the map canvas as you go on. You no longer need to hit the [Apply] button.
Tip: Add custom tabs to the Layer Styling panel
Using PyQGIS, you can set new tabs to manage layer properties in the Layer Styling Panel. See https://nathanw.
net/2016/06/29/qgis-style-dock-part-2-plugin-panels/ for an example.
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Fig. 8.3: Defining a layer symbology from the layer styling panel
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8.2.4 Statistical Summary Panel
This panel can show some statistics on a specific vector layers. The panel allows users to choose:
• the vector layer;
• the column or the expression;
• filter statistics to selected features;
• refresh the informations;
• the statistics information to display with the bottom right button.
Statistic information available are (depending on the field’s type):
Statistics
String
Integer
Float
Date
Count
Count Distinct Value
Count Missing value
Sum
Mean
Standard Deviation
Standard Deviation on Sample
Minimal value
Maximal value
Range
Minority
Majority
Variety
First Quartile
Third Quartile
Inter Quartile Range
Minimum Length
Maximum Length
Table: Statistics available for each field type
8.2.5 QGIS Overview Panel
In QGIS, you can use an overview panel that provides a full extent view of layers added to it. Within the view is
a rectangle showing the current map extent. This allows you to quickly determine which area of the map you are
currently viewing. Note that labels are not rendered to the map overview even if the layers in the map overview
have been set up for labelling. If you click and drag the red rectangle in the overview that shows your current
extent, the main map view will update accordingly.
8.2.6 Log Messages Panel
When loading or processing some operations, you can track and follow messages that appear in different tabs
using the
8.2. Panels
Log Messages Panel. It can be activated using the most right icon in the bottom status bar.
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Fig. 8.4: Show statistics on a field
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8.2.7 Undo/Redo Panel
For each layer being edited, this panel shows the list of actions done, allowing to quickly undo a set of actions by
simply selecting the action listed above. More details at Undo and Redo edits.
8.3 Rendering
By default, QGIS renders all visible layers whenever the map canvas is refreshed. The events that trigger a refresh
of the map canvas include:
• Adding a layer
• Panning or zooming
• Resizing the QGIS window
• Changing the visibility of a layer or layers
QGIS allows you to control the rendering process in a number of ways.
8.3.1 Scale Dependent Rendering
Scale-dependent rendering allows you to specify the minimum and maximum scales at which a layer (raster or
vector) will be visible. To set scale-dependent rendering, open the Properties dialog by double-clicking on the
layer in the legend. On the Rendering tab, tick the
(exclusive) and Maximum (inclusive) scale values.
Scale dependent visibility checkbox and enter the Minimum
You can also activate the scale dependent visibility on a layer from the Layers panel. Right-click on the layer and
in the context menu, select Set Layer Scale Visibility.
The
Set to current canvas scale
button helps you use the current map canvas scale as boundary of the range visibility.
Note: When a layer is not rendered in the map canvas due to the map scale out of its visibility scale range, the
layer is greyed in the Layers panel and a new option Zoom to Visible Scale appears in the layer context menu.
Select it and the map is zoomed to the layer’s nearest visibility scale.
8.3.2 Controlling Map Rendering
Map rendering can be controlled in various ways, as described below.
Suspending Rendering
To suspend rendering, click the
Render checkbox in the lower right corner of the status bar. When the
Render checkbox is not checked, QGIS does not redraw the canvas in response to any of the events described in
section Rendering. Examples of when you might want to suspend rendering include:
• Adding many layers and symbolizing them prior to drawing
• Adding one or more large layers and setting scale dependency before drawing
• Adding one or more large layers and zooming to a specific view before drawing
• Any combination of the above
Checking the
Render checkbox enables rendering and causes an immediate refresh of the map canvas.
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Setting Layer Add Option
You can set an option to always load new layers without drawing them. This means the layer will be added to
the map, but its visibility checkbox in the legend will be unchecked by default. To set this option, choose menu
By default new layers added to the
option Settings → Options and click on the Rendering tab. Uncheck the
map should be displayed checkbox. Any layer subsequently added to the map will be off (invisible) by default.
Stopping Rendering
To stop the map drawing, press the ESC key. This will halt the refresh of the map canvas and leave the map
partially drawn. It may take a bit of time between pressing ESC and the time the map drawing is halted.
Note: It is currently not possible to stop rendering — this was disabled in the Qt4 port because of User Interface
(UI) problems and crashes.
Influence Rendering Quality
QGIS has an option to influence the rendering quality of the map. Choose menu option Settings → Options, click
on the Rendering tab and select or deselect
performance.
Make lines appear less jagged at the expense of some drawing
Speed-up rendering
There are some settings that allow you to improve rendering speed. Open the QGIS options dialog using Settings
→ Options, go to the Rendering tab and select or deselect the following checkboxes:
•
Use render caching where possible to speed up redraws
•
Render layers in parallel using many CPU cores and then set the
Max cores to use.
Map Update interval, the content
• The map renders in the background onto a separate image and each
from this (off-screen) image will be taken to update the visible screen representation. However, if rendering
finishes faster than this duration, it will be shown instantaneously.
• With
Enable Feature simplification by default for newly added layers, you simplify features’ geometry
(less nodes) and as a result, they quickly display. Be aware that you can also face rendering inconsistencies.
8.4 Save and Share Layer Properties
8.4.1 Managing Custom Styles
When a vector layer is added to map canvas, QGIS uses by default a random symbol/color to render its features.
You can however set a default symbol in Project → Project Properties → Default styles that will be applied to
each newly added layer according to its geometry type.
But, most of the time, you’d prefer to have a custom and more complex style that can be applied automatically or
manually (with less efforts) to the layers. You can achieve this goal using the Style combobox at the bottom of the
Layer Properties dialog. This combobox provides you with functions to create, load and manage styles.
A style stores any information set in the layer properties dialog to render or interact with the features (including symbology, labeling, action, diagram. . . settings) for vector layer, or the pixels (band or color rendering,
transparency, pyramids, histogram . . . ) for raster.
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Fig. 8.5: Vector layer style combobox options
By default, the style applied to a loaded layer is named default. Once you have got the ideal and appropriate
rendering for your layer, you can save it by clicking the
Style combobox and choose:
• Rename Current: The active style gets renamed and updated with the current options
• Add: A new style is created using the current options. By default, it will be saved in the QGIS project file.
See below to save the style in another file or a database
• Remove: delete unwanted style, in case you have more than one style defined for the layer.
At the bottom of the Style drop-down list, you see the styles set for the layer and the active one is checked.
Note that each time you validate the layer properties dialog, the active style is updated with the changes you’ve
done.
You can create as many styles as you wish for a layer but only one can be active at a time. In combination with
Map Themes (see Configuring map themes), this offers a quick and powerful way to manage complex projects
without the need to duplicate any layer in the map legend.
Tip: Manage styles from layer context menu
Right-click on the layer in Layers Panel to add, rename or remove layer style.
8.4.2 Storing Style in a File or a Database
While created styles from the Style combobox are by default saved inside the project and can be copied and pasted
from layer to layer in the project, it’s also possible to save them outside the project so that they can be loaded in
another project.
Save in plain text file
Clicking the
Style → Save Style, you can save the style as a:
• QGIS layer style file (.qml)
• or SLD file (.sld), only available for vector layers.
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Used on file based format layers (.shp, .tab. . . ), Save as Default generates a .qml file along the layer (with
the same name). SLDs can be exported from any type of renderer – single symbol, categorized, graduated or
rule-based – but when importing an SLD, either a single symbol or rule-based renderer is created. That means that
categorized or graduated styles are converted to rule-based. If you want to preserve those renderers, you have to
stick to the QML format. On the other hand, it can be very handy sometimes to have this easy way of converting
styles to rule-based.
Save in database
Vector layer style can also be stored in a database if the layer datasource is a database provider. Supported
formats are PostGIS, GeoPackage, SpatiaLite, MSSQL and Oracle. The layer style is saved inside a table (named
layer_styles) of the database. Click on Save Style → Save in database item then fill in the dialog to define a
style name, add a description, a .ui file if applicable and check if the style should be the default style.
You can save several styles for a single table in the database. However each table can have only one default
style. Default style can be saved in the layer database or in the QGIS local database, a SQLite database in the
~/.qgis2/ directory (where QGIS stores its local settings).
Fig. 8.6: Save Style in database Dialog
Tip: Sharing style files between databases
You can only save your style in a database if the layer comes from such a database. You can’t mix databases (layer
in Oracle and style in MSSQL for instance). Use instead a plain text file if you want the style to be shared among
databases.
Note: You may encounter issues to restore the layer_styles table from a PostgreSQL database backup.
Follow QGIS layer_style table and database backup to fix that.
Load style
When loading a layer in QGIS, if a default style already exists for this layer, QGIS loads the layer with this style.
Also Style → Restore Default looks for and loads that file when pressed, replacing current style of the layer.
The Style → Load Style helps you apply any saved style to a layer. While plain text file style (.sld or .qml) can
be applied to any layer whatever its format is, loading styles stored in database is only possible if the layer is from
the same database or the style is stored in the QGIS local database.
The Load Style from Database dialog displays a list of related styles to the layer found in the database and all the
other styles saved in it, with name and description.
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Tip: Quickly share a layer style within the project
You can also share layer style within a project without importing a file or database style: right-click on the layer
in the Layers Panel and, from the Styles combobox , copy the style of a layer and paste it to a group or a selection
of layers: the style is applied to all the layers that are of the same type (vector vs raster) as the original layer and,
in case of vector, have the same geometry type (point, line or polygon).
8.5 Color Selector
The select color dialog will appear whenever you push the
icon to choose a color. The
features of this dialog depends on the state of the Use native color chooser dialogs parameter checkbox in Settings
→ Options → General menu. When checked, the color dialog used is the one of the OS being used. Otherwise,
QGIS custom color chooser is used.
Tip: Dynamically change the color with the live-updating option
Check the Use live-updating color chooser dialogs option in the Settings → Options → General menu to have the
color applied to your items as soon as you pick it in the color chooser dialog.
The custom color chooser dialog has four different tabs which allow you to select colors by
color wheel
,
color swatches
or
color picker
(not available under
color ramp
,
).
Whatever method you use, the selected color is always described through color sliders for HSV (Hue, Saturation,
Value) and RGB (Red, Green, Blue) values. The color is also identifiable as a HTML notation. Finally, there is an
opacity slider to set transparency level.
Modifying a color is as simple as clicking in the color wheel or ramp or in any of the color parameters sliders. You
can adjust such parameters with the spinbox beside or, handy, scrolling the mouse wheel over the corresponding
slider. You can also typeset the color html notation.
The dialog also provides a visual comparison between the current (applied to widget) and the new (being selected)
colors. Thanks to drag-and-drop, any of these colors can be saved in a slot for an easy access.
Fig. 8.7: Color selector ramp tab
8.5. Color Selector
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With
In the
color ramp
or
color swatches
color wheel
tab, you can browse to all possible color combinations and apply it to the item.
tab, you can choose from a preselected list of color palettes:
• Recent colors,
• Standard colors, a user-defined list of colors set under Settings → Options → Colors menu
• or Project colors, a user-defined list of colors set under Project → Project Properties → Default Styles.
and
The latest palettes can be modified thanks to the
nearby the palette combobox also offers several options to:
buttons at the bottom of the frame. The . . . button
• copy, paste, import or export colors
• create, import or remove color palettes. Check the Show in Color Buttons option to add the custom palette
to the color selector widget (see figure_color_selector).
Fig. 8.8: Color selector switcher tab
color picker
which allows you to sample a color from under your mouse pointer at
Another option is to use the
any part of QGIS or even from another application by pressing the space bar. Please note that the color picker is
OS dependent and is currently not supported by macOS.
Tip: Quick color modification
Click the drop-down arrow at the right of the
color box button to display a widget for a quick
color selection, either in the color wheel or from existing color palettes. You can also use it to copy or paste a
color.
8.6 Blending Modes
QGIS offers different options for special rendering effects with these tools that you may previously only know
from graphics programs. Blending modes can be applied on layers, on features but also on print layout items:
• Normal: This is the standard blend mode, which uses the alpha channel of the top pixel to blend with the
pixel beneath it. The colors aren’t mixed.
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Fig. 8.9: Quick color selector menu
8.6. Blending Modes
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• Lighten: This selects the maximum of each component from the foreground and background pixels. Be
aware that the results tend to be jagged and harsh.
• Screen: Light pixels from the source are painted over the destination, while dark pixels are not. This mode
is most useful for mixing the texture of one item with another item (e.g., you can use a hillshade to texture
another layer).
• Dodge: Dodge will brighten and saturate underlying pixels based on the lightness of the top pixel. So,
brighter top pixels cause the saturation and brightness of the underlying pixels to increase. This works best
if the top pixels aren’t too bright; otherwise the effect is too extreme.
• Addition: This blend mode simply adds pixel values of one item with the other. In case of values above
one (in the case of RGB), white is displayed. This mode is suitable for highlighting features.
• Darken: This creates a resultant pixel that retains the smallest components of the foreground and background pixels. Like lighten, the results tend to be jagged and harsh.
• Multiply: Here, the numbers for each pixel of the top item are multiplied with the corresponding pixels for
the bottom item. The results are darker pictures.
• Burn: Darker colors in the top item cause the underlying items to darken. Burn can be used to tweak and
colorise underlying layers.
• Overlay: This mode combines the multiply and screen blending modes. In the resulting picture, light parts
become lighter and dark parts become darker.
• Soft light: This is very similar to overlay, but instead of using multiply/screen it uses color burn/dodge.
This is supposed to emulate shining a soft light onto an image.
• Hard light: Hard light is also very similar to the overlay mode. It’s supposed to emulate projecting a very
intense light onto an image.
• Difference: Difference subtracts the top pixel from the bottom pixel, or the other way around, to always get
a positive value. Blending with black produces no change, as the difference with all colors is zero.
• Subtract: This blend mode simply subtracts pixel values of one item from the other. In case of negative
values, black is displayed.
8.7 Zooming and Panning
QGIS provides tools to zoom and pan to your area of interest.
pan
zoom-in
zoom-out
and
/
icons on the toolbar with the mouse, navigating can also
Apart from using the
be done with the mouse wheel, spacebar and the arrow keys. A Zoom factor can be set under the Settings →
Options → Map tools menu to define the scale behavior while zooming.
8.7.1 With the mouse wheel
You can press the mouse wheel to pan inside of the main window (on macOS, you may need to hold cmd key).
You can roll the mouse wheel to zoom in and out on the map; the mouse cursor position will be the center of the
zoomed area of interest. Holding down Ctrl while rolling the mouse wheel results in a finer zoom.
8.7.2 With the arrow keys
Panning the map is possible with the arrow keys. Place the mouse cursor inside the map area, and click on the
right arrow key to pan east, left arrow key to pan west, up arrow key to pan north, and down arrow key to pan
south.
You can also use the space bar to temporarily cause mouse movements to pan the map. The PgUp and PgDown
keys on your keyboard will cause the map display to zoom in or out following the zoom factor set. Pressing Ctrl
+ or Ctrl - also performs an immediate zoom in/out on the map canvas.
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When certain map tools are active (Identify, Measure. . . ), you can perform a zoom by holding down Shift and
dragging a rectangle on the map to zoom to that area. This is enabled for the map tools which are not selection
tools (since they use Shift for adding to selection) nor edit tools.
8.8 Measuring
8.8.1 General information
QGIS provides four means of measuring geometries:
• the interactive measurement tools
• measuring in the
Field Calculator
,
,
• derived measures in the Identify Features tool,
• and a vector analysis tool: Vector → Geometry Tools → Export/Add Geometry Columns
Measuring works within projected coordinate systems (e.g., UTM) and unprojected data. The first three measuring
tools behave equally to global project settings:
• If “on the fly” CRS transformation (see Define On The Fly (OTF) CRS Transformation) is enabled, the
default measurement metric is - different from most other GIS - ellipsoidal, using the ellipsoid defined in
File → Project properties → General. This is true both when geographic and projected coordinate systems
are defined for the project.
• If you want to calculate the projected / planimetric area or distance using cartesian maths, the measurement ellipsoid has to be set to “None / Planimetric” (File → Project properties → CRS). However, with a
geographic (= unprojected) CRS defined for the data and project, area and distance measurement will be
ellipsoidal.
• If “on the fly” CRS transformation is disabled, the measurement metric is planimetric when the project coordinate system is projected and ellipsoidal when the project coordinate system is unprojected / geographic.
However, neither the identify tool nor the field calculator will transform your data to the project CRS before
measuring. If you want to achieve this, you have to use the vector analysis tool: Vector → Geometry Tools →
Export/Add Geometry Columns. Here, measurement is by default planimetric except if you choose the ellipsoidal
measure.
8.8.2 Measure length, areas and angles interactive
Click the
icon in the Attribute toolbar to begin measurements. The downward arrow near the icon helps you
length,
area or
switch to the convenient tool to measure
the one set in Project → Project Properties → General menu.
angle. The default unit used in the dialog is
Note: Configuring the measure tool
While measuring length or area, clicking the Configuration button at the bottom of the widget helps you define
in menu Settings → Options → Map Tools the rubberband color, the precision of the measurements and the unit
behavior. You can also choose your preferred measurement or angle units but keep in mind that those values are
superseded in the current project by options made in Project → Project Properties → General menu.
All measuring modules use the snapping settings from the digitizing module (see section Setting the Snapping
Tolerance and Search Radius). So, if you want to measure exactly along a line feature, or around a polygon
feature, first set its layer snapping tolerance. Now, when using the measuring tools, each mouse click (within the
tolerance setting) will snap to that layer.
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Measure Line
By default,
: QGIS measures real distances between given points according to a defined ellipsoid.
The tool then allows you to click points on the map. Each segment length, as well as the total, shows up in the
measure window. To stop measuring, click your right mouse button.
Note that you can use the drop-down list near the total to interactively change the measurement units while measuring. This unit is kept for the widget until a new or another project is opened.
The Info section in the dialog explains how calculations are made according to CRS settings available.
Fig. 8.10: Measure Distance
Measure Area
: Areas can also be measured. In the measure window, the accumulated area size appears. Rightclick to stop drawing. The Info section is also available as well as the ability to switch between different area
units.
Fig. 8.11: Measure Area
Measure Angle
: You can also measure angles. The cursor becomes cross-shaped. Click to draw the first segment
of the angle you wish to measure, then move the cursor to draw the desired angle. The measure is displayed in a
pop-up dialog.
Fig. 8.12: Measure Angle
8.9 Selecting features
QGIS provides several tools to select features in the map canvas. Selection tools are available in View → Select
menu or in the Attributes toolbar.
Note: Selection tools work with the currently active layer.
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8.9.1 Selecting manually in the map canvas
To select one or several features with the mouse, you can use one of the following tools:
•
Select Features by area or single click
•
Select Features by Polygon
•
Select Features by Freehand
•
Select Features by Radius
Select Features by Polygon
tool, these manual selection tools allow you to select feature(s) in the
Note: Except the
map canvas with a single click.
While using the
Select Feature(s) tool, holding Shift or Ctrl toggles whether feature is selected (ie either
adds to the current selection or remove from it).
For the other tools, different behaviors can be performed holding:
• Shift: add features to the current selection
• Ctrl: substract features from the current selection
• Ctrl + Shift: intersect with current selection, ie only keep overlapping features from the current selection
• Alt: select features that are totally within the selection shape. Combined to Shift or Ctrl keys, you can
add or substract features to/from the current selection.
8.9.2 Automatic selection
The other selection tools, also available from the Attribute table, perform a selection based on feature’s attribute or
its selection state (note that attribute table and map canvas show the same information, so if you select one feature
in attribute table, it will be selected in map canvas also):
•
Select By Expression. . .
•
Select Features By Value. . .
•
Deselect Features from All Layers
•
Select All Features
•
Invert Feature Selection
allows user to select features using expression dialog.
or press F3
or press Ctrl+Shift+A to deselect all selected features in all layers.
or press Ctrl+A to select all features in the current layer.
to invert the selection in the current layer.
For example, if you want to find regions that are boroughs from regions.shp of the QGIS sample data, you
Select features using an Expression
can use the
icon. Then, you open the Fields and Values menu and choose the field
that you want to query. Double-click the field ‘TYPE_2’ and also click [Load all unique values] in the right
panel. From the list, choose and double-click ‘Borough’. In the Expression field, then you’d write the following
query:
"TYPE_2"
=
'Borough'
From the expression builder dialog, you can also use the Function list → Recent (Selection) to make a selection
that you used before. The dialog remembers the last 20 used expressions. See Expressions chapter for more
information and some example.
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Tip: Save your selection into a new file
Users can save selected features into a New Temporary Scratch Layer or a New Vector Layer using Edit →
Copy Features and Edit → Paste Features as in the wanted format.
8.9.3 Select Features By Value
This selection tool opens the layer’s feature form allowing the user to choose, for each field, which value to look
for, if the search should be case sensitive, and the operation that should be used.
Fig. 8.13: Filter/Select features using form dialog
Alongside each field, there is a drop-down list with the operation options to control the search behaviour. The
common options are:
• Exclude Field - The field will not be used for searching
• Equal to (=)
• Not equal to
• Is missing (null)
• Is not missing (not null)
For numeric and datetime fields, the additional options are:
• Greater than (>)
• Less than (<)
• Greater than or equal to (>=)
• Less than or equal to (<=)
• Between (inclusive)
• Is not between (inclusive)
For text fields, the additional options are:
• Contains
• Does not contain
For the text options above, it is also possible to use the
Case sensitive option.
After setting all search options, you can use the Select features button to select the matching features. The dropdown options are:
• Select features
• Add to current selection
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• Filter current selection
• Remove from current current selection
You can also clean all search options using the Reset form button.
8.10 Data defined override setup
Beside many options in the vector layer properties dialog or settings in the print layout, you can find a
icon. Thanks to expressions based on layer attributes or item settings, prebuild or custom functions
and variables, this tool allows you to set dynamic value for the concerned parameter. When enabled, the value
returned by this widget is applied to the parameter regardless its normal value (checkbox, textbox, slider. . . ).
Data defined override
Clicking the
Data defined override
icon shows following entries:
• Description. . . that indicates if the option is enabled, which input is expected, the valid input type and the
current definition. Hovering over the widget also pops up these information;
• Store data in the project: a button allowing to store the property thanks to the Auxiliary storage mechanism;
• Field type: an entry to select from the layer’s fields that match the valid input type;
• an entry to list the Variable available;
• Edit. . . button to create or edit the expression to use;
• Paste and Copy buttons;
• Clear button to remove the setup.
Parameters that can be used with data-defined tools are:
• Style and symbols parameters
• Labels parameters
• Layout parameters
Tip: Use right-click to (de)activate the data overriding
You can enable or disable a configured
right button.
data-defined override
button by simply clicking the widget with the mouse
Note: When the data-defined override option is setup correctly the icon is yellow
icon is red
or
or
; if it is broken, the
.
8.11 Identify Features
The Identify tool allows you to interact with the map canvas and get information on features in a pop-up window.
To identify features, use:
• View → Identify Features menu,
• or press Ctrl + Shift + I (or
• or click the
Identify Features
Cmd + Shift + I),
icon on the Attributes toolbar.
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8.11.1 Using the Identify Features tool
QGIS offers two ways to identify features with the
Identify Features
tool:
• left click will identify features according to the mode set in the Identify Results panel
• right click will fetch all the snapped features from all the visible layers. This will open a context menu,
allowing the user to choose more precisely the features to identify.
Tip: Filter the layers to query with the Identify Features tool
Uncheck the Identifiable column in Project → (or
Settings →), Project Properties → Identify layers menu in
Identify Features
front of a layer to avoid it being queried when using the
in a mode other than Current Layer.
This is a handy way to return features from only layers that are of interest for you.
If you click on feature(s), the Identify Results dialog will list information about the clicked feature(s). The default
view is a tree view where the first item is the name of the layer and its children are its identified feature(s). Each
feature is described by the name of a field along with its value. This field is the one set in Layer Properties →
Display. Then follows all the other information about the feature.
8.11.2 Feature informations
The Identify Results dialog can be customized to display custom fields, but by default it will display three kinds
of information:
• Actions: Actions can be added to the identify feature windows. The action is run by clicking on the action
label. By default, only one action is added, namely View feature form for editing. You can define
more actions in the layer’s properties dialog (see Actions Properties).
• Derived: This information is calculated or derived from other information. This includes:
– general information about the feature and its geometry: feature id, length or perimeter and area in map
units depending on its geometry, the count of spatial parts and the number of the clicked part in case
of multi-geometry, the count of vertices in the feature and the number of the closest one to the point
clicked
– coordinates information: the X and Y (and Z/M if available) coordinate values of the clicked point,
the feature closest vertex and its first and last vertices. In case you click on a curved line using the info
tool, QGIS will also display the radius of that section in the panel result.
• Data attributes: This is the list of attribute fields and values for the feature that has been clicked.
Note: Links in feature’s attributes are clickable from the Identify Results panel and will open in your default web
browser.
8.11.3 The Identify Results dialog
At the top of the window, you have seven icons:
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•
Expand tree
•
Collapse tree
•
Default behavior
•
View the feature form
to define whether next identified features information should be collapsed or expanded
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Fig. 8.14: Identify Results dialog
•
Clear Results
•
Copy selected feature to clipboard
•
Print selected HTML response
At the bottom of the window, you have the Mode and View comboboxes. With the Mode combobox you can define
from which layers features should be identified:
• Current layer : only features from the selected layer are identified. The layer may not be visible in the
canvas.
• Top down, stop at first: for only features from the upper visible layer.
• Top down: for all features from the visible layers. The results are shown in the panel.
• and Layer selection: opens a context menu where the user selects the layer to identify features from.
Operates like a right-click. Only the chosen features will be shown in the result panel.
Note: Identify tool configuration
You can configure the identify feature in Project → Project Properties in the Identify layers tab. The table allows
user to select layer(s) that can be used by this tool to identify features (column Identifiable). You can also put this
layer in read-only mode with the checkbox in the last column.
The View can be set as Tree, Table or Graph. ‘Table’ and ‘Graph’ views can only be set for raster layers.
The identify tool allows you to
Auto open a form. If checked, each time a single feature is identified QGIS will
open a form showing its attributes. This is a handy way to quickly edit a feature’s attributes.
Other functions can be found in the context menu of the identified item. For example, from the context menu you
can:
• View the feature form
• Zoom to feature
• Copy feature: Copy all feature geometry and attributes
• Toggle feature selection: Adds identified feature to selection
• Copy attribute value: Copy only the value of the attribute that you click on
• Copy feature attributes: Copy the attributes of the feature
• Clear result: Remove results in the window
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• Clear highlights: Remove features highlighted on the map
• Highlight all
• Highlight layer
• Activate layer: Choose a layer to be activated
• Layer properties: Open layer properties window
• Expand all
• Collapse all
8.12 Annotation Tools
Text Annotation
tool in the attribute toolbar provides the possibility to place formatted text in a balloon on the
The
QGIS map canvas. Use the Text Annotation tool and click into the map canvas.
Fig. 8.15: Annotation text dialog
Double clicking on the item opens a dialog with various options. There is the text editor to enter the formatted text
and other item settings. For instance, there is the choice of having the item placed on a map position (displayed
by a marker symbol) or to have the item on a screen position (not related to the map). The item can be moved by
map position (by dragging the map marker) or by moving only the balloon. The icons are part of the GIS theme,
and they are used by default in the other themes, too.
The
Move Annotation
tool allows you to move the annotation on the map canvas.
8.12.1 Html annotations
Html Annotation
The
tools in the attribute toolbar provides the possibility to place the content of an html file in a
balloon on the QGIS map canvas. Using the Html Annotation tool, click into the map canvas and add the path to
the html file into the dialog.
8.12.2 SVG annotations
SVG Annotation
tool in the attribute toolbar provides the possibility to place an SVG symbol in a balloon on
The
the QGIS map canvas. Using the SVG Annotation tool, click into the map canvas and add the path to the SVG file
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into the dialog.
8.12.3 Form annotations
Form Annotation
tool is useful to display
Additionally, you can also create your own annotation forms. The
attributes of a vector layer in a customized Qt Designer form (see figure_custom_annotation). This is similar
to the designer forms for the Identify features tool, but displayed in an annotation item. Also see this video
https://youtu.be/0pDBuSbQ02o?t=2m25s from Tim Sutton for more information.
Fig. 8.16: Customized qt designer annotation form
Note: If you press Ctrl+T while an Annotation tool is active (move annotation, text annotation, form annotation), the visibility states of the items are inverted.
8.13 Spatial Bookmarks
Spatial Bookmarks allow you to “bookmark” a geographic location and return to it later. By default, bookmarks
are saved on the computer, meaning that they are available from any project in the same computer. If you wish to
store the bookmark in the project file (.qgs) then you can do this by selecting the In Project checkbox.
8.13.1 Creating a Bookmark
To create a bookmark:
1. Zoom or pan to the area of interest.
2. Select the menu option View → New Bookmark or press Ctrl-B. The Spatial Bookmark panel opens with
the newly created bookmark.
3. Enter a descriptive name for the bookmark (up to 255 characters).
4. Check the In Project box if you wish to save the bookmark in the project file.
5. Press Enter to add the bookmark or click elsewhere.
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Note that you can have multiple bookmarks with the same name.
8.13.2 Working with Bookmarks
To use or manage bookmarks, select the menu option View → Show Bookmarks. The Spatial Bookmarks panel
allows you to:
• Zoom to a Bookmark: select the desired bookmark and then click Zoom To Bookmark. You can also zoom
to a bookmark by double-clicking on it.
• Delete a Bookmark: select the bookmark and click Delete Bookmark. Confirm your choice.
• Import or Export a bookmark: To share or transfer your bookmarks between computers you can use the
Import/Export Bookmarks pull down menu in the Spatial Bookmarks dialog. All the bookmarks are transferred.
8.14 Nesting Projects
Sometimes, you’d like to keep in different projects a bunch of layers with the same style. You can either create a
default style for these layers or embed them from another project to save you tons of work.
Embed layers and groups from an existing project has some advantages over styling:
• all types of layers (vector or raster, local or online. . . ) can be added
• fetching groups and layers, you can keep the same tree structure of the “background” layers in your different
projects
• While the embedded layers are editable, you can’t change their properties such as symbology, labels, forms,
default values, actions. . . This ensures homogeneity throughout the projects
• modify the items in the original project and changes are propagated to all the other projects.
If you want to embed content from other project files into your project, select Layer → Embed Layers and Groups
and:
1. Press
to look for a project; you can see the content of the project (see figure_embed_dialog).
2. Press Ctrl ( or
Cmd) and click on the layers and groups you wish to retrieve.
3. Press [OK]. The selected layers and groups are embedded in the Layer panel and can be visualized in the
map canvas now. Names of embedded items appear in italic to distinguish them from regular layers and
groups.
Fig. 8.17: Select layers and groups to embed
Like any other layer, an embedded layer can be removed from the project by right-click on the layer and choose
Remove
.
Tip: Change rendering of an embedded layer
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It’s not possible to change rendering of an embedded layer, unless you make the changes in the original project
file. However, right-click on a layer and select Duplicate creates a layer which is fully-featured and not dependent
to the original project. You can then safely remove the linked layer.
8.15 Decorations
The Decorations of QGIS include the Grid, the Copyright Label, the North Arrow and the Scale Bar. They are
used to ‘decorate’ the map by adding cartographic elements.
8.15.1 Grid
Grid
allows you to add a coordinate grid and coordinate annotations to the map canvas.
Fig. 8.18: The Grid Dialog
1. Select from menu View → Decorations → Grid. The dialog starts (see figure_decorations_grid).
2. Activate the
canvas.
Enable grid checkbox and set grid definitions according to the layers loaded in the map
3. Activate the
Draw annotations checkbox and set annotation definitions according to the layers loaded
in the map canvas.
4. Click [Apply] to verify that it looks as expected or [OK] if you’re satisfied.
8.15.2 Copyright Label
Copyright label
adds a copyright label using the text you prefer to the map.
1. Select from menu View → Decorations → Copyright Label.
ure_decorations_copyright).
2. Make sure the
The dialog starts (see fig-
Enable Copyright Label checkbox is checked.
3. Enter the text you want to place on the map. You can use HTML as shown in the example.
4. Choose the placement of the label from the Placement
8.15. Decorations
combo box.
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Fig. 8.19: The Copyright Dialog
5. You can refine the placement of the item by setting a Horizontal and/or Vertical Marging from (Canvas)
Edge. These values can be a distance in Millimeter or Pixels or set as Percentage of the width or height of
the map canvas.
6. You can change the color to apply.
7. Click [Apply] to verify that it looks as expected or [OK] if you’re satisfied.
In the example above, which is the default, QGIS places a copyright symbol followed by the date in the lower
right-hand corner of the map canvas.
8.15.3 North Arrow
North Arrow
places a simple north arrow on the map canvas. Currently, there is only one style available. You can
adjust the angle of the arrow or let QGIS set the direction automatically. If you choose to let QGIS determine the
direction, it makes its best guess as to how the arrow should be oriented. For placement of the arrow, you have
four options, corresponding to the four corners of the map canvas. You can refine the placement of the arrow by
setting a Horizontal and/or Vertical Marging from (Canvas) Edge. These values can be a distance in Millimeter
or Pixels or set as Percentage of the width or height of the map canvas.
Fig. 8.20: The North Arrow Dialog
8.15.4 Scale Bar
Scale Bar
adds a simple scale bar to the map canvas. You can control the style and placement, as well as the
labelling of the bar.
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Fig. 8.21: The Scale Bar Dialog
QGIS only supports displaying the scale in the same units as your map frame. So if the units of your layers are in
meters, you can’t create a scale bar in feet. Likewise, if you are using decimal degrees, you can’t create a scale
bar to display distance in meters.
To add a scale bar:
1. Select from menu View → Decorations → Scale Bar. The dialog starts (see figure_decorations_scale).
2. Make sure the
Enable scale bar checkbox is checked.
3. Choose the style from the Scale bar style
4. Select the color for the bar Color of bar
5. Set the Size of bar
6. Optionally, check
combo box.
or use the default black color.
.
Automatically snap to round number on resize to display values easy-to-read.
7. Choose the placement from the Placement
combo box.
8. You can refine the placement of the item by setting a Horizontal and/or Vertical Marging from (Canvas)
Edge. These values can be a distance in Millimeter or Pixels or set as Percentage of the width or height of
the map canvas.
9. Click [Apply] to verify that it looks as expected or [OK] if you’re satisfied.
Tip: Settings of Decorations
When you save a .qgs project, any changes you have made to Grid, North Arrow, Scale Bar and Copyright will
be saved in the project and restored the next time you load the project.
8.16 Authentication
QGIS has facility to store/retrieve authentication credentials in a secure manner. Users can securely save credentials into authentication configurations, which are stored in a portable database, can be applied to server or
database connections, and safely referenced by their ID tokens in project or settings files. For more information
see Authentication System.
A master password needs to be set up when initializing the authentication system and its portable database.
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8.17 Variables
In QGIS, you can use variables to store useful recurrent values (e.g. the project’s title, or the user’s full name) that
can be used in expressions. Variables can be defined at the application’s global level, project level, layer level,
composition level, and layout item’s level. Just like CSS cascading rules, variables can be overwritten - e.g., a
project level variable will overwrite any application’s global level variables set with the same name. You can use
these variables to build text strings or other custom expressions using the @ character before the variable name.
For example in print layout creating a label with this content:
This map was made using QGIS [% @qgis_version %]. The project file for this
map is: [% @project_path %]
Will render the label like this:
This map was made using QGIS 2.14. The project file for this map is:
/gis/qgis-user-conference-2015.qgs
Besides the preset read-only variables, you can define your own custom variables for any of the levels mentioned
above. You can manage:
• global variables from the Settings → Options menu;
• project’s variables from Project properties (see Project Properties);
• vector layer’s variables from the Layer Properties dialog (see The Vector Properties Dialog);
• composition’s variables from the Composition panel in the Print layout (see Composition Panel);
• and layout item’s variables from the Item properties panel in the Print layout (see Layout Items Common
Options).
To differentiate from editable variables, read-only variable’s names and values are emphasized in italic. On the
other hand, higher level variables overwritten by lower level ones are strike through.
Fig. 8.22: Variables editor at the project’s level
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Note: You can read more about variables and find some examples in Nyall Dawson’s Exploring variables in
QGIS 2.12, part 1, part 2 and part 3 blog posts.
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CHAPTER
9
QGIS Configuration
QGIS is highly configurable through the Settings menu. Choose between Options, Project Properties and Customization.
Note: QGIS follows desktop guidelines for the location of options and project properties item. Consequently
related to the OS you are using, location of some of items described below could be in the Project or the Settings
menu.
9.1 Options
Some basic options for QGIS can be selected using the Options dialog. Select the menu option Settings →
Options. You can modify the options according to your needs. Some of the changes may require a restart of QGIS
before they will be effective.
The tabs where you can customize your options are described below.
Note: Plugins can embed their settings within the Options dialog
While only Core settings are presented below, note that this list can be extended by installed plugins implementing
their own options into the standard Options dialog. This avoids each plugin having their own config dialog with
extra menu items just for them. . .
9.1.1 General Settings
Locale Settings
• Check
Override system locale if you want to use a language different from your system’s and pick the
replacement in Locale to use instead combobox.
• Information about active system locale are provided.
Application
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• Select the Style (QGIS restart required)
‘Plastique’ and ‘Cleanlooks’.
and choose between ‘Oxygen’, ‘Windows’, ‘Motif’, ‘CDE’,
• Define the UI theme
. It can be ‘default’ or ‘Night Mapping’.
• Define the Icon size
.
• Define the Font and its Size. The font can be
Qt default or a user-defined one.
• Change the Timeout for timed messages or dialogs
.
•
Hide splash screen at startup
•
Show tips at startup
•
Check QGIS version at startup to keep you informed if a newer version is released
•
QGIS-styled group boxes
•
Use native color chooser dialogs (see Color Selector)
•
Use live-updating color chooser dialogs (see Color Selector)
•
Canvas rotation support (restart required)
Project files
• Open project on launch
(choose between ‘New’, ‘Most recent’, ‘Welcome Page’, and ‘Specific’).
When choosing ‘Specific’ use the
to define the project to use by default. The ‘Welcome Page’ displays
a list of recent projects with screenshot.
•
Create new project from default project. You have the possibility to press on Set current project as
default or on Reset default. You can browse through your files and define a directory where you find your
user-defined project templates. This will be added to Project → New From Template. If you first activate
Create new project from default project and then save a project in the project templates folder.
•
Prompt to save project and data source changes when required
•
Prompt for confirmation when a layer is to be removed
•
Warn when opening a project file saved with an older version of QGIS
• Enable macros
. This option was created to handle macros that are written to perform an action on
project events. You can choose between ‘Never’, ‘Ask’, ‘For this session only’ and ‘Always (not recommended)’.
9.1.2 System Settings
SVG paths
Add or Remove Path(s) to search for Scalable Vector Graphic (SVG) symbols. These SVG files are then available
to symbolize features or decorate your map composition.
Plugin paths
Add or Remove Path(s) to search for additional C++ plugin libraries
QSettings
It helps you Reset user interface to default settings (restart required) if you made any customization.
Environment
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System environment variables can now be viewed, and many configured, in the Environment group (see figure_environment_variables). This is useful for platforms, such as Mac, where a GUI application does not necessarily inherit the user’s shell environment. It’s also useful for setting and viewing environment variables for
the external tool sets controlled by the Processing toolbox (e.g., SAGA, GRASS), and for turning on debugging
output for specific sections of the source code.
•
Use custom variables (restart required - include separators). You can [Add] and [Remove] variables.
Already-defined environment variables are displayed in Current environment variables, and it’s possible to
filter them by activating
Show only QGIS-specific variables.
Fig. 9.1: System environment variables in QGIS
9.1.3 CRS Settings
Default CRS for new projects
•
Don’t enable ‘on the fly’ reprojection
•
Automatically enable ‘on the fly’ reprojection if layers have different CRS
•
Enable ‘on the fly’ reprojection by default
• Select a CRS and Always start new projects with this CRS
CRS for new layers
This area allows you to define the action to take when a new layer is created, or when a layer without a CRS is
loaded.
•
Prompt for CRS
•
Use project CRS
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•
Use default CRS
Default datum transformations
•
Ask for datum transformation when no default is defined
• With the ‘on-the-fly’ CRS transformation enabled and the above option checked, adding layers of different
CRS opens the Select datum transformations dialog. This offers you to select the most appropriate transformation settings. Validating this dialog with the ‘Remember selection’ option checked populates the table
under CRS → Default datum transformations with information about ‘Source CRS’ and ‘Destination CRS’
as well as ‘Source datum transform’ and ‘Destination datum transform’. From now, QGIS automatically
uses the selected datum transformation for further transformation between these two CRSs until you
remove it from the list.
button to add a datum transformation if you know its parameters (source and destination
You can use the
ellipsoids and the numbers from the transformation table). You then need to manually enter each setting.
Note: For more information on how QGIS handles layers projection, please read the dedicated section at Working
with Projections.
9.1.4 Data Sources Settings
Feature attributes and table
•
Open attribute table in a dock window
• Copy features as ‘Plain text, no geometry’, ‘Plain text, WKT geometry’, or ‘GeoJSON’ when pasting features in other applications.
• Attribute table behavior
: set filter on the attribute table at the opening. There are three possibilities:
‘Show all features’, ‘Show selected features’ and ‘Show features visible on map’.
• Default view: define the view mode of the attribute table at every opening. It can be ‘Remember last view’,
‘Table view’ or ‘Form view’.
• Attribute table row cache
. This row cache makes it possible to save the last loaded N attribute rows
so that working with the attribute table will be quicker. The cache will be deleted when closing the attribute
table.
• Representation for NULL values. Here, you can define a value for data fields containing a NULL value.
Tip: Improve opening of big data attribute table
When working with layers with big amount of records, opening the attribute table may be slow as the dialog
request all the rows in the layer. Setting the Attribute table behavior to Show features visible on map will make
QGIS request only the features in the current map canvas when opening the table, allowing a quick data loading.
Note that data in this attribute table instance will be always tied to the canvas extent it was opened with, meaning
that selecting Show All Features within such a table will not display new features. You can however update the
set of displayed features by changing the canvas extent and selecting Show Features Visible On Map option in
the attribute table.
Data source handling
• Scan for valid items in the browser dock
contents’.
. You can choose between ‘Check extension’ and ‘Check file
• Scan for contents of compressed files (.zip) in browser dock
possible.
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• Prompt for raster sublayers when opening. Some rasters support sublayers — they are called subdatasets
in GDAL. An example is netCDF files — if there are many netCDF variables, GDAL sees every variable
as a subdataset. The option allows you to control how to deal with sublayers when a file with sublayers is
opened. You have the following choices:
– ‘Always’: Always ask (if there are existing sublayers)
– ‘If needed’: Ask if layer has no bands, but has sublayers
– ‘Never’: Never prompt, will not load anything
– ‘Load all’: Never prompt, but load all sublayers
•
Ignore shapefile encoding declaration. If a shapefile has encoding information, this will be ignored by
QGIS.
•
Add PostGIS layers with double click and select in extended mode
•
Add Oracle layers with double click and select in extended mode
•
Execute expressions on server-side if possible
Hidden Browser Path
This widget lists all the folder you chose to hide from the Browser panel. Removing a folder from the list will
make it available in the Browser panel.
9.1.5 Rendering Settings
Fig. 9.2: Rendering tab of Project Properties dialog
Rendering behavior
•
By default new layers added to the map should be displayed
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•
Use render caching where possible to speed up redraws
•
Render layers in parallel using many CPU cores
•
Max cores to use
• Map update interval (default to 250 ms)
•
Enable feature simplification by default for newly added layers
• Simplification threshold
• Simplification algorithm: This option performs a local “on-the-fly” simplification on feature’s and speeds
up geometry rendering. It doesn’t change the geometry fetched from the data providers. This is important
when you have expressions that use the feature geometry (e.g. calculation of area) - it ensures that these
calculations are done on the original geometry, not on the simplified one. For this purpose, QGIS provides
three algorithms: ‘Distance’ (default), ‘SnapToGrid’ and ‘Visvalingam’.
•
Simplify on provider side if possible: the geometries are simplified by the provider (PostGIS, Oracle. . . )
and unlike the local-side simplification, geometry-based calculations may be affected
• Maximum scale at which the layer should be simplified
•
Magnification level (see the magnifier)
Note: Besides the global setting, feature simplification can be set for any specific layer from its Layer properties
→ Rendering menu.
Rendering quality
•
Make lines appear less jagged at the expense of some drawing performance
Curve segmentation
• Segmentation tolerance: this setting controls the way circular arcs are rendered. The smaller maximum angle (between the two consecutive vertices and the curve center, in degrees) or maximum difference (distance
between the segment of the two vertices and the curve line, in map units), the more straight line segments
will be used during rendering.
• Tolerance type: it can be ‘Maximum angle’ or ‘Maximum distance’
Rasters
• With RGB band selection, you can define the number for the Red, Green and Blue band.
Contrast enhancement
• Single band gray
. A single band gray can have ‘No stretch’, ‘Stretch to MinMax’, ‘Stretch and Clip
to MinMax’ and also ‘Clip to MinMax’.
• Multi band color (byte/band)
MinMax’ and ‘Clip to MinMax’.
. Options are ‘No stretch’, ‘Stretch to MinMax’, ‘Stretch and Clip to
• Multi band color (>byte/band)
MinMax’ and ‘Clip to MinMax’.
. Options are ‘No stretch’, ‘Stretch to MinMax’, ‘Stretch and Clip to
• Limits (minimum/maximum)
‘Mean +/- standard deviation’.
. Options are ‘Cumulative pixel count cut’, ‘Minimum/Maximum’,
• Cumulative pixel count cut limits
• Standard deviation multiplier
Debugging
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•
Map canvas refresh
9.1.6 Canvas and Legend Settings
Default map appearance (overridden by project properties)
• Define a Selection color and a Background color.
Layer legend
• Double click action in legend
. You can either ‘Open layer properties’, ‘Open attribute table’ or ‘Open
layer styling dock’ with the double click.
• The following Legend item styles are possible:
–
Capitalise layer names
–
Bold layer names
–
Bold group names
–
Display classification attribute names
–
Create raster icons (may be slow)
– you can also set the WMS getLegendGraphic Resolution
9.1.7 Map tools Settings
This tab offers some options regarding the behavior of the Identify tool.
• Search radius for identifying and displaying map tips is a tolerance distance within which the identify tool
will depict results as long as you click within this tolerance.
• Highlight color allows you to choose with which color should features being identified be highlighted.
• Buffer determines a buffer distance to be rendered from the outline of the identify highlight.
• Minimum width determines how thick should the outline of a highlighted object be.
Measure tool
• Define Rubberband color for measure tools
• Define Decimal places
•
Keep base unit to not automatically convert large numbers (e.g., meters to kilometers)
• Preferred distance units
‘Map Units’ )
(‘Meters’, ‘Kilometers’, ‘Feet’, ‘Yards’, ‘Miles’, ‘Nautical Miles’, ‘Degrees’ or
• Preferred area units
(‘Square meters’, ‘Square kilometers’, ‘Square feet’, ‘Square yards’, ‘Square miles’,
‘Hectares’, ‘Acres’, ‘Square nautical miles’, ‘Square degrees” or ‘Map Units’)
• Preferred angle units
‘Turns/revolutions’)
(‘Degrees’, ‘Radians’, ‘Gon/gradians’, ‘Minutes of arc’, ‘Seconds of arc’ or
Panning and zooming
• Define a Zoom factor for zoom tools or wheel mouse
Predefined scales
Here, you find a list of predefined scales. With the
and
buttons you can add or remove your personal
scales. You can also import or export scales from/to a .XML file. Note that you still have the possibility to remove
your changes and reset to the predefined list.
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9.1.8 Colors Settings
This menu allows you to add some custom color that you can find in each color dialog window of the renderers.
You will see a set of predefined colors in the tab: you can delete or edit all of them. Moreover you can add the
color you want and perform some copy and paste operations. Finally you can export the color set as a gpl file or
import them.
9.1.9 Digitizing Settings
This tab helps you configure general settings when editing vector layer (attributes and geometry).
Feature creation
•
Suppress attribute form pop-up after feature creation
•
Reuse last entered attribute values
• Validate geometries. Editing complex lines and polygons with many nodes can result in very slow rendering.
This is because the default validation procedures in QGIS can take a lot of time. To speed up rendering, it is
possible to select GEOS geometry validation (starting from GEOS 3.3) or to switch it off. GEOS geometry
validation is much faster, but the disadvantage is that only the first geometry problem will be reported.
Rubberband
• Define Rubberband Line width, Line color and Fill color
• Don’t update rubberband during vertex editing
Snapping
•
Open snapping options in a dock window (QGIS restart required)
• Define Default snap mode
(‘To vertex’, ‘To segment’, ‘To vertex and segment’, ‘Off’)
• Define Default snapping tolerance in map units or pixels
• Define the Search radius for vertex edits in map units or pixels
Vertex markers
•
Show markers only for selected features
• Define vertex Marker style
(‘Cross’ (default), ‘Semi transparent circle’ or ‘None’)
• Define vertex Marker size
Curve offset tool
Offset Curve
The next 3 options refer to the
tool in Advanced digitizing. Through the various settings, it is
possible to influence the shape of the line offset. These options are possible starting from GEOS 3.3.
• Join style: ‘Round’, ‘Mitre’ or ‘Bevel’
• Quadrant segments
• Miter limit
9.1.10 Layout Settings
Composition defaults
You can define the Default font used within the print layout.
Grid appearance
• Define the Grid style
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• Define the Grid color
Grid and guide defaults
• Define the Grid spacing
• Define the Grid offset
for x and y
• Define the Snap tolerance
Layout Paths
• Define Path(s) to search for extra print templates: a list of folders with custom layout templates to use while
creating new one.
9.1.11 GDAL Settings
GDAL is a data exchange library for raster files. In this tab, you can Edit create options and Edit Pyramids Options
of the raster formats. You can define which GDAL driver is to be used for a raster format, as in some cases more
than one GDAL driver is available.
9.1.12 Variables Settings
The Variables tab lists all the variables available at the global-level.
It also allows the user to manage global-level variables. Click the
button to add a new custom global-level
variable. Likewise, select a custom global-level variable from the list and click the
button to remove it.
More information about variables in the Variables section.
9.1.13 Authentication Settings
In the Authentication tab you can set authentication configurations and manage PKI certificates. See Authentication System for more details.
9.1.14 Network Settings
General
• Define
WMS
search
address,
search=\%1\&type=rss
default
is
http://geopole.org/wms/search?
• Define Timeout for network requests (ms) - default is 60000
• Define Default expiration period for WMS Capabilities (hours) - default is 24
• Define Default expiration period for WMSC/WMTS tiles (hours) - default is 24
• Define Max retry in case of tile or feature request errors
• Define User-Agent
Cache settings
Define the Directory and a Size for the cache.
•
Use proxy for web access and define ‘Host’, ‘Port’, ‘User’, and ‘Password’.
• Set the Proxy type
according to your needs.
– Default Proxy: Proxy is determined based on the application proxy set using
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Fig. 9.3: Proxy-settings in QGIS
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– Socks5Proxy: Generic proxy for any kind of connection. Supports TCP, UDP, binding to a port (incoming connections) and authentication.
– HttpProxy: Implemented using the “CONNECT” command, supports only outgoing TCP connections;
supports authentication.
– HttpCachingProxy: Implemented using normal HTTP commands, it is useful only in the context of
HTTP requests.
– FtpCachingProxy: Implemented using an FTP proxy, it is useful only in the context of FTP requests.
Excluding some URLs can be added to the text box below the proxy settings (see Figure_Network_Tab).
If you need more detailed information about the different proxy settings, please refer to the manual of the underlying QT library documentation at http://doc.qt.io/qt-4.8/qnetworkproxy.html#ProxyType-enum
Tip: Using Proxies
Using proxies can sometimes be tricky. It is useful to proceed by ‘trial and error’ with the above proxy types, to
check if they succeed in your case.
9.1.15 Advanced Settings
Depending on your OS, all the settings related to QGIS (UI, tools, data providers, default values, plugins options. . . ) are saved:
•
in a text file: $HOME/.config/QGIS/QGIS2.conf
•
in the properties list file: $HOME/Library/Preferences/org.qgis.qgis.plist
•
in the registry under: HKEY\CURRENT_USER\Software\QGIS\qgis
The Advanced tab offers you in a single place, regardless your OS, means to manage these settings through the
Advanced Settings Editor. After you promise to be careful, the widget is populated with a tree of all QGIS settings,
which you can directly edit.
Warning: Avoid using the Advanced tab settings blindly
Be careful while modifying items in this dialog given that changes are automatically applied. Doing changes
without knowledge can break your QGIS installation in various ways.
9.1.16 Processing
The
Processing tab provides you with general settings of tools and data providers that are used in the QGIS
Processing framework. More information at QGIS processing framework.
9.2 Project Properties
In the properties window for the project under Project → Project Properties, you can set project-specific options.
The project-specific options overwrite their equivalent in the Options dialog described above.
• In the General tab, the general settings let you:
– give a title to the project beside the project file path
– choose the color to use for features when they are selected
– choose the background color: the color to use for the map canvas
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– set whether the path to layers in the project should be saved as absolute (full) or as relative to the
project file location. You may prefer relative path when both layers and project files can be moved or
shared or if the project is accessed from computers on different platforms.
– choose to avoid artifacts when project is rendered as map tiles. Note that checking this option can lead
to performance degradation.
Calculating areas and distances is a common need in GIS. However, these values are really tied to the
underlying projection settings. The Measurements frame lets you control these parameters. You can indeed
choose:
– the ellipsoid to use: it can be an existing one, a custom one (you’ll need to set values of the semi-major
and semi-minor axis) or None/Planimetric.
– the units for distance measurements for length and perimeter and the units for area measurements.
These settings, which default to the units set in QGIS options but then overrides it for the current
project, are used in:
* Attribute table field update bar
* Field calculator calculations
* Identify tool derived length, perimeter and area values
* Default unit shown in measure dialog
The Coordinate display allows you to choose and customize the format of units to use to display the mouse
coordinate in the status bar and the derived coordinates shown via the identify tool.
Finally, you can define a project scale list, which overrides the global predefined scales.
Fig. 9.4: General tab of Project Properties dialog
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• The CRS tab enables you to choose the Coordinate Reference System for this project, and to enable onthe-fly re-projection of raster and vector layers when displaying layers from a different CRS. For more
information on projection’s handling in QGIS, please read Working with Projections section.
• The Default Styles tab lets you control how new layers will be drawn when they do not have an existing
.qml style defined. You can also set the default transparency level for new layers and whether symbols
should have random colors assigned to them. There is also an additional section where you can define
specific colors for the running project. You can find the added colors in the drop down menu of the color
dialog window present in each renderer.
• With the Identify layers tab, you set (or disable) which layers will respond to the identify tool. By default,
layers are set queryable.
You can also set whether a layer should appear as read-only, meaning that it can not be edited by the
user, regardless of the data provider’s capabilities. Although this is a weak protection, it remains a quick
and handy configuration to avoid end-users modifying data when working with file-based layers.
• In the Data Sources tab, you can:
–
Evaluate default values on provider side: When adding new features in a PostGreSQL table, fields
with default value constraint are evaluated and populated at the form opening, and not at the commit
moment. This means that instead of an expression like nextval('serial'), the field in the Add
Feature form will display expected value (e.g., 25).
–
Automatically create transaction groups where possible: When this mode is turned on, all (postgres) layers from the same database are synchronised in their edit state, i.e. when one layer is put into
edit state, all are, when one layer is committed or one layer is rolled back, so are the others. Also,
instead of buffering edit changes locally, they are directly sent to a transaction in the database which
gets committed when the user clicks save layer. Note that you can (de)activate this option only if no
layer is being edited in the project.
• The Relations tab is used to define 1:n relations. The relations are defined in the project properties dialog.
Once relations exist for a layer, a new user interface element in the form view (e.g. when identifying a
feature and opening its form) will list the related entities. This provides a powerful way to express e.g. the
inspection history on a length of pipeline or road segment. You can find out more about 1:n relations support
in Section Creating one or many to many relations.
• The Variables tab lists all the variables available at the project’s level (which includes all global variables).
Besides, it also allows the user to manage project-level variables. Click the
button to add a new custom
project-level variable. Likewise, select a custom project-level variable from the list and click the
to remove it. More information on variables usage in the General Tools Variables section.
button
• The Macros tab is used to edit Python macros for projects. Currently, only three macros are available:
openProject(), saveProject() and closeProject().
• The tab QGIS Server allows you to configure your project in order to publish it online. Here you can
define information about the QGIS Server WMS and WFS capabilities, extent and CRS restrictions. More
information available in section Configure your project and subsequent.
9.3 Customization
The customization dialog lets you (de)activate almost every element in the QGIS user interface. This can be very
useful if you want to provide your end-users with a ‘light’ version of QGIS, containing only the icons, menus or
panels they need.
Note: Before your changes are applied, you need to restart QGIS.
Ticking the
Enable customization checkbox is the first step on the way to QGIS customization. This enables
the toolbar and the widget panel from which you can uncheck and thus disable some GUI items.
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Fig. 9.5: Macro settings in QGIS
Fig. 9.6: The Customization dialog
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The configurable item can be:
• a Menu or some of its sub-menus from the Menu Bar
• a whole Panel (see Panels and Toolbars)
• the Status bar described in Status Bar or some of its items
• a Toolbar: the whole bar or some of its icons
• or any widget from any dialog in QGIS: label, button, combobox. . .
Switch to catching widgets in main application
With
, you can click on an item in QGIS interface that you want to be hidden
and QGIS automatically unchecks the corresponding entry in the Customization dialog.
Once you setup your configuration, click [Apply] or [Ok] to validate your changes. This configuration becomes
the one used by default by QGIS at the next startup.
The modifications can also be saved in a .ini file using
Save To File
button. This is a handy way to share a
Load from File
from the destination computer in
common QGIS interface among multiple users. Just click on
order to import the .ini file. You can also run command line tools and save various setups for different use cases
as well.
Tip: Easily restore predefined QGIS
The initial QGIS GUI configuration can be restored by one of the methods below:
• unchecking
Enable customization option in the Customization dialog or click the
Check All
button
• pressing the [Reset] button in the QSettings frame under Settings → Options menu, System tab
• launching QGIS at a command prompt with the following command line qgis --nocustomization
• setting to false the value of UI → Customization → Enabled variable under Settings → Options menu,
Advanced tab (see the warning).
In most cases, you need to restart QGIS in order to have the change applied.
9.4 Keyboard shortcuts
QGIS provides default keyboard shortcuts for many features. You can find them in section Menu Bar. Additionally, the menu option Settings →
Keyboard Shortcuts. . . allows you to change the default keyboard shortcuts
and add new ones to QGIS features.
Configuration is very simple. Use the search box at the top of the dialog to find a particular action, select it from
the list and click on :
• [Change] and press the new combination you want to assign as new shortcut
• [Set none] to clear any assigned shortcut
• or [Set default] to backup the shortcut to its original and default value.
Proceed as above for any other tools you wish to customize. Once you have finished your configuration, simply
[Close] the dialog to have your changes applied. You can also [Save] the changes as an .XML file and [Load]
them into another QGIS installation.
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Fig. 9.7: Define shortcut options
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10
Working with Projections
QGIS allows users to define a global and project-wide CRS (coordinate reference system) for layers without a
pre-defined CRS. It also allows the user to define custom coordinate reference systems and supports on-the-fly
(OTF) projection of vector and raster layers. All of these features allow the user to display layers with different
CRSs and have them overlay properly.
10.1 Overview of Projection Support
QGIS has support for approximately 2,700 known CRSs. Definitions for each CRS are stored in a SQLite database
that is installed with QGIS. Normally, you do not need to manipulate the database directly. In fact, doing so may
cause projection support to fail. Custom CRSs are stored in a user database. See section Custom Coordinate
Reference System for information on managing your custom coordinate reference systems.
The CRSs available in QGIS are based on those defined by the European Petroleum Search Group (EPSG) and
the Institut Geographique National de France (IGNF) and are largely abstracted from the spatial reference tables
used in GDAL. EPSG identifiers are present in the database and can be used to specify a CRS in QGIS.
In order to use OTF projection, either your data must contain information about its coordinate reference system or
you will need to define a global, layer or project-wide CRS. For PostGIS layers, QGIS uses the spatial reference
identifier that was specified when the layer was created. For data supported by OGR, QGIS relies on the presence
of a recognized means of specifying the CRS. In the case of shapefiles, this means a file containing the well-known
text (WKT) specification of the CRS. This projection file has the same base name as the shapefile and a .prj
extension. For example, a shapefile named alaska.shp would have a corresponding projection file named
alaska.prj.
Whenever you select a new CRS, the layer units will automatically be changed in the General tab of the
Properties dialog under the Project (or
Project
Settings) menu.
10.2 Global Projection Specification
QGIS starts each new project using the global default projection. The global default CRS is EPSG:4326 - WGS 84
(proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs), and it comes predefined in QGIS. This
Select CRS
default can be changed via the
button in the first section, which is used to define the default coordinate
reference system for new projects, as shown in figure_projection_options. This choice will be saved for use in
subsequent QGIS sessions.
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Fig. 10.1: CRS tab in the QGIS Options Dialog
When you use layers that do not have a CRS, you need to define how QGIS responds to these layers. This can be
done globally or project-wide in the CRS tab under Settings → Options.
The options shown in figure_projection_options are:
•
Prompt for CRS
•
Use project CRS
•
Use default CRS displayed below
If you want to define the coordinate reference system for a certain layer without CRS information, you can also
do that in the Source tab of the raster and vector properties dialog (see Source Properties for rasters and Source
Properties for vectors). If your layer already has a CRS defined, it will be displayed as shown in Source tab in
vector layers properties dialog.
Tip: CRS in the Layers Panel
Right-clicking on a layer in the Layers Panel (section Layers Panel) provides two CRS shortcuts. Set layer CRS
takes you directly to the Coordinate Reference System Selector dialog (see figure_projection_project). Set project
CRS from Layer redefines the project CRS using the layer’s CRS.
10.3 Define On The Fly (OTF) CRS Transformation
QGIS supports on the fly CRS transformation for both raster and vector data. However, OTF is not activated by
default. When OTF is off, each layer is drawn using the coordinates as read from the data source. When OTF is
on, each layer’s coordinates are transformed to the CRS of the project.
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There are three ways to enable On The Fly CRS Transformation:
• Select
Project Properties → CRS from the Project ( or
Settings) menu. You can then activate the
Enable on the fly CRS transformation (OTF) checkbox in the
(see Coordinate Reference System Selector)
• Click on the
dialog.
CRS status
CRS tab and select the CRS to use
icon in the lower right-hand corner of the status bar, leading you to the previous
CRS tab of the Settings → Options dialog by selecting
Enable ‘on
• Turn OTF on by default in the
the fly’ reprojection by default or Automatically enable ‘on the fly’ reprojection if layers have different CRS.
If you have already loaded a layer and you want to enable OTF reprojection, the best practice is to open the
CRS tab of the Project Properties dialog, activate the
Enable ‘on the fly’ CRS transformation checkbox, and
CRS status
select a CRS. The
icon will no longer be greyed out, and all layers will be OTF projected to the CRS
shown next to the icon.
Fig. 10.2: Project Properties Dialog
10.4 Coordinate Reference System Selector
This dialog helps you assign a Coordinate Reference System to a project or a layer, provided a set of projection
databases. Items in the dialog are:
• Filter: If you know the EPSG code, the identifier, or the name for a coordinate reference system, you can
use the search feature to find it. Enter the EPSG code, the identifier or the name.
• Recently used coordinate reference systems: If you have certain CRSs that you frequently use in your
everyday GIS work, these will be displayed in this list. Click on one of these items to select the associated
CRS.
• Coordinate reference systems of the world: This is a list of all CRSs supported by QGIS, including
Geographic, Projected and Custom coordinate reference systems. To define a CRS, select it from the list by
expanding the appropriate node and selecting the CRS. The active CRS is preselected.
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• PROJ.4 text: This is the CRS string used by the PROJ.4 projection engine. This text is read-only and
provided for informational purposes.
Tip: Looking for a layer CRS? Use the CRS selector.
Sometimes, you receive a layer and you don’t know its projection. Assuming that you have another layer with a
valid crs that should overlaps with it, enable the OTF reprojection and, in the Source tab of the Layer properties
dialog, use the Coordinate Reference System selector to assign a projection. Your layer position is then moved
accordingly. You may have to do some trial and error in order to find the right position, hence its original CRS.
Note: When operating across layers, for example, computing intersections between two layers, it is important
that both layers have the same CRS. To change the projection of an existing layer, it is insufficient to simply
change the CRS in that layer’s properties. Instead you must save the layer as a new layer, and choose the desired
CRS for the new layer.
10.5 Custom Coordinate Reference System
If QGIS does not provide the coordinate reference system you need, you can define a custom CRS. To define a
CRS, select
Custom CRS. . . from the Settings menu. Custom CRSs are stored in your QGIS user database.
In addition to your custom CRSs, this database also contains your spatial bookmarks and other custom data.
Defining a custom CRS in QGIS requires a good understanding of the PROJ.4 projection library. To begin, refer to
“Cartographic Projection Procedures for the UNIX Environment - A User’s Manual” by Gerald I. Evenden, U.S.
Geological Survey Open-File Report 90-284, 1990 (available at ftp://ftp.remotesensing.org/proj/OF90-284.pdf).
This manual describes the use of the proj.4 and related command line utilities. The cartographic parameters
used with proj.4 are described in the user manual and are the same as those used by QGIS.
The Custom Coordinate Reference System Definition dialog requires only two parameters to define a user CRS:
1. A descriptive name
2. The cartographic parameters in PROJ.4 format
To create a new CRS, click the
Add new CRS
button and enter a descriptive name and the CRS parameters.
Note that the Parameters must begin with a +proj= block, to represent the new coordinate reference system.
You can test your CRS parameters to see if they give sane results. To do this, enter known WGS 84 latitude and
longitude values in North and East fields, respectively. Click on [Calculate], and compare the results with the
known values in your coordinate reference system.
10.6 Default datum transformations
OTF depends on being able to transform data into a ‘default CRS’, and QGIS uses WGS84. For some CRS there
are a number of transforms available. QGIS allows you to define the transformation used otherwise QGIS uses a
default transformation.
In the CRS tab under Settings →
Options you can:
• set QGIS to ask you when it needs define a transformation using
default is defined
Ask for datum transformation when no
• edit a list of user defaults for transformations.
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Fig. 10.3: Custom CRS Dialog
10.6. Default datum transformations
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QGIS asks which transformation to use by opening a dialogue box displaying PROJ.4 text describing the source
and destination transforms. Further information may be found by hovering over a transform. User defaults can be
Remember selection.
saved by selecting
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11
Managing Data Source
11.1 Opening Data
As part of an Open Source Software ecosystem, QGIS is built upon different libraries that, combined with its own
providers, offer capabilities to read and often write a lot of formats:
• Vector data formats include ESRI formats (shapefiles, geodatabases. . . ), MapInfo and MicroStation file
formats, AutoCAD DWG/DXF, GeoPackage, GeoJSON, GRASS, GPX, KML, Comma Separated Values,
and many more. . . Read the complete list of OGR vector supported formats;
• Raster data formats include ArcInfo Binary Grid, ArcInfo ASCII Grid, JPEG, GeoTIFF, ERDAS IMAGINE,
MBTiles, R or Idrisi rasters, ASCII Gridded XYZ, GDAL Virtual, SRTM, Sentinel Data, and many more. . .
Read the complete list of raster supported formats;
• Database formats include PostgreSQL/PostGIS, SQLite/SpatiaLite, Oracle, DB2 or MSSQL Spatial,
MySQL. . . ;
• Support of web data services (WM(T)S, WFS, WCS, CSW, ArcGIS Servers. . . ) is also handled by QGIS
providers (see QGIS as OGC Data Client);
• You can also read supported files from archived folders and use QGIS native formats such as virtual and
memory layers.
As of the date of this document, more than 80 vector and 140 raster formats are supported by the GDAL/OGR and
QGIS native providers.
Note: Not all of the listed formats may work in QGIS for various reasons. For example, some require external
proprietary libraries, or the GDAL/OGR installation of your OS may not have been built to support the format you
want to use. To have a list of available formats, run the command line ogrinfo --formats (for vector) or
check settings → Options → GDAL menu (for raster) in QGIS.
11.1.1 The Browser Panel
QGIS Browser is one of the main panels of QGIS that lets you quickly and easily add your data to projects. It
helps you navigate in your filesystem and manage geodata, regardless the type of layer (raster, vector, table), or
the datasource format (plain or compressed files, database, web services).
To add a layer into a project:
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1. right-click on QGIS toolbar and check
Browser Panel to activate it or select it from the menu View →
Settings → Panels);
Panels (or
2. a browser tree with your filesystem, databases and web services is displayed;
3. find the layer in the list;
4. right-click on its name and select Add selected layer(s). Your layer is now added to the Layers Panel and
can be viewed in the map canvas.
Note: You can also add a layer or open a QGIS project directly from the Browser panel by double-clicking its
name or by drag-and-drop into the map canvas.
Once a file is loaded, you can zoom around it using the map navigation tools. To change the style of a layer,
open the Layer Properties dialog by double clicking on the layer name or by right-clicking on the name in the
legend and choosing Properties from the context menu. See section Symbology Properties for more information
on setting symbology of vector layers.
At the top of the Browser panel, you find some icons that help you to:
•
Add Selected Layers
: you can also add data into the map canvas by selecting Add selected layer(s) from the
layer’s context menu;
•
Refresh
•
Filter Browser
the browser tree;
to search for specific data. Enter a search word or wildcard and the browser will filter the tree
to only show paths to matching DB tables, filenames or folders – other data or folders won’t be displayed.
See the Browser Panel(2) example on the figure_browser_panels. The comparison can be case-sensitive or
not. It can also be set to:
– normal: return any item containing the search text;
– using wildcard(s): fine tune the search using ? and/or * characters to specify the position of the search
text;
– using a regular expression.
•
•
Collapse All
the whole tree;
Enable/disable properties widget
: when toggled on, a new widget is added at the bottom of the panel showing, if
applicable, metadatas of the selected item.
Right-click an item in the browser tree helps you to:
• in case of file or table, display its metadata or open it in your project. Tables can even be renamed, deleted
or truncated;
• in case of folder, bookmark it into your favourites, hide it from the browser tree. Hidden folders can be
managed from the Settings → Options → Data Sources tab;
• create connection to databases or web servers;
• refresh, rename or delete schema.
You can also import files into databases or copy tables from one schema/database to another one with a simple
drag-and-drop. There is a second browser panel available to avoid long scrolling while dragging. Just select the
file and drag-and-drop from one panel to the other.
Tip: Add layers to QGIS by simple drag-and-drop from your OS file browser
You can also add file(s) to the project by drag-and-dropping them from your operating system file browser to the
Layers Panel or the map canvas.
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Fig. 11.1: QGIS Browser panels side-by-side
11.1.2 The DB Manager
The DB Manager Plugin is another one of the main and native tools to integrate and manage spatial database
formats supported by QGIS (PostGIS, SpatiaLite, GeoPackage, Oracle Spatial, MSSQL, DB2, Virtual layers) in
one user interface. It can be activated from the Plugins → Manage and Install Plugins. . . menu.
The
DB Manager
Plugin provides several features:
• connect to databases and display its structure and contents;
• preview tables of databases;
• add layers to map canvas, either by double-click or drag-and-drop;
• add layers to a database from the QGIS Browser or from another database;
• create and add output of SQL queries to the map canvas;
• create virtual layers.
More information on DB Manager capabilities are exposed in DB Manager Plugin.
11.1.3 Provider-based loading tools
Beside Browser Panel and DB Manager, the main tools provided by QGIS to add layers regardless the format,
you’ll also find tools that are specific to data providers.
Note: Some external plugins also propose tools to open specific format files in QGIS.
Loading a layer from a file
To load a layer from a file, you can:
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Fig. 11.2: DB Manager dialog
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• for vector data (like Shapefile, Mapinfo or dxf layer), click on
Add Vector Layer
toolbar button, select the
Layer → Add Layer →
Add Vector Layer menu option or press Ctrl+Shift+V. This will bring up a
new window (see figure_vector_add) from which you can check
File and click on [Browse]. You can
also specify the encoding for the file if desired.
Fig. 11.3: Add Vector Layer Dialog
Add Raster Layer
icon, select the Layer → Add Layer →
• for raster layers, click on the
menu option or type Ctrl+Shift+R.
Add Raster Layer
That will bring up a standard open file dialog (see figure_vector_open), which allows you to navigate the file
system and load a shapefile, a geotiff or other supported data source. The selection box Filter
allows you
to preselect some supported file formats. Only the formats that have been well tested appear in the list. Other
untested formats can be loaded by selecting All files (*.*).
Fig. 11.4: Open an OGR Supported Vector Layer Dialog
Selecting a file from the list and clicking [Open] loads it into QGIS. More than one layer can be loaded at the same
time by holding down the Ctrl or Shift key and clicking on multiple items in the dialog. Figure_vector_loaded
shows QGIS after loading the alaska.shp file.
Note: Because some formats like MapInfo (e.g., .tab) or Autocad (.dxf) allow mixing different types of
geometry in a single file, loading such format in QGIS opens a dialog to select geometries to use in order to have
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Fig. 11.5: QGIS with Shapefile of Alaska loaded
one geometry per layer.
Using the
Add Vector Layer
tool:
• You can also load specific formats like ArcInfo Binary Coverage, UK. National Transfer
Format, as well as the raw TIGER format of the US Census Bureau or OpenfileGDB. To do that,
Directory as Source type. In this case a directory can be selected in the dialog after
you’d need to select
pressing [Browse].
• With the
Database source type you can select an existing database connection or create one to
the selected database type. Available database types are ODBC, OGDI Vectors, Esri Personal
Geodatabase, MySQL as well as PostgreSQL or MSSQL.
Pressing the [New] button opens the Create a New OGR Database Connection dialog whose parameters are
among the ones you can find in Creating a stored Connection. Pressing [Open] you can select from the
available tables for example of the PostGIS enabled database.
Protocol, enables to open data from the web using for example GeoJSON or
• The last source type,
CouchDB format. After selecting the type you have to fill URI of the source.
Tip: Load layers and projects from mounted external drives on macOS
On macOS, portable drives that are mounted beside the primary hard drive do not show up as expected under File
→ Open Project. We are working on a more macOS-native open/save dialog to fix this. As a workaround, you can
type /Volumes in the File name box and press Enter. Then you can navigate to external drives and network
mounts.
Importing a delimited text file
Delimited text file (e.g. .csv, .txt) can be loaded in QGIS using the tools described above. However, loaded
this way, it’ll show up like a simple table data. Sometimes, delimited text files can contain geometric data you’d
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want to visualize; this is what the
Add Delimited Text Layer is designed for.
Add Delimited Text Layer
Click the toolbar icon
in the Manage layers toolbar to open the Create a Layer from a
Delimited Text File dialog, as shown in figure_delimited_text.
Fig. 11.6: Delimited Text Dialog
First, select the file to import (e.g., qgis_sample_data/csv/elevp.csv) by clicking on the [Browse]
button. Once the file is selected, QGIS attempts to parse the file with the most recently used delimiter. To enable
QGIS to properly parse the file, it is important to select the correct delimiter. You can specify a delimiter by
activating:
•
•
CSV (comma separated values);
Custom delimiters, choosing among some predefined delimiters like comma, space, tab,
semicolon. . . ;
• or Regular expression delimiter and entering text into the Expression field. For example, to change the
delimiter to tab, use \t (this is a regular expression for the tab character).
Once the file is parsed, set Geometry definition to
Point coordinates and choose the X and Y fields from the
dropdown lists. If the coordinates are defined as degrees/minutes/seconds, activate the
checkbox.
DMS coordinates
Finally, enter a layer name (e.g., elevp), as shown in figure_delimited_text. To add the layer to the map, click
[OK]. The delimited text file now behaves as any other map layer in QGIS.
There is also a helper option that allows you to trim leading and trailing spaces from fields —
Also, it is possible to
by activating
Trim fields.
Discard empty fields. If necessary, you can force a comma to be the decimal separator
Decimal separator is comma.
If spatial information is represented by WKT, activate the
Well Known Text option and select the field with the
No geometry
WKT definition for point, line or polygon objects. If the file contains non-spatial data, activate
(attribute only table) and it will be loaded as an ordinal table.
Additionally, you can enable:
•
Use spatial index to improve the performance of displaying and spatially selecting features;
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•
Use subset index;
•
Watch file to watch for changes to the file by other applications while QGIS is running.
Importing a DXF or DWG file
DXF files can be added to QGIS by simple drag-and-drop from the common Browser Panel. You’ll be prompted
to select the sublayers you’d like to add to the project. Layers are added with random style properties.
Note: DXF files containing several geometry types (point, line and/or polygon), the name of the layer will be
made from <filename.dxf> entities <geometry type>.
To keep the dxf/dwg structure and its symbology in QGIS, you may want to use the dedicated DWG/DXF Import. . .
tool.
Importing OpenStreetMap Vectors
In recent years, the OpenStreetMap project has gained popularity because in many countries no free geodata such
as digital road maps are available. The objective of the OSM project is to create a free editable map of the world
from GPS data, aerial photography or local knowledge. To support this objective, QGIS provides support for OSM
data.
Using the Browser Panel, you can load a .osm file to the map canvas, in which case you’ll get a dialog to select
sublayers based on the geometry type. The loaded layers will contain all the data of that geometry type in the file
and keep the osm file data structure.
To avoid working with a such complex data structure, and be able to select only features you need based on their
tags, QGIS provides a core and fully integrated OpenStreetMap import tool:
• To connect to the OSM server and download data, open the menu Vector → OpenStreetMap → Download
data. . . . You can skip this step if you already obtained an .osm XML file using JOSM, Overpass API or
any other source;
• The menu Vector → OpenStreetMap → Import Topology from XML. . . will convert your .osm file into a
SpatiaLite database and create a corresponding database connection;
• The menu Vector → OpenStreetMap → Export Topology to SpatiaLite. . . then allows you to open the
database connection, select the type of data you want (points, lines, or polygons) and choose tags to import. This creates a SpatiaLite geometry layer that you can add to your project by clicking on the
Add SpatiaLite Layer
toolbar button or by selecting the
(see section SpatiaLite Layers).
Add SpatiaLite Layer. . . option from the Layer menu
GPS
Loading GPS data in QGIS can be done using the core plugin: GPS Tools. Instructions are described in Section
GPS Plugin.
GRASS
Working with GRASS vector data is described in Section GRASS GIS Integration.
SpatiaLite Layers
The first time you load data from a SpatiaLite database, begin by:
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• clicking on the
• selecting the
Add SpatiaLite Layer
toolbar button;
Add SpatiaLite Layer. . . option from the Layer → Add Layer menu;
• or by typing Ctrl+Shift+L.
This will bring up a window that will allow you either to connect to a SpatiaLite database already known to QGIS,
which you can choose from the drop-down menu, or to define a new connection to a new database. To define a
new connection, click on [New] and use the file browser to point to your SpatiaLite database, which is a file with
a .sqlite extension.
QGIS also supports editable views in SpatiaLite.
Database related tools
Creating a stored Connection
In order to read and write tables from the many database formats QGIS supports you’ll need to create a connection
to that database. While QGIS Browser Panel is the simplest and recommanded way to connect and use databases,
QGIS provides other tools to connect to each of them and load their tables:
•
Add PostGIS Layer. . . or by typing Ctrl+Shift+D;
•
Add MSSQL Spatial Layer or by typing Ctrl+Shift+M;
•
Add Oracle Spatial Layer. . . or by typing Ctrl+Shift+O;
•
Add DB2 Spatial Layer. . . or by typing Ctrl+Shift+2.
These tools are accessible either from the Manage Layers Toolbar or the Layer → Add Layer → menu. Connecting
to SpatiaLite database is described at SpatiaLite Layers.
Tip: Create connection to database from the QGIS Browser Panel
Select the corresponding database format in the Browser tree, right-click and choose connect will provide you
with the database connection dialog.
Most of the connection dialogs follow a common basis that will be described below using the PostGreSQL
database tool as example. For additional settings specific to other providers, you can find corresponding description at:
• Connecting to MSSQL Spatial;
• Connecting to Oracle Spatial;
• Connecting to DB2 Spatial.
The first time you use a PostGIS data source, you must create a connection to a database that contains the data.
Begin by clicking the appropriate button as exposed above, opening an Add PostGIS Table(s) dialog (see figure_add_postgis_tables). To access the connection manager, click on the [New] button to display the Create a
New PostGIS Connection dialog.
The parameters required for a PostGIS connection are exposed below. For the other database types, see their
differences at Particular Connection requirements.
• Name: A name for this connection. It can be the same as Database.
• Service: Service parameter to be used alternatively to hostname/port (and potentially database). This can be
defined in pg_service.conf. Check the PostgreSQL Service connection file section for more details.
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Fig. 11.7: Create a New PostGIS Connection Dialog
• Host: Name of the database host. This must be a resolvable host name such as would be used to open a
TCP/IP connection or ping the host. If the database is on the same computer as QGIS, simply enter localhost
here.
• Port: Port number the PostgreSQL database server listens on. The default port for PostGIS is 5432.
• Database: Name of the database.
• SSL mode: How the SSL connection will be negotiated with the server. Note that massive speed-ups in
PostGIS layer rendering can be achieved by disabling SSL in the connection editor. The following options
are available:
– Disable: Only try an unencrypted SSL connection;
– Allow: Try a non-SSL connection. If that fails, try an SSL connection;
– Prefer (the default): Try an SSL connection. If that fails, try a non-SSL connection;
– Require: Only try an SSL connection.
• Username: User name used to log in to the database.
• Password: Password used with Username to connect to the database.
You can save any or both of the username and password parameters, in which case they will be used by
default each time you need to connect to this database. If not saved, you’ll be prompted to fill the missing
credentials to connect to the database in next QGIS sessions; meanwhile the connection parameters you
entered are stored in a temporary internal cache and returned whenever a username/password for the same
database is requested, until you close the current QGIS process.
Warning: QGIS User Settings and Security
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In the Authentication tab, saving username and password will keep unprotected credentials in the
connection configuration. Those credentials will be visible if, for instance, you shared the project
file with someone. Therefore, it’s advisable to save your credentials in a Authentication configuration
instead (Configurations tab - See Authentication System for more details) or in a service connection file
(see PostgreSQL Service connection file for example).
Optionally, depending on the type of database, you can activate the following checkboxes:
•
Only show layers in the layer registries
•
Don’t resolve type of unrestricted columns (GEOMETRY)
•
Only look in the ‘public’ schema
•
Also list tables with no geometry
•
Use estimated table metadata
Tip: Use estimated table metadata to speed up operations
When initializing layers, various queries may be needed to establish the characteristics of the geometries stored
in the database table. When the Use estimated table metadata option is checked, these queries examine only a
sample of the rows and use the table statistics, rather than the entire table. This can drastically speed up operations
on large datasets, but may result in incorrect characterization of layers (eg. the feature count of filtered layers
will not be accurately determined) and may even cause strange behaviour in case columns that are supposed to be
unique actually are not.
Once all parameters and options are set, you can test the connection by clicking on the [Test connection] button
or apply it hitting [OK]. From the Add PostGIS Table(s), click now on [Connect] and the dialog is filled with
tables from the selected database (as shown in figure_add_postgis_tables).
Particular Connection requirements
Because of database type particularities, provided options are all the same for all the databases. Below are exposed
these connection specificities.
PostgreSQL Service connection file
The service connection file allows PostgreSQL connection parameters to be associated with a single service name.
That service name can then be specified by a client and the associated settings will be used.
It’s called .pg_service.conf under *nix systems (GNU/Linux, macOS etc.) and pg_service.conf on
Windows.
The service file looks like:
[water_service]
host=192.168.0.45
port=5433
dbname=gisdb
user=paul
password=paulspass
[wastewater_service]
host=dbserver.com
dbname=water
user=waterpass
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Note: There are two services in the above example: water_service and wastewater_service. You can
use these to connect from QGIS, pgAdmin etc. by specifying only the name of the service you want to connect to
(without the enclosing brackets). If you want to use the service with psql you need to do something like export
PGSERVICE=water_service before doing your psql commands.
Note: You can find all the parameters here
Note: If you don’t want to save the passwords in the service file you can use the .pg_pass option.
On *nix operating systems (GNU/Linux, macOS etc.) you can save the .pg_service.conf file in the user’s
home directory and the PostgreSQL clients will automatically be aware of it. For example, if the logged user is
web, .pg_service.conf should be saved in the /home/web/ directory in order to directly work (without
specifying any other environment variables).
You can specify the location of the service file by creating a PGSERVICEFILE environment variable (e.g. run
the export PGSERVICEFILE=/home/web/.pg_service.conf command under your *nix OS to temporarily set the PGSERVICEFILE variable)
You can also make the service file available system-wide (all users) either by placing the .pg_service.conf
file at pg_config --sysconfdir or by adding the PGSYSCONFDIR environment variable to specify the
directory containing the service file. If service definitions with the same name exist in the user and the system file,
the user file takes precedence.
Warning: There are some caveats under Windows:
• The service file should be saved as pg_service.conf and not as .pg_service.conf.
• The service file should be saved in Unix format in order to work. One way to do it is to open it with
Notepad++ and Edit → EOL Conversion → UNIX Format → File save.
• You can add environmental variables in various ways; a tested one, known to work reliably, is Control
Panel → System and Security → System → Advanced system settings → Environment Variables adding
PGSERVICEFILE and the path of the type C:\Users\John\pg_service.conf
• After adding an environment variable you may also need to restart the computer.
Connecting to Oracle Spatial
The spatial features in Oracle Spatial aid users in managing geographic and location data in a native type within
an Oracle database. In addition to some of the options in Creating a stored Connection, the connection dialog
proposes:
• Database: SID or SERVICE_NAME of the Oracle instance;
• Port: Port number the Oracle database server listens on. The default port is 1521;
• Workspace: Workspace to switch to.
Optionally, you can activate following checkboxes:
•
Only look in metadata table: restricts the displayed tables to those that are in the
all_sdo_geom_metadata view. This can speed up the initial display of spatial tables;
•
Only look for user’s tables: when searching for spatial tables, restrict the search to tables that are owned
by the user;
•
Also list tables with no geometry: indicates that tables without geometry should also be listed by default;
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•
Use estimated table statistics for the layer metadata: when the layer is set up, various metadata are
required for the Oracle table. This includes information such as the table row count, geometry type and
spatial extents of the data in the geometry column. If the table contains a large number of rows, determining
this metadata can be time-consuming. By activating this option, the following fast table metadata operations
are done: Row count is determined from all_tables.num_rows. Table extents are always determined
with the SDO_TUNE.EXTENTS_OF function, even if a layer filter is applied. Table geometry is determined
from the first 100 non-null geometry rows in the table;
•
Only existing geometry types: only list the existing geometry types and don’t offer to add others;
•
Include additional geometry attributes.
Tip: Oracle Spatial Layers
Normally, an Oracle Spatial layer is defined by an entry in the USER_SDO_METADATA table.
Connecting to DB2 Spatial
In addition to some of the options described in Creating a stored Connection, the connection to a DB2 database
(see DB2 Spatial Layers for more information) can be specified using either a Service/DSN name defined to
ODBC or using the driver, host and port information.
An ODBC Service/DSN connection requires the service name defined to ODBC.
A driver/host/port connection requires:
• Driver: Name of the DB2 driver. Typically this would be IBM DB2 ODBC DRIVER.
• DB2 Host: Name of the database host. This must be a resolvable host name such as would be used to
open a TCP/IP connection or ping the host. If the database is on the same computer as QGIS, simply enter
localhost here.
• DB2 Port: Port number the DB2 database server listens on. The default DB2 LUW port is 50000. The
default DB2 z/OS port is 446.
Tip: DB2 Spatial Layers
A DB2 Spatial layer is defined by a row in the DB2GSE.ST_GEOMETRY_COLUMNS view.
Note: In order to work effectively with DB2 spatial tables in QGIS, it is important that tables have an INTEGER
or BIGINT column defined as PRIMARY KEY and if new features are going to be added, this column should also
have the GENERATED characteristic.
It is also helpful for the spatial column to be registered with a specific spatial reference identifier (most often 4326
for WGS84 coordinates). A spatial column can be registered by calling the ST_Register_Spatial_Column
stored procedure.
Connecting to MSSQL Spatial
In addition to some of the options in Creating a stored Connection, creating a new MSSQL connection dialog
proposes you to fill a Provider/DSN name. You can also display available databases.
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Loading a Database Layer
Once you have one or more connections defined to a database (see section Creating a stored Connection), you
can load layers from it. Of course, this requires having available data. See e.g. section Importing Data into
PostgreSQL for a discussion on importing data into a PostGIS database.
To load a layer from a database, you can perform the following steps:
1. Open the “Add <database> table(s)” dialog (see Creating a stored Connection),
2. Choose the connection from the drop-down list and click [Connect].
3. Select or unselect
Also list tables with no geometry.
4. Optionally, use some
Search Options to reduce the list of tables to those matching your search. You can
also set this option before you hit the [Connect] button, speeding this way the database fetching.
5. Find the layer(s) you wish to add in the list of available layers.
6. Select it by clicking on it. You can select multiple layers by holding down the Shift key while clicking.
7. If applicable, use the [Set Filter] button (or double-click the layer) to start the Query builder dialog (See
section Query Builder) and define which features to load from the selected layer. The filter expression
appears in the sql column. This restriction can be removed or edited in the Layer Properties → General
→ Provider Feature Filter frame.
8. The checkbox in the Select at id column that is activated by default gets the features ids without the
attributes and speed in most cases the data loading.
9. Click on the [Add] button to add the layer to the map.
Fig. 11.8: Add PostGIS Table(s) Dialog
Tip: Load database table(s) from the Browser Panel
Like simple files, connected database are also listed in the Browser Panel. Hence, you can load tables from
databases using the Browser:
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1. Find the layer to use with the
the search options);
Filter Browser
tool at the top the browser panel (see The Browser Panel for
2. select and drag-and-drop it in the map canvas.
11.1.4 QGIS Custom formats
QGIS proposes two custom formats you can load in the application using their own loading tool:
• Temporary Scratch Layer: a memory layer that is bound to the project it’s opened with (see Creating a new
Temporary Scratch Layer for more information)
• Virtual Layers: a layer resulting from a query on other layer(s) (see Creating virtual layers for more information)
11.1.5 Connecting to web services
With QGIS you can have access to different types of OGC web services (WM(T)S, WFS(-T), CSW . . . ). Thanks
to QGIS Server, you can also publish these services. Description of these capabilities and how-to are provided in
chapter Working with OGC Data.
11.2 Creating Layers
Layers can be created in many ways, including:
• empty layers from scratch;
• layers from existing layers;
• layers from the clipboard;
• layers as a result of an SQL-like query based on one or many layers: the virtual layer.
QGIS also provides tools to import/export different formats.
11.2.1 Creating new vector layers
QGIS allows you to create new Shapefile layers, new SpatiaLite layers, new GPX layers and new Temporary
Scratch layers. Creation of a new GRASS layer is supported within the GRASS plugin. (Please refer to section
Creating a new GRASS vector layer for more information on creating GRASS vector layers.)
Creating a new GeoPackage layer
New GeoPackage Layer. . . . The New GeoPackage
To create a new GeoPackage layer go to Layer → New →
Layer dialog will be displayed as shown in figure_create_geopackage.
The first step is to select an existing GeoPackage or create a new one. This can be done by pressing the ellipses
[. . . ] button at the right of the Database field. Then, give a name for the new layer, define the layer type and
specify the coordinate reference system with [Specify CRS].
To define an attribute table for the new GeoPackage layer, add the names of the attribute columns you want to
create with the corresponding column type, and click on the [Add to fields list] button. Once you are happy with
the attributes, click [OK]. QGIS will automatically add the new layer to the legend, and you can edit it in the same
way as described in section Digitizing an existing layer.
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Fig. 11.9: Creating a New GeoPackage layer dialog
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Creating a new Shapefile layer
New Shapefile Layer. . . from the Layer menu. The
To create a new Shapefile layer, choose Create Layer →
New Shapefile Layer dialog will be displayed as shown in figure_create_shapefile. The first step is to provide a
path and name for the Shapefile. QGIS will automatically add the .shp extension to the name you specify Next,
choose the type of layer (point, line or polygon) and optional Z or M dimensions, as well as the CRS (coordinate
reference system).
Fig. 11.10: Creating a new Shapefile layer dialog
To complete the creation of the new Shapefile layer, add the desired attributes by specifying a name and type
for each attribute and clicking on the [Add to fields list] button. A first ‘id’ column is added by default but
can be removed, if not wanted. Only Decimal number
, Whole number
, Text data
and Date
attributes are supported. Additionally, depending on the attribute type, you can also define the length and
precision of the new attribute column. Once you are happy with the attributes, click [OK]. Once the Shapefile has
been created, it will be added to the map as a new layer, and you can edit it in the same way as described in section
Digitizing an existing layer.
Creating a new SpatiaLite layer
New SpatiaLite Layer. . . from the Layer menu.
To create a new SpatiaLite layer for editing, choose New →
The New SpatiaLite Layer dialog will be displayed as shown in Figure_create_spatialite.
The first step is to select an existing SpatiaLite database or to create a new SpatiaLite database. This can be done
with the browse button
to the right of the database field. Then, add a name for the new layer, define the
layer type, and specify the coordinate reference system with [Specify CRS]. If desired, you can select
an autoincrementing primary key.
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Create
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Fig. 11.11: Creating a New SpatiaLite layer dialog
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To define an attribute table for the new SpatiaLite layer, add the names of the attribute columns you want to create
with the corresponding column type, and click on the [Add to attribute list] button. Once you are happy with the
attributes, click [OK]. QGIS will automatically add the new layer to the legend, and you can edit it in the same
way as described in section Digitizing an existing layer.
Further management of SpatiaLite layers can be done with the DB Manager. See DB Manager Plugin.
Creating a new GPX layer
To create a new GPX file, you need to load the GPS plugin first. Plugins →
Plugin Manager Dialog. Activate the
Plugin Manager. . . opens the
GPS Tools checkbox.
When this plugin is loaded, choose New →
Create new GPX Layer. . . from the Layer menu. In the Save new
GPX file as dialog, choose where to save the new file and press [Save]. Three new layers are added to the Layers
Panel: waypoints, routes and tracks with predefined structure.
Creating a new Temporary Scratch Layer
Temporary Scratch Layers are in-memory layers, meaning that they are not saved on disk and will be discarded
when QGIS is closed. They can be handy to store features you temporarily need or as intermediate layers during
geoprocessing operations.
Empty, editable temporary scratch layers can be defined using Layer → Create Layer → New Temporary Scratch
Layer or
New temporary scratch layer
button from the Manage Layers Toolbar. Here you can create a:
• No geometry type layer, served as simple table,
• Point or MultiPoint layer,
• LineString/CompoundCurve or MultiLineString/MultiCurve layer,
• Polygon/CurvePolygon or MultiPolygon/MultiSurface layer.
Fig. 11.12: Creating a new Temporary Scratch layer dialog
By default, a new temporary scratch layer is created without any attribute. But you can also create prepopulated
temporary scratch layers using e.g. the clipboard. See Creating new layers from the clipboard.
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11.2.2 Creating new layers from an existing layer
Both raster and vector layers can be saved in a different format and/or reprojected to a different coordinate reference system (CRS) using the Save As. . . function in the layer context menu (by right-clicking in the layer in the
layer tree) or in the Layer → Save As. . . menu.
Common parameters
The Save As dialog shows several parameters to change the behavior when saving the layer. Common parameters
for raster and vector are:
• Format
• File name
• CRS can be changed to reproject the data
• Add saved file to map to add the new layer to the canvas
• Extent (possible values are layer, Map view or user-defined extent)
However, some parameters are specific to raster and vector formats:
Raster specific parameters
• Output mode (it can be raw data or rendered image)
• Resolution
• Create Options: advanced options (file compression, block sizes, colorimetry. . . ) to fine tune the output
file. See the gdal-ogr driver documentation.
• Pyramids creation
• VRT Tiles
• No data values
Vector specific parameters
Depending on the format of export, some of these options are available or not:
• Encoding
• Save only selected features
• Select fields to export and their export options. In case you set your fields behavior with some Edit widgets,
e.g. value map, you can keep the displayed values in the layer by checking
fields values by displayed values.
Replace all selected raw
• Symbology export: can be used mainly for DXF export and for all file formats who manage OGR feature
styles (see note below) as DXF, KML, tab file formats:
– No symbology: default style of the application that reads the data
– Feature symbology: save style with OGR Feature Styles (see note below)
– Symbol Layer symbology: save with OGR Feature Styles (see note below) but export the same
geometry multiple times if there are multiple symbology symbol layers used
– A Scale value can be applied to the latest options.
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Fig. 11.13: Saving as a new raster layer
Note: OGR Feature Styles are a way to store style directly in the data as a hidden attribute. Only some formats
can handle this kind of information. KML, DXF and TAB file formats are such formats. For advanced users, you
can read the OGR Feature Styles specification document.
• Geometry: you can configure the geometry capabilities of the output layer
– geometry type: keep the original geometry of the features when set to Automatic, otherwise removes
or overrides it with any type. You can add an empty geometry column to an attribute table, remove the
geometry column of a spatial layer.
– Force multi-type: force creation of multi-geometry features in the layer
– Include z-dimension to geometries.
Tip: Overriding layer geometry type makes it possible to do things like save a geometryless table (e.g. .csv
file) into a shapefile WITH any type of geometry (point, line, polygon), so that geometries can then be manually
added to rows with the
Add Part
tool .
• Datasources Options, Layer Options or Custom Options which allow you to configure some advanced parameters. See the gdal-ogr driver documentation.
When saving a vector layer into an existing file, depending on the capabilities of the output format (Geopackage,
SpatiaLite, FileGDB. . . ), the user can decide whether to:
• overwrite the whole file
• overwrite only the target layer (the layer name is configurable)
• append features to the existing target layer
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Fig. 11.14: Saving as a new vector layer
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• append features, add new fields if there are any.
For formats like ESRI Shapefile, MapInfo .tab, feature append is also available.
11.2.3 Creating new DXF files
Besides the Save As. . . dialog which provides options to export a single layer to another format, including *.DXF,
QGIS provides another tool to export multiple layers as a single DXF layers. It’s accessible in the Project → DXF
Export. . . menu.
The DXF Export dialog allows the user to:
• indicate the destination layer file;
• choose the symbology mode and scale (see the OGR Feature Styles note);
• select the encoding and CRS;
• check the loaded layers to include in the DXF files or pick them from an existing map theme.
For each layer, you can choose a field whose values are used to split features in generated destination layers
in the DXF output. You can also choose to
Use the layer title as name if set and keep features grouped.
• choose to only Export features intersecting the current map extent.
11.2.4 Creating new layers from the clipboard
Features that are on the clipboard can be pasted into a new layer. To do this, Select some features, copy them to
the clipboard, and then paste them into a new layer using Edit → Paste Features as → and choosing:
• New Vector Layer. . . : you need to select the layer CRS, poping up the Save vector layer as. . . dialog
from which you can select any supported data format (see Creating new layers from an existing layer for
parameters);
• or Temporary Scratch Layer. . . : you need to select the layer CRS and give a name.
A new layer, filled with selected features and their attributes is created and added to map canvas if asked.
Note: Creating layers from clipboard applies to features selected and copied within QGIS and also to features
from another source defined using well-known text (WKT).
11.2.5 Creating virtual layers
Virtual layers are a special kind of vector layer. They allow you to define a layer as the result of an SQL query involving any number of other vector layers that QGIS is able to open. Virtual layers do not carry data by themselves
and can be seen as views to other layers.
To create a virtual layer, open the virtual layer creation dialog by clicking on Add Virtual Layer in the Layer menu
or from the corresponding toolbar.
The dialog allows you to specify a Layer name and an SQL Query. The query can use the name (or id) of loaded
vector layers as tables, as well as their field names as columns.
For example, if you have a layer called airports, you can create a new virtual layer called
public_airports with an SQL query like:
SELECT *
FROM airports
WHERE USE = "Civilian/Public"
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The SQL query will be executed, regardless of the underlying provider of the airports layer, even if this
provider does not directly support SQL queries.
Fig. 11.15: Create virtual layers dialog
Joins and complex queries can also be created, for example, to join airports and country information:
SELECT airports.*, country.population
FROM airports
JOIN country
ON airports.country = country.name
Note: It’s also possible to create virtual layers using the SQL window of DB Manager Plugin.
Embedding layers for use in queries
Besides the vector layers available in the map canvas, the user can add layers to the Embedded layers list, which
he can use in queries without the need to have them showing in the map canvas or Layers panel.
To embed a layer, click Add and provide the Local name, Provider, Encoding and the path to the Source.
The Import button allows adding layers loaded in the map canvas into the Embedded layers list. This allows to
later remove those layers from the Layers panel without breaking any existent query.
Supported query language
The underlying engine uses SQLite and SpatiaLite to operate.
It means you can use all of the SQL your local installation of SQLite understands.
Functions from SQLite and spatial functions from SpatiaLite can also be used in a virtual layer query. For instance,
creating a point layer out of an attribute-only layer can be done with a query similar to:
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SELECT id, MakePoint(x, y, 4326) as geometry
FROM coordinates
Functions of QGIS expressions can also be used in a virtual layer query.
To refer the geometry column of a layer, use the name geometry.
Contrary to a pure SQL query, all the fields of a virtual layer query must be named. Don’t forget to use the as
keyword to name your columns if they are the result of a computation or function call.
Performance issues
With default parameters set, the virtual layer engine will try its best to detect the type of the different columns of
the query, including the type of the geometry column if one is present.
This is done by introspecting the query when possible or by fetching the first row of the query (LIMIT 1) at last
resort. Fetching the first row of the result just to create the layer may be undesirable for performance reasons.
The creation dialog allows to specify different parameters:
• Unique identifier column: this option allows specifying which field of the query represents unique integer
values that QGIS can use as row identifiers. By default, an autoincrementing integer value is used. Defining
a unique identifier column allows to speed up the selection of rows by id.
• No geometry: this option forces the virtual layer to ignore any geometry field. The resulting layer is an
attribute-only layer.
• Geometry Column: this option allows to specify the name of the column that is to be used as the geometry
of the layer.
• Geometry Type: this option allows to specify the type of the geometry of the virtual layer.
• Geometry CRS: this option allows to specify the coordinate reference system of the virtual layer.
Special comments
The virtual layer engine tries to determine the type of each column of the query. If it fails, the first row of the
query is fetched to determine column types.
The type of a particular column can be specified directly in the query by using some special comments.
The syntax is the following: /*:type*/. It has to be placed just after the name of a column. type can be either
int for integers, real for floating point numbers or text.
For instance:
SELECT id+1 as nid /*:int*/
FROM table
The type and coordinate reference system of the geometry column can also be set thanks to special comments
with the following syntax /*:gtype:srid*/ where gtype is the geometry type (point, linestring,
polygon, multipoint, multilinestring or multipolygon) and srid an integer representing the
EPSG code of a coordinate reference system.
Use of indexes
When requesting a layer through a virtual layer, indexes of this source layer will be used in the following ways:
• if an = predicate is used on the primary key column of the layer, the underlying data provider will be asked
for a particular id (FilterFid)
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• for any other predicates (>, <=, !=, etc.) or on a column without a primary key, a request built from an
expression will be used to request the underlying vector data provider. It means indexes may be used on
database providers if they exist.
A specific syntax exists to handle spatial predicates in requests and triggers the use of a spatial index: a hidden
column named _search_frame_ exists for each virtual layer. This column can be compared for equality to a
bounding box. Example:
SELECT *
FROM vtab
WHERE _search_frame_=BuildMbr(-2.10,49.38,-1.3,49.99,4326)
Spatial binary predicates like ST_Intersects are significantly sped up when used in conjunction with this
spatial index syntax.
11.3 Exploring Data Formats and Fields
11.3.1 Raster data
Raster data in GIS are matrices of discrete cells that represent features on, above or below the earth’s surface. Each
cell in the raster grid has the same size, and cells are usually rectangular (in QGIS they will always be rectangular).
Typical raster datasets include remote sensing data, such as aerial photography, or satellite imagery and modelled
data, such as an elevation matrix.
Unlike vector data, raster data typically do not have an associated database record for each cell. They are geocoded
by pixel resolution and the x/y coordinate of a corner pixel of the raster layer. This allows QGIS to position the
data correctly in the map canvas.
QGIS makes use of georeference information inside the raster layer (e.g., GeoTiff) or in an appropriate world file
to properly display the data.
11.3.2 Vector Data
Many of the features available in QGIS work the same, regardless the vector data source. However, because of the
differences in formats specifications (ESRI shapefiles, MapInfo and MicroStation file formats, AutoCAD DXF,
PostGIS, SpatiaLite, DB2, Oracle Spatial and MSSQL Spatial databases, and many more), QGIS may handle
differently some of their properties. This section describes how to work with these specificities.
Note: QGIS supports (multi)point, (multi)line, (multi)polygon, CircularString, CompoundCurve, CurvePolygon,
MultiCurve, MultiSurface feature types, all with Z and/or M values.
You should note also that some drivers don’t support some of these feature types like CircularString, CompoundCurve, CurvePolygon, MultiCurve, MultiSurface feature type. QGIS will convert them to (multi)polygon feature.
ESRI Shapefiles
The ESRI shapefile is still one of the most used vector file format in QGIS. However, this file format has some
limitation that some other file format have not (like Geopackage, spatialite). Support is provided by the OGR
Simple Feature Library.
A shapefile actually consists of several files. The following three are required:
1. .shp file containing the feature geometries
2. .dbf file containing the attributes in dBase format
3. .shx index file
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Shapefiles also can include a file with a .prj suffix, which contains the projection information. While it is very
useful to have a projection file, it is not mandatory. A shapefile dataset can contain additional files. For further
details, see the ESRI technical specification at http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf.
Improving Performance for Shapefiles
To improve the performance of drawing a shapefile, you can create a spatial index. A spatial index will improve
the speed of both zooming and panning. Spatial indexes used by QGIS have a .qix extension.
Use these steps to create the index:
• Load a shapefile (see The Browser Panel);
• Open the Layer Properties dialog by double-clicking on the shapefile name in the legend or by right-clicking
and choosing Properties from the context menu.
• In the Source tab, click the [Create Spatial Index] button.
Problem loading a shape .prj file
If you load a shapefile with a .prj file and QGIS is not able to read the coordinate reference system from that
file, you will need to define the proper projection manually within the Layer Properties → Source tab of the layer
Select CRS
button. This is due to the fact that .prj files often do not provide the complete
by clicking the
projection parameters as used in QGIS and listed in the CRS dialog.
For the same reason, if you create a new shapefile with QGIS, two different projection files are created: a .prj
file with limited projection parameters, compatible with ESRI software, and a .qpj file, providing the complete
parameters of the used CRS. Whenever QGIS finds a .qpj file, it will be used instead of the .prj.
Delimited Text Files
Tabular data is a very common and widely used format because of its simplicity and readability – data can be
viewed and edited even in a plain text editor. A delimited text file is an attribute table with each column separated
by a defined character and each row separated by a line break. The first row usually contains the column names. A
common type of delimited text file is a CSV (Comma Separated Values), with each column separated by a comma.
Such data files can also contain positional information in two main forms:
• As point coordinates in separate columns
• As well-known text (WKT) representation of geometry
QGIS allows you to load a delimited text file as a layer or ordinal table. But first check that the file meets the
following requirements:
1. The file must have a delimited header row of field names. This must be the first line in the text file.
2. The header row must contain field(s) with geometry definition. These field(s) can have any name.
3. The X and Y coordinates (if geometry is defined by coordinates) must be specified as numbers. The coordinate system is not important.
4. If you have any data that is not a string (text) and the file is a CSV file, you must have a CSVT file (see
section CSVT Files).
As an example of a valid text file, we import the elevation point data file elevp.csv that comes with the QGIS
sample dataset (see section Sample Data):
X;Y;ELEV
-300120;7689960;13
-654360;7562040;52
1640;7512840;3
[...]
Some items to note about the text file:
1. The example text file uses ; (semicolon) as delimiter. Any character can be used to delimit the fields.
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2. The first row is the header row. It contains the fields X, Y and ELEV.
3. No quotes (") are used to delimit text fields.
4. The X coordinates are contained in the X field.
5. The Y coordinates are contained in the Y field.
CSVT Files
When loading CSV files, the OGR driver assumes all fields are strings (i.e. text) unless it is told otherwise. You
can create a CSVT file to tell OGR (and QGIS) what data type the different columns are:
Type
Whole number
Decimal number
Date
Time
Date & Time
Name
Integer
Real
Date (YYYY-MM-DD)
Time (HH:MM:SS+nn)
DateTime (YYYY-MM-DD HH:MM:SS+nn)
Example
4
3.456
2016-07-28
18:33:12+00
2016-07-28 18:33:12+00
The CSVT file is a ONE line plain text file with the data types in quotes and separated by commas, e.g.:
"Integer","Real","String"
You can even specify width and precision of each column, e.g.:
"Integer(6)","Real(5.5)","String(22)"
This file is saved in the same folder as the .csv file, with the same name, but .csvt as the extension.
You can find more information at GDAL CSV Driver.
Others valuable informations for advanced users
Features with curved geometries (CircularString, CurvePolygon and CompoundCurve) are supported. Here are
three examples of such geometry types as a delimited text with WKT geometries:
Label;WKT_geom
CircularString;CIRCULARSTRING(268 415,227 505,227 406)
CurvePolygon;CURVEPOLYGON(CIRCULARSTRING(1 3, 3 5, 4 7, 7 3, 1 3))
CompoundCurve;COMPOUNDCURVE((5 3, 5 13), CIRCULARSTRING(5 13, 7 15,
9 13), (9 13, 9 3), CIRCULARSTRING(9 3, 7 1, 5 3))
Delimited Text supports also Z and M coordinates in geometries:
LINESTRINGM(10.0 20.0 30.0, 11.0 21.0 31.0)
PostGIS Layers
PostGIS layers are stored in a PostgreSQL database. The advantages of PostGIS are its spatial indexing, filtering
and querying capabilities it provides. Using PostGIS, vector functions such as select and identify work more
accurately than they do with OGR layers in QGIS.
Tip: PostGIS Layers
Normally, a PostGIS layer is defined by an entry in the geometry_columns table. QGIS can load layers that do
not have an entry in the geometry_columns table. This includes both tables and views. Defining a spatial view
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provides a powerful means to visualize your data. Refer to your PostgreSQL manual for information on creating
views.
This section contains some details on how QGIS accesses PostgreSQL layers. Most of the time, QGIS should
simply provide you with a list of database tables that can be loaded, and it will load them on request. However,
if you have trouble loading a PostgreSQL table into QGIS, the information below may help you understand any
QGIS messages and give you direction on changing the PostgreSQL table or view definition to allow QGIS to
load it.
Primary key
QGIS requires that PostgreSQL layers contain a column that can be used as a unique key for the layer. For tables,
this usually means that the table needs a primary key, or a column with a unique constraint on it. In QGIS, this
column needs to be of type int4 (an integer of size 4 bytes). Alternatively, the ctid column can be used as primary
key. If a table lacks these items, the oid column will be used instead. Performance will be improved if the column
is indexed (note that primary keys are automatically indexed in PostgreSQL).
QGIS offers a checkbox Select at id that is activated by default. This option gets the ids without the attributes
which is faster in most cases.
View
If the PostgreSQL layer is a view, the same requirement exists, but views do not always have primary keys or
columns with unique constraints on them. You have to define a primary key field (has to be integer) in the QGIS
dialog before you can load the view. If a suitable column does not exist in the view, QGIS will not load the layer.
If this occurs, the solution is to alter the view so that it does include a suitable column (a type of integer and either
a primary key or with a unique constraint, preferably indexed).
As for table, a checkbox Select at id is activated by default (see above for the meaning of the checkbox). It can
make sense to disable this option when you use expensive views.
QGIS layer_style table and database backup
If you want to make a backup of your PostGIS database using the pg_dump and pg_restore commands, and
the default layer styles as saved by QGIS fail to restore afterwards, you need to set the XML option to DOCUMENT
before the restore command:
SET XML OPTION DOCUMENT;
Filter database side
QGIS allows to filter features already on server side. Check the Settings → Options → Data Sources →
Execute expressions on postgres server-side if possible checkbox to do so. Only supported expressions will be sent
to the database. Expressions using unsupported operators or functions will gracefully fallback to local evaluation.
Support of PostgreSQL data types
Most of common data types are supported by the PostgreSQL provider: integer, float, varchar, geometry, timestamp and array.
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Importing Data into PostgreSQL
Data can be imported into PostgreSQL/PostGIS using several tools, including the DB Manager plugin and the
command line tools shp2pgsql and ogr2ogr.
DB Manager
DB Manager
QGIS comes with a core plugin named
. It can be used to load shapefiles and other data formats, and
it includes support for schemas. See section DB Manager Plugin for more information.
shp2pgsql
PostGIS includes an utility called shp2pgsql that can be used to import shapefiles into a PostGIS-enabled database.
For example, to import a shapefile named lakes.shp into a PostgreSQL database named gis_data, use the
following command:
shp2pgsql -s 2964 lakes.shp lakes_new | psql gis_data
This creates a new layer named lakes_new in the gis_data database. The new layer will have a spatial reference identifier (SRID) of 2964. See section Working with Projections for more information on spatial reference
systems and projections.
Tip: Exporting datasets from PostGIS
Like the import tool shp2pgsql, there is also a tool to export PostGIS datasets as shapefiles: pgsql2shp. This is
shipped within your PostGIS distribution.
ogr2ogr
Besides shp2pgsql and DB Manager, there is another tool for feeding geodata in PostGIS: ogr2ogr. This is part
of your GDAL installation.
To import a shapefile into PostGIS, do the following:
ogr2ogr -f "PostgreSQL" PG:"dbname=postgis host=myhost.de user=postgres
password=topsecret" alaska.shp
This will import the shapefile alaska.shp into the PostGIS database postgis using the user postgres with the
password topsecret on host server myhost.de.
Note that OGR must be built with PostgreSQL to support PostGIS. You can verify this by typing (in
)
ogrinfo --formats | grep -i post
If you prefer to use PostgreSQL’s COPY command instead of the default INSERT INTO method, you can export
the following environment variable (at least available on
and
):
export PG_USE_COPY=YES
ogr2ogr does not create spatial indexes like shp2pgsl does. You need to create them manually, using the normal SQL command CREATE INDEX afterwards as an extra step (as described in the next section Improving
Performance).
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Improving Performance
Retrieving features from a PostgreSQL database can be time-consuming, especially over a network. You can
improve the drawing performance of PostgreSQL layers by ensuring that a PostGIS spatial index exists on each
layer in the database. PostGIS supports creation of a GiST (Generalized Search Tree) index to speed up spatial
searches of the data (GiST index information is taken from the PostGIS documentation available at http://postgis.
net).
Tip: You can use the DBManager to create an index to your layer. You should first select the layer and click on
Table → Edit table, go to Indexes tab and click on [Add spatial index].
The syntax for creating a GiST index is:
CREATE INDEX [indexname] ON [tablename]
USING GIST ( [geometryfield] GIST_GEOMETRY_OPS );
Note that for large tables, creating the index can take a long time. Once the index is created, you should perform a
VACUUM ANALYZE. See the PostGIS documentation (POSTGIS-PROJECT Literature and Web References) for
more information.
The following is an example of creating a GiST index:
gsherman@madison:~/current$ psql gis_data
Welcome to psql 8.3.0, the PostgreSQL interactive terminal.
Type:
\copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit
gis_data=# CREATE INDEX sidx_alaska_lakes ON alaska_lakes
gis_data-# USING GIST (the_geom GIST_GEOMETRY_OPS);
CREATE INDEX
gis_data=# VACUUM ANALYZE alaska_lakes;
VACUUM
gis_data=# \q
gsherman@madison:~/current$
Vector layers crossing 180° longitude
Many GIS packages don’t wrap vector maps with a geographic reference system (lat/lon) crossing the 180 degrees
longitude line (http://postgis.refractions.net/documentation/manual-2.0/ST_Shift_Longitude.html). As result, if
we open such a map in QGIS, we will see two far, distinct locations, that should appear near each other. In
Figure_vector_crossing, the tiny point on the far left of the map canvas (Chatham Islands) should be within the
grid, to the right of the New Zealand main islands.
Fig. 11.16: Map in lat/lon crossing the 180° longitude line
A work-around is to transform the longitude values using PostGIS and the ST_Shift_Longitude function. This
function reads every point/vertex in every component of every feature in a geometry, and if the longitude coor11.3. Exploring Data Formats and Fields
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dinate is < 0°, it adds 360° to it. The result is a 0° - 360° version of the data to be plotted in a 180°-centric
map.
Fig. 11.17: Crossing 180° longitude applying the ST_Shift_Longitude function
Usage
• Import data into PostGIS (Importing Data into PostgreSQL) using, for example, the DB Manager plugin.
• Use the PostGIS command line interface to issue the following command (in this example,
“TABLE” is the actual name of your PostGIS table): gis_data=# update TABLE set
the_geom=ST_Shift_Longitude(the_geom);
• If everything went well, you should receive a confirmation about the number of features that were updated.
Then you’ll be able to load the map and see the difference (Figure_vector_crossing_map).
SpatiaLite Layers
If you want to save a vector layer to SpatiaLite format, you can do this by right clicking the layer in the legend.
Then, click on Save as. . . , define the name of the output file, and select ‘SpatiaLite’ as format and the CRS. Also,
you can select ‘SQLite’ as format and then add SPATIALITE=YES in the OGR data source creation option field.
This tells OGR to create a SpatiaLite database. See also http://www.gdal.org/ogr/drv_sqlite.html.
QGIS also supports editable views in SpatiaLite.
If you want to create a new SpatiaLite layer, please refer to section Creating a new SpatiaLite layer.
Tip: SpatiaLite data management Plugins
For SpatiaLite data management, you can also use several Python plugins: QSpatiaLite, SpatiaLite Manager or
DB Manager (core plugin, recommended). If necessary, they can be downloaded and installed with the Plugin
Installer.
DB2 Spatial Layers
IBM DB2 for Linux, Unix and Windows (DB2 LUW), IBM DB2 for z/OS (mainframe) and IBM DashDB products
allow users to store and analyse spatial data in relational table columns. The DB2 provider for QGIS supports the
full range of visualization, analysis and manipulation of spatial data in these databases.
User documentation on these capabilities can be found at the DB2 z/OS KnowledgeCenter, DB2 LUW KnowledgeCenter and DB2 DashDB KnowledgeCenter.
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For more information about working with the DB2 spatial capabilities, check out the DB2 Spatial Tutorial on IBM
DeveloperWorks.
The DB2 provider currently only supports the Windows environment through the Windows ODBC driver.
The client running QGIS needs to have one of the following installed:
• DB2 LUW
• IBM Data Server Driver Package
• IBM Data Server Client
To open a DB2 data in QGIS, you can refer to The Browser Panel or Loading a Database Layer section.
If you are accessing a DB2 LUW database on the same machine or using DB2 LUW as a client, the DB2 executables and supporting files need to be included in the Windows path. This can be done by creating a batch file like
the following with the name db2.bat and including it in the directory %OSGEO4W_ROOT%/etc/ini.
@echo off
REM Point the following to where DB2 is installed
SET db2path=C:\Program Files (x86)\sqllib
REM This should usually be ok - modify if necessary
SET gskpath=C:\Program Files (x86)\ibm\gsk8
SET Path=%db2path%\BIN;%db2path%\FUNCTION;%gskpath%\lib64;%gskpath%\lib;%path%
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CHAPTER
12
Working with Vector Data
12.1 The Symbol Library
12.1.1 The Style Manager
The Style Manager is the place where users can manage and create generic symbols to be used in several QGIS
projects. You can open it with the Settings → Style Manager or from the Style tab in the vector layer’s Properties.
It allows users to:
• create, edit and remove symbols
• organize symbols in custom groups
• export and import symbols.
Fig. 12.1: The Style Manager
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Groups and smart groups
You can organize symbols into different categories. These categories, listed in the panel at the left, can be static
(called Group) or dynamic (named Smart Group). A group is smart when its symbols are dynamically fetched
according to conditions set. See figure_smart_group:
To create a group, right click on an existing group or on the main Groups directory in the left of the dialog. You
can also select a group and click the
Add Group
button. The new group will be a sub-group of the selected one.
Create Smart Group is similar to creating group, but instead select Smart Groups. The dialog box allows user
to choose the expression to select symbols in order to appear in the smart group (contains some tags, member of
a group, have a string in its name, etc.). Any symbol that satisfies the entered condition(s) is automatically added
to the smart group.
Fig. 12.2: Creating a Smart Group
To remove a group or a smart group, right click on the group and select Remove Group or select it and push
Remove Group
button.
Unlike the smart groups that automatically fetch their belonged symbols, simple groups are filled by the user. To
add a symbol into a group, you can either right click on a symbol, choose Apply Group and then the group name.
There is a second way to add several symbols into a group: just select the group, click
and choose Group
Symbols. All symbols display a checkbox that allows you to add the symbol into the selected groups. When
selection is finished, you can click the same button, and choose Finish Grouping.
All the symbols that are not placed under a custom group belong to a default group named Ungrouped.
Add, Edit, Remove Symbol
Selecting a group returns in the right panel, if applicable, the list of symbols of the group (including its subgroups).
These symbols are organized in four different tabs:
• Marker for point symbols
• Line for linear symbols
• Fill for surface symbols
• and Color Ramp.
To delete a symbol you no longer need, just select it and click
The symbol will be deleted from the local symbols database.
The symbol list can be modified by adding new symbols with
Edit item
124
Remove item
Add item
(also available through right-click).
button or modifying existing ones with
. See The Symbol Selector for further information.
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Share symbols
Share item
tool, at the right bottom of the Style Library dialog, offers options to easily share symbols with
The
others: users can indeed export their symbols and import symbols to their library.
Exporting symbols
You can export the selected symbols to PNG, SVG or XML file formats. Exporting to PNG or SVG (both not
available for color ramp symbols) creates a file for each selected symbol, and the SVG folder can be added to
SVG Paths in Settings → Options to e.g. share these symbols on a network. The XML format generates a single
file containing all the selected symbols. This file can then be imported in another user’s style library.
Importing symbols
You can extend your symbols library by importing new symbols. Just select
at the right bottom of the dialog. In the new dialog, you’ll need to :
Import from the drop-down list
• indicate the source of the symbols (it can be a .xml file on the disk or an url),
• give the name of the group under which the symbols will be put
• select the symbols you want to add to your library
• and press Import.
Note that import and export options are also available through right-click.
Fig. 12.3: Importing symbols
Color Ramp
The Color ramp tab in the Style Manager presents different types of color ramps you can use to style layers.
To create a custom color ramp, activate the Color ramp tab and click the
drop-down list to choose the ramp type:
Add item
button. The button reveals a
• Gradient: given a start and end colors, generate a color ramp which can be continuous or discrete. With
double-clicking the ramp preview, you can add as many intermediate color stops as you want.
• Random: creates a random set of colors based on range of values for hue, saturation, value and opacity and
a number of colors (classes)
• ColorBrewer: a set of predefined discrete color gradients you can custom the number of colors in the ramp
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• or cpt-city: an access to a whole catalog of color gradients to locally save as gradient color.
Tip: Easily custom the color stops of the gradient color ramp
Double-clicking the ramp preview or drag-and-drop a color from the color spot onto the ramp preview adds a new
color stop. Each color stop can be tweaked using the Color Selector widgets or by plotting each of its parameters.
You can also reposition it using the mouse, the arrow keys (combine with Shift key for a larger move) or the
Relative position spinbox. Pressing Delete stop as well as DEL key removes the selected color stop.
You can use the
Invert option while classifying the data with a color ramp. See figure_color_custom_ramp for
an example of a custom color ramp and figure_color_cpt_city for the cpt-city Colors dialog.
Fig. 12.4: Example of custom gradient color ramp with multiple stops
The cpt-city option opens a new dialog with hundreds of themes included ‘out of the box’.
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Fig. 12.5: cpt-city dialog with hundreds of color ramps
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12.1.2 The Symbol Selector
The Symbol selector is the main dialog to design a symbol. You can create or edit Marker, Line or Fill Symbols.
Fig. 12.6: Designing a Marker symbol
Two main components structure the symbol selector dialog:
• the symbol tree, showing symbol layers that are combined afterwards to shape a new global symbol
• and settings to configure the selected symbol layer in the tree.
The symbol layer tree
A symbol can consist of several Symbol layers. The symbol tree shows the overlay of these symbol layers that are
combined afterwards to shape a new global symbol. Besides, a dynamic symbol representation is updated as soon
as symbol properties change.
A set of tools is available to manage the symbol tree items and according to the level selected, you’ll get enabled
different tools at the bottom of the dialog to:
•
add new symbol layer: you can stack as many symbols as you want
•
remove the selected symbol layer
• lock colors of symbol layer: a
(or upper) symbol level
•
locked color stays unchanged when user changes the color at the global
duplicate a (group of) symbol layer(s)
• move up or down the symbol layer
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Configuring a symbol
In QGIS, configuring a symbol is done in two steps: the symbol and then the symbol layer.
The symbol
At the top level of the tree, it depends on the layer geometry and can be of Marker, Line or Fill type. Each
symbol can embed one or more symbols (including, of any other type) or symbol layers.
You can setup some parameters that apply to the global symbol:
• Unit: it can be Millimeter, Pixels or Map unit
• Transparency
• Color: when this parameter is changed by the user, its value is echoed to all unlocked sub-symbols color
• Size and Rotation for marker symbols
• Width for line symbols
Note: The Data-defined override button beside the last layer-related parameters is inactive when setting the
symbol from the Style manager dialog. When the symbol is connected to a map layer, this button offers access to
the size assistant dialog which helps to create proportional or multivariate analysis rendering.
The symbols used at this level are items you can pick from the symbols library. A list of available symbols of the
same type from your symbol library is shown and can be filtered by selecting a group in the drop-down list just
above. Click the Save button to add the designed symbol to your symbol library.
With the Advanced
option, you can:
• set the symbol levels: defining the way symbol layers are connected to each other in the map canvas (see
Symbols levels for more information)
• and for line and fill symbols, clip features to canvas extent.
Tip: Note that once you have set the size in the lower levels of the Symbol layers dialog, the size of the whole
symbol can be changed with the Size (for marker symbols) or the Width (for line symbols) menu in the first level
again. The size of the lower levels changes accordingly, while the size ratio is maintained.
The symbol layer
At a lower level of the tree, you can customize the symbol layers. The available symbol layer types depend on the
upper symbol type. You can apply on the symbol layer
paint effects to enhance its rendering.
Because describing all the options of all the symbol layer types would not be possible, only particular and significative ones are mentioned below.
Common parameters
Some common options and widgets are available to build a symbol layer, regardless it’s of marker, line or fill
sub-type:
• the color selector widget to ease color manipulation
• Units: it can be Millimeter, Pixels or Map unit
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data-defined override
• the
widget near almost all options, extending capabilities of customizing each symbol
(see Data defined override setup for more information)
Note: While the description below assumes that the symbol layer type is bound to the feature geometry, keep
in mind that you can embed symbol layers in each others. In that case, the lower level symbol layer parameter
(placement, offset. . . ) might be bound to the upper-level symbol, and not to the feature geometry itself.
Marker Symbols
Appropriate for point geometry features, marker symbols have several Symbol layer types:
• Simple marker (default);
• Ellipse marker: a simple marker symbol layer, with customizable width and height;
• Filled marker: similar to the simple marker symbol layer, except that it uses a fill sub symbol to render the
marker. This allows use of all the existing QGIS fill (and stroke) styles for rendering markers, e.g. gradient
or shapeburst fills;
• Font marker: use installed fonts as marker symbols;
• Geometry generator (see The Geometry Generator);
• Vector Field marker (see The Vector Field Marker);
• SVG marker: provides you with images from your SVG paths (set in Settings → Options → System menu)
to render as marker symbol. Each SVG file colors and stroke can be adapted.
Note: Requirements for a customizable SVG marker symbol
To have the possibility to change the colors of a SVG marker, you have to add the placeholders
param(fill) for fill color, param(outline) for stroke color and param(outline-width) for
stroke width. These placeholders can optionally be followed by a default value, e.g.:
<svg width="100%" height="100%">
<rect fill="param(fill) #ff0000" stroke="param(outline) #00ff00" stroke-width=
˓→"param(stroke-width) 10" width="100" height="100">
</rect>
</svg>
For each marker symbol layer type, you can set some of the following properties:
• Color for the fill and/or stroke, using all the capabilities of the Color Selector widget;
• Size
• Stroke style
• Stroke width
• Join style
• Rotation
• Offset X,Y: You can shift the symbol in the x- or y- direction;
• Anchor point.
In most of the marker symbols dialog, you also have a frame with previews of predefined symbols you can choose
from.
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Line Symbols
Appropriate for line geometry features, line marker symbols have following symbol layer types:
• Simple line (default): available settings are:
– Color
– Stroke width
– Stroke style
– Join style
– Cap style
– Offset
–
Use custom dash pattern: overrides the Stroke style setting with a custom dash.
• Arrow: draws lines as curved (or not) arrows with a single or a double head with configurable width, length
and thickness. To create a curved arrow the line feature must have at least three vertices. It also uses a fill
symbol such as gradients or shapeburst to render the arrow body. Combined with the geometry generator,
this type of layer symbol helps you representing flow maps;
• Geometry generator (see The Geometry Generator);
• Marker line: displays a marker symbol along the line. It can be at a regular distance or based on its
geometry: first, last or each vertex, on central point or on every curve point. You can set an offset along the
line for the marker symbol, or offset the line itself. The Rotate marker option allows you to set whether the
marker symbol should follow the line orientation or not.
Fill Symbols
Appropriate for polygon geometry features, fill symbols have also several symbol layer types:
• Simple fill (default): the following settings are available:
– Fill color
– Stroke color
– Fill style
– Stroke style
– Stroke width
– Join style
– Offset X,Y
• Centroid fill: places a marker symbol at the centroid of the visible feature. The position of the marker may
however not be the real centroid of the feature because calculation takes into account the polygon(s) clipped
to area visible in map canvas for rendering and ignores holes. Use The Geometry Generator if you want the
exact centroid.
The marker can be placed on every part of a multi-part feature or only on its biggest part, and forced to be
inside the polygon;
• Geometry generator (see geometry_generator_symbol);
• Gradient fill: uses a radial, linear or conical gradient, based on either simple two color gradients or a
predefined gradient color ramp to fill polygon layers. Gradient can be rotated and applied on a single
feature basis or across the whole map extent. Also start and end points can be set via coordinates or using
the centroid (of feature or map);
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• Line pattern fill: fills the polygon with a hatching pattern of line symbol layer. You can set the spacing
between lines and an offset from the feature boundary;
• Point pattern fill: fills the polygon with a hatching pattern of marker symbol layer. You can set the spacing
between lines and an offset from the feature boundary;
• Raster image fill: you can fill polygons with a tiled raster image. Options include (data defined) file name,
opacity, image size (in pixels, mm or map units), coordinate mode (feature or view) and rotation;
• SVG fill: fills the polygon using SVG markers;
• Shapeburst fill: this option buffered a gradient fill, where a gradient is drawn from the boundary of a
polygon towards the polygon’s centre. Configurable parameters include distance from the boundary to
shade, use of color ramps or simple two color gradients, optional blurring of the fill and offsets;
• Outline: Arrow: uses a line arrow symbol layer to represent the polygon boundary;
• Outline: Marker line: uses a marker line symbol layer to represent the polygon boundary;
• Outline: simple line: uses a simple line symbol layer to represent the polygon boundary. The Draw line
only inside polygon option helps polygon borders inside the polygon and can be useful to clearly represent
adjacent polygon boundaries.
Note: When geometry type is polygon, you can choose to disable the automatic clipping of lines/polygons to the
canvas extent. In some cases this clipping results in unfavourable symbology (e.g. centroid fills where the centroid
must always be the actual feature’s centroid).
The Geometry Generator
Available with all types of symbols, the geometry generator symbol layer allows to use expression syntax to
generate a geometry on the fly during the rendering process. The resulting geometry does not have to match with
the original geometry type and you can add several differently modified symbol layers on top of each other.
Some examples:
-- render the centroid of a feature
centroid( $geometry )
-- visually overlap features within a 100 map units distance from a point
-- feature, i.e generate a 100m buffer around the point
buffer( $geometry, 100 )
-- Given polygon layer1( id1, layer2_id, ...) and layer2( id2, fieldn...)
-- render layer1 with a line joining centroids of both where layer2_id = id2
make_line( centroid( $geometry ),
centroid( geometry( get_feature( 'layer2', 'id2', attribute(
$currentfeature, 'layer2_id') ) )
)
The Vector Field Marker
The vector field marker is used to display vector field data such as earth deformation, tidal flows, and the like.
It displays the vectors as lines (preferably arrows) that are scaled and oriented according to selected attributes of
data points. It can only be used to render point data; line and polygon layers are not drawn by this symbology.
The vector field is defined by attributes in the data, which can represent the field either by:
• cartesian components (x and y components of the field)
• or polar coordinates: in this case, attributes define Length and Angle. The angle may be measured either
clockwise from north, or Counterclockwise from east, and may be either in degrees or radians.
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• or as height only data, which displays a vertical arrow scaled using an attribute of the data. This is appropriate for displaying the vertical component of deformation, for example.
The magnitude of field can be scaled up or down to an appropriate size for viewing the field.
12.2 The Vector Properties Dialog
The Layer Properties dialog for a vector layer provides general settings to manage appearance of layer features
in the map (symbology, labeling, diagrams), interaction with the mouse (actions, map tips, form design). It also
provides information about the layer.
To access the Layer Properties dialog, double-click on a layer in the legend or right-click on the layer and select
Properties from the pop-up menu.
Note: Depending on the external plugins you have installed, new tabs may be added to the layer properties dialog.
Those are not presented below.
Tip: Live update rendering
The Layer Styling Panel provides you with some of the common features of the Layer properties dialog and is a
good modeless widget that you can use to speed up the configuration of the layer styles and automatically view
your changes in the map canvas.
Note: Because properties (symbology, label, actions, default values, forms. . . ) of embedded layers (see Nesting
Projects) are pulled from the original project file and to avoid changes that may break this behavior, the layer
properties dialog is made unavailable for these layers.
12.2.1 Information Properties
The Information tab is read-only and represents an interesting place to quickly grab summarized information
and metadata on the current layer. Provided information are :
• based on the provider of the layer (format of storage, path, geometry type encoding, extent. . . );
• picked from the filled metadata (access, links, contacts, history. . . ) ;
• or related to its geometry (spatial extent, CRS. . . ) or its attributes (number of fields, characteristics of
each. . . )
12.2.2 Source Properties
Use this tab to define general settings for the vector layer. Available options are:
Settings
• Set the Layer name to display in the Layers Panel
• Display the Layer source of the vector layer
• Define the Data source encoding to define provider-specific options and to be able to read the file
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Coordinate Reference System
• Displays the layer’s Coordinate Reference System (CRS) as a PROJ.4 string. You can change the layer’s
Select CRS
button (see Coordinate
CRS, selecting a recently used one in the drop-down list or clicking on
Reference System Selector). Use this process only if the CRS applied to the layer is a wrong one or if none
was applied. If you wish to reproject your data into another CRS, rather use layer reprojection algorithms
from Processing or Save it into another layer.
• Create a Spatial Index (only for OGR-supported formats)
• Update Extents information for a layer
Query Builder
Under the Provider Feature Filter frame, the Query Builder allows you to define a subset of the features in the
layer using a SQL-like WHERE clause and to display the result in the main window. As long as the query is
active, only the features corresponding to its result are available in the project. The query result can be saved as a
new vector layer.
The Query Builder is accessible through the eponym term at the bottom of the Source tab in the Layer Properties.
Under Feature subset, click on the [Query Builder] button to open the Query builder. For example, if you have
a regions layer with a TYPE_2 field, you could select only regions that are borough in the Provider specific
filter expression box of the Query Builder. Figure_vector_querybuilder shows an example of the Query Builder
populated with the regions.shp layer from the QGIS sample data. The Fields, Values and Operators sections
help you to construct the SQL-like query.
Fig. 12.7: Query Builder
The Fields list contains all attribute columns of the attribute table to be searched. To add an attribute column to
the SQL WHERE clause field, double click its name in the Fields list. Generally, you can use the various fields,
values and operators to construct the query, or you can just type it into the SQL box.
The Values list lists the values of an attribute table. To list all possible values of an attribute, select the attribute in
the Fields list and click the [all] button. To list the first 25 unique values of an attribute column, select the attribute
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column in the Fields list and click the [Sample] button. To add a value to the SQL WHERE clause field, double
click its name in the Values list.
The Operators section contains all usable operators. To add an operator to the SQL WHERE clause field, click
the appropriate button. Relational operators ( = , > , . . . ), string comparison operator (LIKE), and logical operators
(AND, OR, . . . ) are available.
The [Test] button shows a message box with the number of features satisfying the current query, which is useful
in the process of query construction. The [Clear] button clears the text in the SQL WHERE clause text field.
The [OK] button closes the window and selects the features satisfying the query. The [Cancel] button closes the
window without changing the current selection.
QGIS treats the resulting subset acts as if it were the entire layer. For example if you applied the filter above for
‘Borough’, you can not display, query, save or edit Anchorage, because that is a ‘Municipality’ and therefore not
part of the subset.
The only exception is that unless your layer is part of a database, using a subset will prevent you from editing the
layer.
Fig. 12.8: Source tab in vector layers properties dialog
12.2.3 Symbology Properties
The Symbology tab provides you with a comprehensive tool for rendering and symbolizing your vector data.
You can use tools that are common to all vector data, as well as special symbolizing tools that were designed
for the different kinds of vector data. However all types share the following dialog structure: in the upper part,
you have a widget that helps you prepare the classification and the symbol to use for features and at the bottom
the:ref:layer_rendering widget.
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Tip: Switch quickly between different layer representations
Using the Styles → Add combobox at the bottom of the Layer Properties dialog, you can save as many combinations of layer properties settings (symbology, labeling, diagram, fields form, actions. . . ) as you want. Then,
simply switch between styles from the context menu of the layer in Layers Panel to automatically get different
representations of your data.
Tip: Export vector symbology
You have the option to export vector symbology from QGIS into Google *.kml, *.dxf and MapInfo *.tab files.
Just open the right mouse menu of the layer and click on Save As. . . to specify the name of the output file and its
format. In the dialog, use the Symbology export menu to save the symbology either as Feature symbology → or as
Symbol layer symbology →. If you have used symbol layers, it is recommended to use the second setting.
Features rendering
The renderer is responsible for drawing a feature together with the correct symbol. Regardless layer geometry
type, there are four common types of renderers: single symbol, categorized, graduated and rule-based. For point
layers, there are a point displacement and a heatmap renderers available while polygon layers can also be rendered
with the inverted polygons and 2.5 D renderers.
There is no continuous color renderer, because it is in fact only a special case of the graduated renderer. The
categorized and graduated renderers can be created by specifying a symbol and a color ramp - they will set the
colors for symbols appropriately. For each data type (points, lines and polygons), vector symbol layer types are
available. Depending on the chosen renderer, the dialog provides different additional sections.
Note: If you change the renderer type when setting the style of a vector layer the settings you made for the
symbol will be maintained. Be aware that this procedure only works for one change. If you repeat changing the
renderer type the settings for the symbol will get lost.
Single Symbol Renderer
The
Single Symbol Renderer is used to render all features of the layer using a single user-defined symbol. See
The Symbol Selector for further information about symbol representation.
Tip: Edit symbol directly from layer panel
If in your Layers Panel you have layers with categories defined through categorized, graduated or rule-based style
mode, you can quickly change the fill color of the symbol of the categories by right-clicking on a category and
color wheel
choose the color you prefer from a
menu. Right-clicking on a category will also give you access to
the options Hide all items, Show all items and Edit symbol.
No Symbols Renderer
The
No Symbols renderer is a special use case of the Single Symbol renderer as it applies the same rendering
to all features. Using this renderer, no symbol will be drawn for features, but labeling, diagrams and other nonsymbol parts will still be shown.
Selections can still be made on the layer in the canvas and selected features will be rendered with a default symbol.
Features being edited will also be shown.
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Fig. 12.9: Single symbol line properties
This is intended as a handy shortcut for layers which you only want to show labels or diagrams for, and avoids the
need to render symbols with totally transparent fill/border to achieve this.
Categorized Renderer
The
Categorized Renderer is used to render the features of a layer, using a user-defined symbol whose aspect
reflects the discrete values of a field or an expression. The Categorized menu allows you to
• select an existing field (using the Column listbox) or
• type or build an expression using the
of any type; it can for example:
Set column expression
. The expression used to classify features can be
– be a comparison, e.g. myfield >= 100, $id = @atlas_featureid, myfield % 2 = 0,
within( $geometry, @atlas_geometry ). In this case, QGIS returns values 1 (True) and
0 (False).
– combine different fields, e.g. concat( field1, ' ', field2 ) particularly useful when you
want to process classification on two or more fields simultaneously.
– be a calculation on fields, e.g. myfield % 2, year( myfield ) field_1 + field_2.
– be used to transform linear values in discrete classes, e.g.:
CASE WHEN x > 1000 THEN 'Big' ELSE 'Small' END
– combine several discrete values in one single category, e.g.:
CASE
WHEN building IN ('residence', 'mobile home') THEN 'residential'
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WHEN building IN ('commercial', 'industrial') THEN 'Commercial and
˓→Industrial'
END
Note: While you can use any kind of expression to categorize features, for some complex expressions it
might be simpler to use rule-based rendering.
• the symbol (using the The Symbol Selector dialog) which will be used as base symbol for each class;
• the range of colors (using the Color ramp listbox) from which color applied to the symbol is selected.
Then click on [Classify] button to create classes from the distinct value of the attribute column. Each class can be
disabled unchecking the checkbox at the left of the class name.
To change symbol, value and/or label of the class, just double click on the item you want to change.
Right-click shows a contextual menu to Copy/Paste, Change color, Change transparency, Change output unit,
Change symbol width.
The example in figure_categorized_symbology shows the category rendering dialog used for the rivers layer of the
QGIS sample dataset.
Fig. 12.10: Categorized Symbolizing options
Tip: Select and change multiple symbols
The Symbology allows you to select multiple symbols and right click to change color, transparency, size, or width
of selected entries.
Tip: Match categories to symbol name
In the [Advanced] menu, under the classes, you can choose one of the two first actions to match symbol name to
a category name in your classification. Matched to saved symbols match category name with a symbol name from
your Style Manager. Match to symbols from file match category name to a symbol name from an external file.
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Graduated Renderer
Graduated Renderer is used to render all the features from a layer, using an user-defined symbol whose
The
color or size reflects the assignment of a selected feature’s attribute to a class.
Like the Categorized Renderer, the Graduated Renderer allows you to define rotation and size scale from specified
columns.
Also, analogous to the Categorized Renderer, it allows you to select:
• The attribute (using the Column listbox or the
Set column expression
function)
• The symbol (using the Symbol selector dialog)
• The legend format and the precision
• The method to use to change the symbol: color or size
• The colors (using the color Ramp list) if the color method is selected
• The size (using the size domain and its unit)
Then you can use the Histogram tab which shows an interactive histogram of the values from the assigned field or
expression. Class breaks can be moved or added using the histogram widget.
Note: You can use Statistical Summary panel to get more information on your vector layer. See Statistical
Summary Panel.
Back to the Classes tab, you can specify the number of classes and also the mode for classifying features within
the classes (using the Mode list). The available modes are:
• Equal Interval: each class has the same size (e.g. values from 0 to 16 and 4 classes, each class has a size of
4);
• Quantile: each class will have the same number of element inside (the idea of a boxplot);
• Natural Breaks (Jenks): the variance within each class is minimal while the variance between classes is
maximal;
• Standard Deviation: classes are built depending on the standard deviation of the values;
• Pretty Breaks: Computes a sequence of about n+1 equally spaced nice values which cover the range of the
values in x. The values are chosen so that they are 1, 2 or 5 times a power of 10. (based on pretty from the
R statistical environment http://astrostatistics.psu.edu/datasets/R/html/base/html/pretty.html)
The listbox in the center part of the Style tab lists the classes together with their ranges, labels and symbols that
will be rendered.
Click on Classify button to create classes using the chosen mode. Each classes can be disabled unchecking the
checkbox at the left of the class name.
To change symbol, value and/or label of the class, just double click on the item you want to change.
Right-click shows a contextual menu to Copy/Paste, Change color, Change transparency, Change output unit,
Change symbol width.
The example in figure_graduated_symbology shows the graduated rendering dialog for the rivers layer of the
QGIS sample dataset.
Tip: Thematic maps using an expression
Categorized and graduated thematic maps can be created using the result of an expression. In the properties dialog
for vector layers, the attribute chooser is extended with a
12.2. The Vector Properties Dialog
Set column expression
function. So you don’t need to write
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Fig. 12.11: Graduated Symbolizing options
the classification attribute to a new column in your attribute table if you want the classification attribute to be a
composite of multiple fields, or a formula of some sort.
Proportional Symbol and Multivariate Analysis
Proportional Symbol and Multivariate Analysis are not rendering types available from the Style rendering dropdown list. However with the Size Assistant options applied over any of the previous rendering options, QGIS
allows you to display your point and line data with such representation. Creating proportional symbol
Proportional rendering is done by first applying to the layer the Single Symbol Renderer. Once you set the symbol,
at the upper level of the symbol tree, the
Data-defined override button available beside Size or Width options
(for point or line layers respectively) provides tool to create proportional symbology for the layer. An assistant is
moreover accessible through the
menu to help you define size expression.
The assistant lets you define:
• The attribute to represent, using the Field listbox or the
Set column expression
function (see Expressions)
• the scale method of representation which can be ‘Flannery’, ‘Surface’ or ‘Radius’
• The minimum and maximum size of the symbol
• The range of values to represent: The down pointing arrow helps you fill automatically these fields with the
minimum (or zero) and maximum values returned by the chosen attribute or the expression applied to your
data.
• An unique size to represent NULL values.
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Fig. 12.12: Varying size assistant
To the right side of the dialog, you can preview the features representation within a live-update widget. This
representation is added to the layer tree in the layer legend and is also used to shape the layer representation in the
print layout legend item.
The values presented in the varying size assistant above will set the size ‘Data-defined override’ with:
coalesce(scale_exp(Importance, 1, 20, 2, 10, 0.57), 1)
Creating multivariate analysis
A multivariate analysis rendering helps you evaluate the relationship between two or more variables e.g., one can
be represented by a color ramp while the other is represented by a size.
The simplest way to create multivariate analysis in QGIS is to first apply a categorized or graduated rendering on
a layer, using the same type of symbol for all the classes. Then, clicking on the symbol [Change] button above
the classification frame, you get the The Symbol Selector dialog from which, as seen above, you can activate and
set the size assistant option either on size (for point layer) or width (for line layer).
Like the proportional symbol, the size-related symbol is added to the layer tree, at the top of the categorized or
graduated classes symbols. And both representation are also available in the print layout legend item.
Fig. 12.13: Multivariate example
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Rule-based rendering
The
Rule-based Renderer is used to render all the features from a layer, using rule-based symbols whose
aspect reflects the assignment of a selected feature’s attribute to a class. The rules are based on SQL statements.
The dialog allows rule grouping by filter or scale, and you can decide if you want to enable symbol levels or use
only the first-matched rule.
To create a rule:
• Activate an existing row by double-clicking it (by default, QGIS adds a symbol without a rule when the
rendering mode is enabled) or click the
Edit rule
or
Add rule
button;
• In the Edit Rule dialog that opens, you can define a label to help you identify each rule. This is the label that
will be displayed in the Layers Panel and also in the print composer legend;
• Press the
button to open the expression string builder dialog;
• Use the provided functions and the layer attributes to build an expression to filter the features you’d like to
retrieve;
• A longer label can then be used to complete the rule description;
• You can use the
Scale Range option to set when the rule should be applied;
• Finally, configure the symbol to use for these features and press [OK];
• A new row summarizing the rule is added to the Layer Properties dialog.
You can create as many rules as necessary following the steps above or copy pasting an existing one with the right
mouse button. You can also use the ELSE rule that will be run if none of the other rules on that level matches.
Selecting a rule, you can organize its features in subclasses using the Refine selected rules drop-down menu. Rule
refinement can be based on:
• scales;
• categories: applying a categorized renderer;
• or ranges: applying a graduated renderer.
Refined classes appear like sub-items of the rule, in a tree hierarchy and like above, you can set symbology of
each class.
The created rules also appear in a tree hierarchy in the map legend. Double-click the rules in the map legend and
the Symbology tab of the layer properties appears showing the rule that is the background for the symbol in the
tree.
The example in figure_rule_based_symbology shows the rule-based rendering dialog for the rivers layer of the
QGIS sample dataset.
Point displacement
The
Point Displacement renderer works to visualize all features of a point layer, even if they have the same
location. To do this, the symbols of the points are placed on a displacement circle around one center symbol or on
several concentric circles.
Note: You can still render features with other renderer like Single symbol, Graduated, Categorized or Rule-Based
renderer using the Renderer drop-down list then the Renderer Settings. . . button.
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Fig. 12.14: Rule-based Symbolizing options
Fig. 12.15: Point displacement dialog
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Point Cluster
Unlike the
Point Displacement renderer which blows up overlaid point features placement, the
Point
Cluster renderer groups nearby points into a single rendered marker symbol. Based on a specified Distance,
points that fall within from each others are merged into a single symbol. Points aggregation is made based on the
closest group being formed, rather than just assigning them the first group within the search distance.
From the main dialog, you can:
• set the symbol to represent the point cluster in the Cluster symbol; the default rendering displays the number
of aggregated features thanks to the @cluster_size variable on Font marker symbol layer.
• use the Renderer drop-down list to apply any of the other feature rendering types to the layer (single,
categorized, rule-based. . . ). Then, push the [Renderer settings. . . ] button to configure features’ symbology
as usual. Note that this renderer is only visible on features that are not clustered. Also, when the symbol
color is the same for all the point features inside a cluster, that color sets the @cluster_color variable
of the cluster.
Fig. 12.16: Point Cluster dialog
Note: Point displacement and cluster renderers do not alter feature geometry, meaning that points are not moved
from their position. They are still located at their initial place. Changes are only visual, for rendering purpose.
Inverted Polygon
The
Inverted Polygon renderer allows user to define a symbol to fill in outside of the layer’s polygons. As
above you can select subrenderers, namely Single symbol, Graduated, Categorized, Rule-Based or 2.5D renderer.
Heatmap
With the
Heatmap renderer you can create live dynamic heatmaps for (multi)point layers. You can specify
the heatmap radius in pixels, mm or map units, choose and edit a color ramp for the heatmap style and use a slider
for selecting a trade-off between render speed and quality. You can also define a maximum value limit and give a
weight to points using a field or an expression. When adding or removing a feature the heatmap renderer updates
the heatmap style automatically.
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Fig. 12.17: Inverted Polygon dialog
Fig. 12.18: Heatmap dialog
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2.5D
Using the
2.5D renderer it’s possible to create a 2.5D effect on your layer’s features. You start by choosing a
Height value (in map units). For that you can use a fixed value, one of your layer’s fields, or an expression. You
also need to choose an Angle (in degrees) to recreate the viewer position (0° means west, growing in counter clock
wise). Use advanced configuration options to set the Roof Color and Wall Color. If you would like to simulate
Shade walls based on aspect option. You can also
solar radiation on the features walls, make sure to check the
simulate a shadow by setting a Color and Size (in map units).
Fig. 12.19: 2.5D dialog
Tip: Using 2.5D effect with other renderers
Once you have finished setting the basic style on the 2.5D renderer, you can convert this to another renderer (single,
categorized, graduated). The 2.5D effects will be kept and all other renderer specific options will be available for
you to fine tune them (this way you can have for example categorized symbols with a nice 2.5D representation
or add some extra styling to your 2.5D symbols). To make sure that the shadow and the “building” itself do not
interfere with other nearby features, you may need to enable Symbols Levels ( Advanced → Symbol levels. . . ).
The 2.5D height and angle values are saved in the layer’s variables, so you can edit it afterwards in the variables
tab of the layer’s properties dialog.
Layer rendering
From the Style tab, you can also set some options that invariabily act on all features of the layer:
• Layer transparency
: You can make the underlying layer in the map canvas visible with
this tool. Use the slider to adapt the visibility of your vector layer to your needs. You can also make a
precise definition of the percentage of visibility in the the menu beside the slider.
• Layer blending mode and Feature blending mode: You can achieve special rendering effects with these tools
that you may previously only know from graphics programs. The pixels of your overlaying and underlaying
layers are mixed through the settings described in Blending Modes.
• Apply paint effects on all the layer features with the Draw Effects button.
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• Control feature rendering order allows you, using features attributes, to define the z-order in which they
shall be rendered. Activate the checkbox and click on the
dialog in which you:
button beside. You then get the Define Order
– choose a field or build an expression to apply to the layer features
– set in which order the fetched features should be sorted, i.e. if you choose Ascending order, the
features with lower value are rendered under those with upper value.
– define when features returning NULL value should be rendered: first or last.
You can add several rules of ordering. The first rule is applied to all the features in the layer, z-ordering
them according to the value returned. Then, for each group of features with the same value (including those
with NULL value) and thus same z-level, the next rule is applied to sort its items among them. And so on. . .
Fig. 12.20: Layer rendering options
Other Settings
Symbols levels
For renderers that allow stacked symbol layers (only heatmap doesn’t) there is an option to control the rendering
order of each symbol’s levels.
For most of the renderers, you can access the Symbols levels option by clicking the [Advanced] button below
the saved symbols list and choosing Symbol levels. For the Rule-based rendering the option is directly available
through [Symbols levels] button, while for Point displacement renderer the same button is inside the Rendering
settings dialog.
To activate symbols levels, select the
Enable symbol levels. Each row will show up a small sample of the
combined symbol, its label and the individual symbols layer divided into columns with a number next to it. The
numbers represent the rendering order level in which the symbol layer will be drawn. Lower values levels are
drawn first, staying at the bottom, while higher values are drawn last, on top of the others.
Note: If symbols levels are deactivated, the complete symbols will be drawn according to their respective features
order. Overlapping symbols will simply obfuscate to other below. Besides, similar symbols won’t “merge” with
each other.
Draw effects
In order to improve layer rendering and avoid (or at least reduce) the resort to other software for final rendering of
maps, QGIS provides another powerful functionality: the
customizing the visualization of vector layers.
Draw Effects options, which adds paint effects for
The option is available in the Layer Properties –> Style dialog, under the Layer rendering group (applying to the
whole layer) or in symbol layer properties (applying to corresponding features). You can combine both usage.
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Fig. 12.21: Symbol levels dialog
Fig. 12.22: Symbol levels activated (A) and deactivated (B) difference
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Customize effects
Paint effects can be activated by checking the
Draw effects option and clicking the
button,
that will open the Effect Properties Dialog (see figure_effects_source). The following effect types, with custom
options are available:
• Source: Draws the feature’s original style according to the configuration of the layer’s properties. The
transparency of its style can be adjusted.
Fig. 12.23: Draw Effects: Source dialog
• Blur: Adds a blur effect on the vector layer. The options that someone can change are the Blur type (Stack
or Gaussian blur), the strength and transparency of the blur effect.
• Colorize: This effect can be used to make a version of the style using one single hue. The base will always
be a grayscale version of the symbol and you can use the
Grayscale to select how to create it (options
Colorise is selected, it will be possible to mix another
are: ‘lightness’, ‘luminosity’ and ‘average’). If
color and choose how strong it should be. You can also control the Brightness, contrast and saturation
levels of the resulting symbol.
• Drop Shadow: Using this effect adds a shadow on the feature, which looks like adding an extra dimension.
This effect can be customized by changing the offset degrees and radius, determining where the shadow
shifts towards to and the proximity to the source object. Drop Shadow also has the option to change the blur
radius, the transparency and the color of the effect.
• Inner Shadow: This effect is similar to the Drop Shadow effect, but it adds the shadow effect on the inside
of the edges of the feature. The available options for customization are the same as the Drop Shadow effect.
• Inner Glow: Adds a glow effect inside the feature. This effect can be customized by adjusting the spread
(width) of the glow, or the Blur radius. The latter specifies the proximity from the edge of the feature where
you want any blurring to happen. Additionally, there are options to customize the color of the glow, with a
single color or a color ramp.
• Outer Glow: This effect is similar to the Inner Glow effect, but it adds the glow effect on the outside of the
edges of the feature. The available options for customization are the same as the Inner Glow effect.
• Transform: Adds the possibility of transforming the shape of the symbol. The first options available
for customization are the Reflect horizontal and Reflect vertical, which actually create a reflection on the
horizontal and/or vertical axes. The 4 other options are:
– Shear: slants the feature along the x and/or y axis
– Scale: enlarges or minimizes the feature along the x and/or y axis by the given percentage
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Fig. 12.24: Draw Effects: Blur dialog
Fig. 12.25: Draw Effects: Colorize dialog
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Fig. 12.26: Draw Effects: Drop Shadow dialog
Fig. 12.27: Draw Effects: Inner Shadow dialog
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Fig. 12.28: Draw Effects: Inner Glow dialog
Fig. 12.29: Draw Effects: Outer Glow dialog
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– Rotation: turns the feature around its center point
– and Translate changes the position of the item based on a distance given on the x and/or the y axis.
Fig. 12.30: Draw Effects: Transform dialog
There are some common options available for all draw effect types. Transparency and Blend mode options work
similar to the ones described in Layer rendering and can be used in all draw effects except for the transform one.
One or more draw effects can used at the same time. You activate/deactivate an effect using its checkbox in the
effects list. You can change the selected effect type by using the
Move up
and
effects using
Remove effect
buttons.
Move down
Effect type option. You can reorder the
buttons, and also add/remove effects using the
Add effect
and
There is also a
Draw mode option available for every draw effect, and you can choose whether to render
and/or to modify the symbol. Effects render from top to bottom.’Render only’ mode means that the effect will be
visible while the ‘Modify only’ mode means that the effect will not be visible but the changes that it applies will
be passed to the next effect (the one immediately below). The ‘Render and Modify’ mode will make the effect
visible and pass any changes to the next effect. If the effect is in the top of the effects list or if the immediately
above effect is not in modify mode, then it will use the original source symbol from the layers properties (similar
to source).
12.2.4 Labels Properties
The
Labels properties provides you with all the needed and appropriate capabilities to configure smart labeling
Layer Labeling Options
on vector layers. This dialog can also be accessed from the Layer Styling panel, or using the
icon of the Labels toolbar.
Setting a label
The first step is to choose the labeling method from the drop-down list. There are four options available:
• No labels
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• Show labels for this layer
• Rule-based labeling
• and Blocking: allows to set a layer as just an obstacle for other layer’s labels without rendering any labels
of its own.
The next steps assume you select the Show labels for this layer option, enabling following tabs that help you
configure the labeling:
• Text
• Formatting
• Buffer
• Background
• Shadow
• Placement
• Rendering
It also enables the Label with drop-down list, from which you can select an attribute column to use. Click
you want to define labels based on expressions - See Define labels based on expressions.
if
The following steps describe simple labeling without using the Data defined override functions, which are situated
next to the drop-down menus - see Using data-defined override for labeling for a use case.
Fig. 12.31: Layer labeling settings - Text tab
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Text tab
In the Text tab, you can define the Font, Style, and Size of your labels’ text (see Figure_labels). There are options
available to set the labels’ Color and Transparency. Use the Type case option to change the capitalization style of
the text. You have the possibility to render the text as ‘All uppercase’, ‘All lowercase’ or ‘Capitalize first letter’.
In Spacing, you can change the space between words and between individual letters. Finally, use the Blend mode
option to determine how your labels will mix with the map features below them (see more about it in Blending
Modes).
The Apply label text substitutes option gives you ability to specify a list of texts to substitute to texts in feature
labels (e.g., abbreviating street types). Replacement texts are thus used to display labels in the map canvas. Users
can also export and import lists of substitutes to make reuse and sharing easier.
Formatting tab
In the Formatting tab, you can define a character for a line break in the labels with the Wrap on character option.
You can also format the Line Height and the alignment. For the latter, typical values are available (left, right, and
center), plus Follow label placement for point layers. When set to this mode, text alignment for labels will be
dependent on the final placement of the label relative to the point. E.g., if the label is placed to the left of the point,
then the label will be right aligned, while if it is placed to the right, it will be left aligned.
For line vector layers you can include Line directions symbols to help determine the lines directions. They work
particularly well when used with the curved or Parallel placement options from the Placement tab. There are
options to set the symbols position, and to reverse direction.
Use the
Formatted numbers option to format numeric labels. You can set the number of Decimal places. By
default, 3 decimal places will be used. Use the
numbers.
Show plus sign if you want to show the plus sign in positive
Buffer tab
To create a buffer around the labels, activate the
Draw text buffer checkbox in the Buffer tab. You can set the
buffer’s Size, color, and Transparency. The buffer expands from the label’s outline , so, if the
color buffer’s fill
checkbox is activated, the buffer interior is filled. This may be relevant when using partially transparent labels or
with non-normal blending modes, which will allow seeing behind the label’s text. Deactivating
fill checkbox (while using totally transparent labels) will allow you to create outlined text labels.
color buffer’s
Background tab
In the Background tab, you can define with Size X and Size Y the shape of your background. Use Size type to
insert an additional ‘Buffer’ into your background. The buffer size is set by default here. The background then
consists of the buffer plus the background in Size X and Size Y. You can set a Rotation where you can choose
between ‘Sync with label’, ‘Offset of label’ and ‘Fixed’. Using ‘Offset of label’ and ‘Fixed’, you can rotate the
background. Define an Offset X,Y with X and Y values, and the background will be shifted. When applying
Radius X,Y, the background gets rounded corners. Again, it is possible to mix the background with the underlying
layers in the map canvas using the Blend mode (see Blending Modes).
Shadow tab
Use the Shadow tab for a user-defined Drop shadow. The drawing of the background is very variable. Choose
between ‘Lowest label component’, ‘Text’, ‘Buffer’ and ‘Background’. The Offset angle depends on the orientaUse global shadow checkbox, then the zero point of the angle is always
tion of the label. If you choose the
oriented to the north and doesn’t depend on the orientation of the label. You can influence the appearance of the
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shadow with the Blur radius. The higher the number, the softer the shadows. The appearance of the drop shadow
can also be altered by choosing a blend mode.
Placement tab
Choose the Placement tab for configuring label placement and labeling priority. Note that the placement options
differ according to the type of vector layer, namely point, line or polygon.
Placement for point layers
Cartographic placement mode, point labels are generated with a better visual relationship with the
With the
point feature, following ideal cartographic placement rules. Labels can be placed at a set Distance either from the
point feature itself or from the bounds of the symbol used to represent the feature. The latter option is especially
useful when the symbol size isn’t fixed, e.g. if it’s set by a data defined size or when using different symbols in a
categorized renderer.
By default, placements are prioritised in the following order:
1. top right
2. top left
3. bottom right
4. bottom left
5. middle right
6. middle left
7. top, slightly right
8. bottom, slightly left.
Placement priority can, however, be customized or set for an individual feature using a data defined list of prioritised positions. This also allows only certain placements to be used, so e.g. for coastal features you can prevent
labels being placed over the land.
Around point setting places the label in an equal radius (set in Distance) circle around the feature. The
The
placement of the label can even be constrained using the Quadrant option.
With the
Offset from point, labels are placed at a fixed offset from the point feature. You can select the Quadrant
in which to place your label. You are also able to set the Offset X,Y distances between the points and their labels
and can alter the angle of the label placement with the Rotation setting. Thus, placement in a selected quadrant
with a defined rotation is possible.
Placement for line layers
Label options for line layers include
Parallel,
Curved or
Horizontal. For the
Parallel and
Curved
options, you can set the position to
Above line,
On line and
Below line. It’s possible to select several
options at once. In that case, QGIS will look for the optimal label position. For Parallel and curved placement
options, you can also use the line orientation for the position of the label. Additionally, you can define a Maximum
angle between curved characters when selecting the
Curved option (see Figure_labels_placement_line).
For all three placement options, in Repeat, you can set up a minimum distance for repeating labels. The distance
can be in mm or in map units.
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Fig. 12.32: Label placement examples in lines
Placement for polygon layers
You can choose one of the
ure_labels_placement_polygon):
•
Offset from centroid,
•
Horizontal (slow),
•
Around centroid,
•
Free (slow),
•
Using perimeter,
• and
following
options
for
placing
labels
in
polygons
(see
fig-
Using perimeter (curved).
visible polygon or
whole polygon.
In the Offset from centroid settings you can specify if the centroid is of the
That means that either the centroid is used for the polygon you can see on the map or the centroid is determined
for the whole polygon, no matter if you can see the whole feature on the map. You can place your label within a
specific quadrant, and define offset and rotation.
The Around centroid setting places the label at a specified distance around the centroid. Again, you can define
visible polygon or
whole polygon for the centroid.
With the Horizontal (slow) or Free (slow) options, QGIS places at the best position either a horizontal or a rotated
label inside the polygon.
With the Using perimeter option, the label will be drawn next to the polygon boundary. The label will behave like
the parallel option for lines. You can define a position and a distance for the label. For the position,
Above
line,
On line,
Below line and
Line orientation dependent position are possible. You can specify the
distance between the label and the polygon outline, as well as the repeat interval for the label.
The Using perimeter (curved) option helps you draw the label along the polygon boundary, using a curved labeling.
In addition to the parameters available with Using perimeter setting, you can set the Maximum angle between
curved characters polygon, either inside or outside.
In the priority section you can define the priority with which labels are rendered for all three vector layer types
(point, line, polygon). This placement option interacts with the labels from other vector layers in the map canvas.
If there are labels from different layers in the same location, the label with the higher priority will be displayed
and the others will be left out.
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Fig. 12.33: Label placement examples in polygons
Rendering tab
In the Rendering tab, you can tune when the labels can be rendered and their interaction with other labels and
features.
Under Label options, you find the scale-based and the Pixel size-based visibility settings.
The Label z-index determines the order in which labels are rendered, as well in relation with other feature labels in
the layer (using data-defined override expression), as with labels from other layers. Labels with a higher z-index
are rendered on top of labels (from any layer) with lower z-index.
Additionally, the logic has been tweaked so that if 2 labels have matching z-indexes, then:
• if they are from the same layer, the smaller label will be drawn above the larger label
• if they are from different layers, the labels will be drawn in the same order as their layers themselves (ie
respecting the order set in the map legend).
Note that this setting doesn’t make labels to be drawn below the features from other layers, it just controls the
order in which labels are drawn on top of all the layer’s features.
While rendering labels and in order to display readable labels, QGIS automatically evaluates the position of the
labels and can hide some of them in case of collision. You can however choose to
layer (including colliding labels) in order to manually fix their placement.
Show all labels for this
With data-defined expressions in Show label and Always Show you can fine tune which labels should be rendered.
Under Feature options, you can choose to label every part of a multi-part feature and limit the number of features
to be labeled. Both line and polygon layers offer the option to set a minimum size for the features to be labeled,
using Suppress labeling of features smaller than. For polygon features, you can also filter the labels to show
according to whether they completely fit within the feature or not. For line features, you can choose to Merge
connected lines to avoid duplicate labels, rendering a quite airy map in conjunction with the Distance or Repeat
options in Placement tab.
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From the Obstacles frame, you can manage the covering relation between labels and features. Activate the
Discourage labels from covering features option to decide whether features of the layer should act as obstacles
for any label (including labels from other features in the same layer). An obstacle is a feature QGIS tries as far as
possible to not place labels over. Instead of the whole layer, you can define a subset of features to use as obstacles,
using the
data-defined override
control next to the option.
The
priority control slider for obstacles allows you to make labels prefer to overlap features
from certain layers rather than others. A Low weight obstacle priority means that features of the layer are less
considered as obstacles and thus more likely to be covered by labels. This priority can also be data-defined, so
that within the same layer, certain features are more likely to be covered than others.
For polygon layers, you can choose the type of obstacle features could be by minimising the labels placement:
• over the feature’s interior: avoids placing labels over the interior of the polygon (prefers placing labels
totally outside or just slightly inside the polygon)
• or over the feature’s boundary: avoids placing labels over boundary of the polygon (prefers placing labels
outside or completely inside the polygon). E.g., it can be useful for regional boundary layers, where the
features cover an entire area. In this case, it’s impossible to avoid placing labels within these features, and
it looks much better to avoid placing them over the boundaries between features.
Setting the automated placement engine
In the top right corner of the Labels tab, you can use the
global and automated behavior of the labels. Clicking the
Automated placement settings (applies to all layers)
to configure a
button provides you with the following options:
• The Search method combobox provides you with different placement methods for finding good placement
solutions for point, line and polygon labeling.
• The Number of candidates controls set how many label placement candidates should be generated for each
feature type. The more candidates generated, the better the labeling will be - but at a cost of rendering speed.
Smaller number of candidates results in less labels placed but faster redraws.
•
Draw text as outlines: controls whether text labels are drawn (and exported) as either proper text objects
OR as paths only. If they are exported as text objects then they can be edited in external applications
(e.g. Inkscape) as normal text. BUT the side effect is that the rendering quality is decreased, AND there’s
issues with rendering when certain text settings like buffers are in place. That’s why drawing as outlines is
recommended. Note that when exporting a layout to svg there’s actually an override for this setting - so you
can leave the project rendering as outlines but for a .svg export export the labels as text.
•
Show partial labels: controls whether labels which fall partially outside of the map extent should be
rendered. If checked, these labels will be shown (when there’s no way to place them fully within the visible
area). If unchecked then partial visible labels will be skipped.
•
show all labels for all layers (i.e. including colliding objects). Note that this option can be also set per
layer (see Rendering tab)
•
show candidates (for debugging): controls whether boxes should be drawn on the map showing all the
candidates generated for label placement. Like the label says, it’s useful only for debugging and testing the
effect different labeling settings have. This could be handy for a better manual placement with tools from
the label toolbar.
Rule-based labeling
With rule-based labeling multiple label configurations can be defined and applied selectively on the base of expression filters and scale range, as in Rule-based rendering.
To create a rule, select the Rule-based labeling option in the main drop-down list from the Labels tab and click
the
button at the bottom of the dialog. Then fill the new dialog with a description and an expression to filter
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features. You can also set a scale range in which the label rule should be applied. The other options available in
this dialog are the common settings seen beforehand.
Fig. 12.34: Rule settings
A summary of existing rules is shown in the main dialog (see figure_labels_rule_based). You can add multiple
rules, reorder or imbricate them with a drag-and-drop. You can as well remove them with the
them with
button or edit
button or a double-click.
Define labels based on expressions
Whether you choose simple or rule-based labeling type, QGIS allows using expressions to label features.
Click the
icon near the Label with drop-down list in the
Labels tab of the properties dialog. In figure_labels_expression, you see a sample expression to label the alaska regions with name and area size, based on
the field ‘NAME_2’, some descriptive text, and the function $area in combination with format_number()
to make it look nicer.
Expression based labeling is easy to work with. All you have to take care of is that:
• You need to combine all elements (strings, fields, and functions) with a string concatenation function such
as concat, + or ||. Be aware that in some situations (when null or numeric value are involved) not all of
these tools will fit your need.
• Strings are written in ‘single quotes’.
• Fields are written in “double quotes” or without any quote.
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Fig. 12.35: Rule based labeling panel
Fig. 12.36: Using expressions for labeling
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Let’s have a look at some examples:
1. Label based on two fields ‘name’ and ‘place’ with a comma as separator:
"name" || ', ' || "place"
Returns:
John Smith, Paris
2. Label based on two fields ‘name’ and ‘place’ with other texts:
'My name is ' + "name" + 'and I live in ' + "place"
'My name is ' || "name" || 'and I live in ' || "place"
concat('My name is ', name, ' and I live in ', "place")
Returns:
My name is John Smith and I live in Paris
3. Label based on two fields ‘name’ and ‘place’ with other texts combining different concatenation functions:
concat('My name is ', name, ' and I live in ' || place)
Returns:
My name is John Smith and I live in Paris
Or, if the field ‘place’ is NULL, returns:
My name is John Smith
4. Multi-line label based on two fields ‘name’ and ‘place’ with a descriptive text:
concat('My name is ', "name", '\n' , 'I live in ' , "place")
Returns:
My name is John Smith
I live in Paris
5. Label based on a field and the $area function to show the place’s name and its rounded area size in a
converted unit:
'The area of ' || "place" || ' has a size of '
|| round($area/10000) || ' ha'
Returns:
The area of Paris has a size of 10500 ha
6. Create a CASE ELSE condition. If the population value in field population is <= 50000 it is a town,
otherwise it is a city:
concat('This place is a ',
CASE WHEN "population <= 50000" THEN 'town' ELSE 'city' END)
Returns:
This place is a town
As you can see in the expression builder, you have hundreds of functions available to create simple and very
complex expressions to label your data in QGIS. See Expressions chapter for more information and examples on
expressions.
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Using data-defined override for labeling
Data defined override
functions, the settings for the labeling are overridden by entries in the attribute table.
With the
It can be used to set values for most of the labeling options described above. See the widget’s description and
manipulation in Data defined override setup section.
The Label Toolbar
label or
diagram properties, but only if the
The Label Toolbar provides some tools to manipulate
corresponding data-defined option is indicated (otherwise, buttons are disabled). Layer might also need to be in
edit mode.
Fig. 12.37: The Label toolbar
While for readability, label has been used below to describe the Label toolbar, note that when mentioned in
their name, the tools work almost the same way with diagrams:
•
•
Pin/Unpin Labels And Diagrams
that has data-defined position. By clicking or draging an area, you pin label(s).
If you click or drag an area holding Shift, label(s) are unpinned. Finally, you can also click or drag an
area holding Ctrl to toggle the pin status of label(s).
Highlight Pinned Labels And Diagrams
. If the vector layer of the label is editable, then the highlighting is green,
otherwise it’s blue.
•
Move Label And Diagram
•
Show/Hide Labels And Diagrams
that has data-defined position. You just have to drag the label to the desired place.
that has data-defined visbility. If you click or drag an area holding Shift,
then label(s) are hidden. When a label is hidden, you just have to click or drag an area around the feature’s
point to restore its visibility.
•
Rotate Label
•
Change Label
. Click the label and move around and you get the text rotated.
. It opens a dialog to change the clicked label properties; it can be the label itself, its coordinates, angle, font, size. . . as long as this property has been mapped to a field.
Warning: Label tools overwrite current field values
Using the Label toolbar to customize the labeling actually writes the new value of the property in the mapped
field. Hence, be careful to not inadvertently replace data you may need later!
Note: The Auxiliary storage mechanism may be used to customize labeling (position, and so on) without modifying the underlying data source.
Customize the labels from the map canvas
Combined with the Label Toolbar, the data defined override setting helps you manipulate labels in the map canvas
(move, edit, rotate). We now describe an example using the data-defined override function for the
function (see figure_labels_data_defined).
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1. Import lakes.shp from the QGIS sample dataset.
2. Double-click the layer to open the Layer Properties. Click on Labels and Placement. Select
centroid.
Offset from
icon to define the field type for the Coordinate. Choose
3. Look for the Data defined entries. Click the
xlabel for X and ylabel for Y. The icons are now highlighted in yellow.
Fig. 12.38: Labeling of vector polygon layers with data-defined override
4. Zoom into a lake.
5. Set editable the layer using the
Toggle Editing
button.
6. Go to the Label toolbar and click the
icon. Now you can shift the label manually to another position
(see figure_labels_move). The new position of the label is saved in the xlabel and ylabel columns of
the attribute table.
7. Using The Geometry Generator with the expression below, you can also add a linestring symbol layer to
connect each lake to its moved label:
make_line( centroid( $geometry ), make_point( "xlabel", "ylabel" ) )
Fig. 12.39: Moved labels
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Note: The Auxiliary storage mechanism may be used with data-defined properties without having an editable
data source.
12.2.5 Diagrams Properties
The Diagrams tab allows you to add a graphic overlay to a vector layer (see figure_diagrams_attributes).
The current core implementation of diagrams provides support for:
• pie charts, a circular statistical graphic divided into slices to illustrate numerical proportion. The arc length
of each slice is proportional to the quantity it represents,
• text diagrams, a horizontaly divided circle showing statistics values inside
• and histograms.
Tip: Switch quickly between types of diagrams
Given that the settings are almost common to the different types of diagram, when designing your diagram, you
can easily change the diagram type and check which one is more appropriate to your data without any loss.
For each type of diagram, the properties are divided into several tabs:
• Attributes
• Appearance
• Size
• Placement
• Options
• Legend
Attributes
add item
Attributes defines which variables to display in the diagram. Use
button to select the desired fields into
the ‘Assigned Attributes’ panel. Generated attributes with Expressions can also be used.
You can move up and down any row with click and drag, sorting how attributes are displayed. You can also change
the label in the ‘Legend’ column or the attribute color by double-clicking the item.
This label is the default text displayed in the legend of the print layout or of the layer tree.
Appearance
Appearance defines how the diagram looks like. It provides general settings that do not interfere with the statistic
values such as:
• the graphic transparency, its outline width and color
• the width of the bar in case of histogram
• the circle background color in case of text diagram, and the font used for texts
• the orientation of the left line of the first slice represented in pie chart. Note that slices are displayed
clockwise.
In this tab, you can also manage the diagram visibility:
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Fig. 12.40: Diagram properties - Attributes tab
• by removing diagrams that overlap others or Show all diagrams even if they overlap each other
• by selecting a field with Data defined visibility to precisely tune which diagrams should be rendered
• by setting the scale visibility
Size
Size is the main tab to set how the selected statistics are represented. The diagram size units can be ‘Map Units’
or ‘Millimeters’. You can use :
• Fixed size, an unique size to represent the graphic of all the features, except when displaying histogram
• or Scaled size, based on an expression using layer attributes.
Placement
Placement helps to define diagram position. According to the layer geometry type, it offers different options for
the placement:
• ‘Over the point’ or ‘Around the point’ for point geometry. The latter variable requires a radius to follow.
• ‘Over the line’ or ‘Around the line’ for line geometry. Like point feature, the last variable requires a distance
to respect and user can specify the diagram placement relative to the feature (‘above’, ‘on’ and/or ‘below’
the line) It’s possible to select several options at once. In that case, QGIS will look for the optimal position
of the diagram. Remember that here you can also use the line orientation for the position of the diagram.
• ‘Over the centroid’, ‘Around the centroid’ (with a distance set), ‘Perimeter’ and anywhere ‘Inside polygon’
are the options for polygon features.
The diagram can also be placed using feature data by filling the X and Y fields with an attribute of the feature.
The placement of the diagrams can interact with the labeling, so you can detect and solve position conflicts
between diagrams and labels by setting the Priority slider or the z-index value.
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Fig. 12.41: Diagram properties - Appearance tab
Fig. 12.42: Diagram properties - Size tab
Fig. 12.43: Vector properties dialog with diagram properties, Placement tab
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Options
The Options tab has settings only in case of histogram. You can choose whether the bar orientation should be
‘Up’, ‘Down’, ‘Right’ and ‘Left’.
Legend
From the Legend tab, you can choose to display items of the diagram in the Layers Panel, besides the layer
symbology. It can be:
• the represented attributes: color and legend text set in Attributes tab
• and if applicable, the diagram size, whose symbol you can customize.
When set, the diagram legend items are also available in the print layout legend, besides the layer symbology.
Case Study
We will demonstrate an example and overlay on the Alaska boundary layer a text diagram showing temperature
data from a climate vector layer. Both vector layers are part of the QGIS sample dataset (see section Sample Data).
Load Vector
icon, browse to the QGIS sample dataset folder, and load the two vector
1. First, click on the
shape layers alaska.shp and climate.shp.
2. Double click the climate layer in the map legend to open the Layer Properties dialog.
combo box, select ‘Text diagram’.
3. Click on the Diagrams tab and from the Diagram type
4. In the Appearance tab, we choose a light blue as background color, and in the Size tab, we set a fixed size
to 18 mm.
5. In the Position tab, placement could be set to ‘Around Point’.
6. In the diagram, we want to display the values of the three columns T_F_JAN, T_F_JUL and T_F_MEAN.
So, in the Attributes tab first select T_F_JAN and click the
finally T_F_MEAN.
button, then repeat with T_F_JUL and
7. Now click [Apply] to display the diagram in the QGIS main window.
8. You can adapt the chart size in the Size tab. Activate the
Scaled size and set the size of the diagrams on
the basis of the maximum value of an attribute and the Size option. If the diagrams appear too small on the
screen, you can activate the
the diagrams.
Increase size of small diagrams checkbox and define the minimum size of
9. Change the attribute colors by double clicking on the color values in the Assigned attributes field. Figure_diagrams_mapped gives an idea of the result.
10. Finally, click [Ok].
Remember that in the Position tab, a
Data defined position of the diagrams is possible. Here, you can use
attributes to define the position of the diagram. You can also set a scale-dependent visibility in the Appearance
tab.
The size and the attributes can also be an expression. Use the
chapter for more information and example.
button to add an expression. See Expressions
Using data-defined override
As mentioned above, you can use some custom data-defined to tune the diagrams rendering:
• position in Placement tab by filling X and Y fields
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Fig. 12.44: Diagram from temperature data overlayed on a map
• visibility in Appearance tab by filling the Visibility field
See Using data-defined override for labeling for more information.
12.2.6 Source Fields Properties
The Source Fields tab helps you organize the fields of the selected dataset and the way you can interact with
the feature’s attributes. The buttons
New field
and
Delete field
can be used when the dataset is in
Editing mode
.
You can rename fields by double-clicking in the fields name (note that you should switch to editing mode to edit
the field name). This is only supported for data providers like PostgreSQL, Oracle, Memory layer and some OGR
layer depending the OGR data format and version.
You can define some alias to display human readable fields in the feature form or the attribute table. In this case,
you don’t need to switch to editing mode. Alias are saved in project file.
Comments can be added by clicking in the comment field of the column but if you are using a PostgreSQL layer,
comment of the column could be the one in the PostgreSQL table if set. Comments are saved in the QGIS project
file as for the alias.
The dialog also lists read-only characteristics of the field such as its type, type name, length and
precision. When serving the layer as WMS or WFS, you can also check here which fields could be retrieved.
12.2.7 Attributes Form Properties
Configure the field behavior
The Attributes Form tab helps you set the type of widget used to fill or display values of the field, in the
attribute table or the feature form: you can define how user interacts with each field and the values or range of
values that are allowed to be added to each.
Common settings
Regardless the type of widget applied to the field, there are some common properties you can set to control whether
and how a field can be edited:
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Fig. 12.45: Field properties tab
Fig. 12.46: Dialog to select an edit widget for an attribute column
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• Editable: uncheck this to set the field read-only (not manually modifiable) when the layer is in edit mode.
Note that checking this setting doesn’t override any edit limitation from the provider.
• Label on top: places the field name above or beside the widget in the feature form
• Default value: for new features, automatically populates by default the field with a predefined value or an
expression-based one. For example, you can:
– use $x, $length, $area to populate a field with the feature’s x coordinate, length, area or any
geometric information at its creation;
– incremente a field by 1 for each new feature using maximum("field")+1;
– save the feature creation datetime using now();
– use variables in expressions, making it easier to e.g. insert the operator name (@user_full_name),
the project file path (@project_path), . . .
A preview of the resulting default value is displayed at the bottom of the widget.
Note: The Default value option is not aware of the values in any other field of the feature being
created so it won’t be possible to use an expression combining any of those values i.e using an expression
like concat(field1, field2) may not work.
• Constraints: you can constrain the value to insert in the field. This constraint can be:
–
Not null: force the user to provide a value
– based on a custom expression: e.g. regexp_match(col0,'A-Za-z') to ensure that the value
of the field col0 has only alphabetical letter.
A short description of the constraint can be added and will be displayed at the top of the form as a
warning message when the value supplied does not match the constraint.
Edit widgets
The available widgets are:
• Checkbox: Displays a checkbox, and you can define what attribute is added to the column when the checkbox is activated or not.
• Classification: Displays a combo box with the values used for classification, if you have chosen ‘unique
value’ as legend type in the Style tab of the properties dialog.
• Color: Displays a color button allowing user to choose a color from the color dialog window.
• Date/Time: Displays a line field which can open a calendar widget to enter a date, a time or both. Column
type must be text. You can select a custom format, pop-up a calendar, etc.
• Enumeration: Opens a combo box with values that can be used within the columns type. This is currently
only supported by the PostgreSQL provider.
• External Resource: Uses a “Open file” dialog to store file path in a relative or absolute mode. It can also
be used to display a hyperlink (to document path), a picture or a web page.
• File Name: Simplifies the selection by adding a file chooser dialog.
• Hidden: A hidden attribute column is invisible. The user is not able to see its contents.
• Photo: Field contains a filename for a picture. The width and height of the field can be defined.
• Range: Allows you to set numeric values from a specific range. The edit widget can be either a slider or a
spin box.
• Relation Reference: This widget lets you embed the feature form of the referenced layer on the feature
form of the actual layer. See Creating one or many to many relations.
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• Text Edit (default): This opens a text edit field that allows simple text or multiple lines to be used. If you
choose multiple lines you can also choose html content.
• Unique Values: You can select one of the values already used in the attribute table. If ‘Editable’ is activated,
a line edit is shown with autocompletion support, otherwise a combo box is used.
• UUID Generator: Generates a read-only UUID (Universally Unique Identifiers) field, if empty.
• Value Map: A combo box with predefined items. The value is stored in the attribute, the description is
shown in the combo box. You can define values manually or load them from a layer or a CSV file.
• Value Relation: Offers values from a related table in a combobox. You can select layer, key column and
value column. Several options are available to change the standard behaviours: allow null value, order by
value, allow multiple selections and use of autocompleter. The forms will display either a drop-down list or
a line edit field when completer checkbox is enabled.
• Web View: Field contains a URL. The width and height of the field is variable.
Tip: Relative Path in widgets
If the path which is selected with the file browser is located in the same directory as the .qgs project file or below,
paths are converted to relative paths. This increases portability of a .qgs project with multimedia information
attached. This is enabled only for File Name, Photo and Web View at this moment.
Customize a form for your data
Identify Features
tool or switch the attribute table to the form view
By default, when you click on a feature with the
mode, QGIS displays a form with tabulated textboxes (one per field). This rendering is the result of the default
Autogenerate value of the Layer properties → Fields → Attribute editor layout setting. Thanks to the widget
setting, you can improve this dialog.
You can furthermore define built-in forms (see figure_fields_form), e.g. when you have objects with many attributes, you can create an editor with several tabs and named groups to present the attribute fields.
Fig. 12.47: Resulting built-in form with tabs and named groups
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The drag and drop designer
Choose Drag and drop designer from the Attribute editor layout combobox to layout the features form
within QGIS. Then, drag and drop rows from the Fields frame to the Label panel to have fields added to your
custom form.
icon
You can also use categories (tab or group frames) to better structure the form. The first step is to use the
to create a tab in which fields and groups will be displayed (see figure_fields_layout). You can create as many
categories as you want. The next step will be to assign to each category the relevant fields, using the
You’d need to select the targeted category beforehand. You can use the same fields many times.
icon.
Fig. 12.48: Dialog to create categories with the Attribute editor layout
You can configure tabs or groups with a double-click. QGIS opens a form in which you can:
• choose to hide or show the item label
• rename the category
• set over how many columns the fields under the category should be distributed
• enter an expression to control the category visibility. The expression will be re-evaluated everytime values
in the form change and the tab or groupbox shown/hidden accordingly.
• show the category as a group box (only available for tabs)
With a double-click on a field label, you can also specify whether the label of its widget should be visible or not
in the form.
In case the layer is involved in one to many relations (see Creating one or many to many relations),
referencing layers are listed in the Relations frame and their form can be embedded in the current layer form by
drag-and-drop. Like the other items, double-click the relation label to configure some options:
• choose to hide or show the item label
• show the link button
• show the unlink button
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Provide an ui-file
The Provide ui-file option allows you to use complex dialogs made with Qt-Designer. Using a UI-file
allows a great deal of freedom in creating a dialog. Note that, in order to link the graphical objects (textbox,
combobox. . . ) to the layer’s fields, you need to give them the same name.
Use the Edit UI to define the path to the file to use.
You’ll find some example in the Creating a new form lesson of the QGIS-trainingmanual-index-reference.
For more advanced information, see http://nathanw.net/2011/09/05/
qgis-tips-custom-feature-forms-with-python-logic/.
Enhance your form with custom functions
QGIS forms can have a Python function that is called when the dialog is opened. Use this function to add extra
logic to your dialogs. The form code can be specified in three different ways:
• load from the environment: use a function, for example in startup.py or from an installed
plugin)
• load from an external file: a file chooser will appear in that case to allow you to select a Python
file from your filesystem
• provide code in this dialog: a Python editor will appear where you can directly type the function to use.
In all cases you must enter the name of the function that will be called (open in the example below).
An example is (in module MyForms.py):
def open(dialog,layer,feature):
geom = feature.geometry()
control = dialog.findChild(QWidged,"My line edit")
Reference in Python Init Function like so: open
12.2.8 Joins Properties
The Joins tab allows you to join a loaded attribute table to a loaded vector layer. After clicking
, the
Add vector join dialog appears. As key columns, you have to define a join layer you want to connect with the
target vector layer. Then, you have to specify the join field that is common to both the join layer and the target
layer. Now you can also specify a subset of fields from the joined layer based on the checkbox
Choose which
fields are joined. As a result of the join, all information from the join layer and the target layer are displayed in
the attribute table of the target layer as joined information. If you specified a subset of fields only these fields are
displayed in the attribute table of the target layer.
If the target layer is editable, then some icons will be displayed in the attribute table next to fields, in order to
inform their status:
•
: the join layer is not configured to be editable. If you want to be able to edit join features from the target
attribute table, then you have to check the option
•
•
Editable join layer.
: the join layer is well configured to be editable, but its current status is read only.
: the join layer is editable but synchronization mechanisms are not activated. If you want to automatically add a feature in the join layer when a feature is created in the target layer, then you have to check the
option
Upsert on edit. Symmetrically, the option
automatically delete join features.
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Moreover, the
Dynamic form option helps to synchronize join fields on the fly, according to the Target field.
This way, constraints for join fields are also correctly updated. Note that it’s deactivated by default because it may
be very time consuming if you have a lot of features or a myriad of joins.
Otherwise, the
Cache join layer in virtual memory option allows to cache values in memory (without geometries) from the joined layer in order to speed up lookups.
QGIS currently has support for joining non-spatial table formats supported by OGR (e.g., CSV, DBF and Excel),
delimited text and the PostgreSQL provider (see figure_joins).
Fig. 12.49: Join an attribute table to an existing vector layer
Additionally, the add vector join dialog allows you to:
•
Create attribute index on the join field
•
Choose which fields are joined
• Create a
Custom field name prefix
12.2.9 Auxiliary storage
The regular way to customize styling and labeling is to use data-defined properties as described in Data defined
override setup. However, it may not be possible if the underlying data is read only. Moreover, configuring these
data-defined properties may be very time consuming or not desirable! For example, if you want to fully use map
tools coming with The Label Toolbar, then you need to add and configure more than 20 fields in your original data
source (x and y positions, rotation angle, font style, color and so on).
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The Auxiliary Storage mechanism provides the solution to these limitations and awkward configurations. Actually,
auxiliary fields are a roundabout mean to automatically manage and store these data-defined properties (labels,
diagram, symbology. . . ) in a SQLite database thanks to editable joins. This way, data source doesn’t even need to
be editable!
A tab is available in vector layer properties dialog to manage auxiliary storage:
Fig. 12.50: Auxiliary Storage tab
Labeling
Considering that the data source may be customized thanks to data-defined properties without being editable,
labeling map tools described in The Label Toolbar are always available as soon as labeling is activated.
Actually, the auxiliary storage system needs an auxiliary layer to store these properties in a SQLite database (see
Auxiliary storage database). Its creation process is run the first time you click on the map while a labeling map
tool is currently activated. Then, a window is displayed, allowing to indicate the primary key to use for joining (to
ensure that features are uniquely identified):
As soon as an auxiliary layer is configured for the current data source, you can retrieve its information in the tab:
For now, we can see that:
• the primary key used is well id
• there’s 0 feature using an auxiliary field
• there’s 0 auxiliary field
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Fig. 12.51: Auxiliary Layer creation dialog
Fig. 12.52: Auxiliary Layer key
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Now that the auxiliary layer is well created, we just have to edit our labels. If we click on a label while the
Change Label
map tool is activated, then we’re able to update styling properties like sizes, colors and so on. Then, the
corresponding data-defined properties are created and can be retrieved:
Fig. 12.53: Auxiliary Fields
As we are seeing in the previous figure, 21 fields have been automatically created and configured for labeling. For
example, the Color auxiliary field type is a String and is named labeling_color in the underlying SQLite
database. Moreover, we observe that there’s 1 entity which is currently using these auxiliary fields (according to
the current example).
By the way, considering that auxiliary fields are linked to data-defined properties, we can observe that data-defined
override options are setup correctly because of the icon
in the labeling tab:
data-defined override
Otherwise, there’s another way to create an auxiliary field for a specific property thanks to the
button. By clicking on Store data in the project, an auxiliary field is automatically created for the Opacity field. If
you click on this button whereas the auxiliary layer is not created yet, then the window Auxiliary Layer creation
dialog is firstly displayed to select the primary key to use for joining.
Symbology
In the same way than for customizing labels, auxiliary fields may be used to stylize symbols too. To do this, you
just have to click on Store data in the project for a specific symbol property. For example for the Fill color field:
Because you may customize same property for different (levels of) symbols, each setting requires a unique name
to avoid conflict. Thus, by clicking on Store data in the project, a window is displayed, indicating the Type of the
field and providing a way to give the unique name. For the Fill color field, the next window is opened:
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Fig. 12.54: Data-defined properties automatically created
Fig. 12.55: Data-defined property menu for symbol
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Fig. 12.56: Name of the auxiliary field for a symbol
Once created, the auxiliary field can be retrieved in the auxiliary storage tab:
Fig. 12.57: Auxiliary field symbol
Attribute table and widgets
Once created, auxiliary fields may be edited through the attribute table. However, there’s some subtlety about
widgets of auxiliary fields.
For example, auxiliary fields which may be edited through an external tool are not visible in the attribute table.
Change Label
Rotate Label
This way, as the Rotation may be edited through
or
, the auxiliary widget is Hidden by
default (see Edit widgets). However, as the Opacity field cannot be edited thanks to map tools, the corresponding
widget is not Hidden. Moreover, auxiliary fields representing a Color have a widget Color set by default.
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Then, the underlying form will look like the next figure:
Fig. 12.58: Form with auxiliary fields
Management
Some actions are available to manage auxiliary layers thanks to the next combobox:
The first item Create is disabled in this case because the auxiliary layer is already created. But in case of a fresh
work, you can use this action to create an auxiliary layer. As explained in Labeling, a primary key will be needed
then.
The Clear action allows to keep all auxiliary fields, but remove their contents. This way, the number of features
using these fields will fall to 0.
The Delete action completely removes the auxiliary layer. In other words, the corresponding table is deleted from
the underlying SQLite database and properties customization are lost.
Finally, the Export action allows to save the auxiliary layer as a new vector layer. Note that geometries are not
stored in auxiliary storage. However, in this case, geometries are exported from the original data source too.
Auxiliary storage database
When you save your project with the .qgs format, the SQLite database used for auxiliary storage is saved at the
same place but with the extension .qgd.
For convenience, an archive may be used instead thanks to the .qgz format. In this case, .qgd and .qgs files
are both embedded in the archive.
12.2.10 Actions Properties
QGIS provides the ability to perform an action based on the attributes of a feature. This can be used to
perform any number of actions, for example, running a program with arguments built from the attributes of a
feature or passing parameters to a web reporting tool.
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Fig. 12.59: Auxiliary layer management
Fig. 12.60: Overview action dialog with some sample actions
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Actions are useful when you frequently want to run an external application or view a web page based on one or
more values in your vector layer. They are divided into six types and can be used like this:
• Generic, Mac, Windows and Unix actions start an external process.
• Python actions execute a Python expression.
• Generic and Python actions are visible everywhere.
• Mac, Windows and Unix actions are visible only on the respective platform (i.e., you can define three ‘Edit’
actions to open an editor and the users can only see and execute the one ‘Edit’ action for their platform to
run the editor).
There are several examples included in the dialog. You can load them by clicking on [Create default actions].
To edit any of the examples, double-click its row. One example is performing a search based on an attribute value.
This concept is used in the following discussion.
Defining Actions
To define an attribute action, open the vector Layer Properties dialog and click on the Actions tab. In the Actions
tab, click the
Add a new action
to open the Edit Action dialog.
Select the action Type and provide a descriptive name for the action. The action itself must contain the name of
the application that will be executed when the action is invoked. You can add one or more attribute field values as
arguments to the application. When the action is invoked, any set of characters that start with a % followed by the
name of a field will be replaced by the value of that field. The special characters %% will be replaced by the value
of the field that was selected from the identify results or attribute table (see using_actions below). Double quote
marks can be used to group text into a single argument to the program, script or command. Double quotes will be
ignored if preceded by a backslash.
If you have field names that are substrings of other field names (e.g., col1 and col10), you should indicate that
by surrounding the field name (and the % character) with square brackets (e.g., [%col10]). This will prevent
the %col10 field name from being mistaken for the %col1 field name with a 0 on the end. The brackets will be
removed by QGIS when it substitutes in the value of the field. If you want the substituted field to be surrounded
by square brackets, use a second set like this: [[%col10]].
Using the Identify Features tool, you can open the Identify Results dialog. It includes a (Derived) item that contains
information relevant to the layer type. The values in this item can be accessed in a similar way to the other fields
by proceeding the derived field name with (Derived).. For example, a point layer has an X and Y field, and
the values of these fields can be used in the action with %(Derived).X and %(Derived).Y. The derived
attributes are only available from the Identify Results dialog box, not the Attribute Table dialog box.
Two example actions are shown below:
• konqueror http://www.google.com/search?q=%nam
• konqueror http://www.google.com/search?q=%%
In the first example, the web browser konqueror is invoked and passed a URL to open. The URL performs a
Google search on the value of the nam field from our vector layer. Note that the application or script called by the
action must be in the path, or you must provide the full path. To be certain, we could rewrite the first example as: /
opt/kde3/bin/konqueror http://www.google.com/search?q=%nam. This will ensure that the
konqueror application will be executed when the action is invoked.
The second example uses the %% notation, which does not rely on a particular field for its value. When the action
is invoked, the %% will be replaced by the value of the selected field in the identify results or attribute table.
Using Actions
Actions can be invoked from either the Identify Results dialog, an Attribute Table dialog or from Run FeaIdentify Features
Open Attribute Table
ture Action (recall that these dialogs can be opened by clicking
or
or
). To invoke an action, right click on the feature and choose the action from the pop-up menu (they
Run Feature Action
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should have been enabled to be displayed in the attribute table). Actions are listed in the popup menu by the name
you assigned when defining the action. Click on the action you wish to invoke.
If you are invoking an action that uses the %% notation, right-click on the field value in the Identify Results dialog
or the Attribute Table dialog that you wish to pass to the application or script.
Here is another example that pulls data out of a vector layer and inserts it into a file using bash and the echo command (so it will only work on or perhaps ). The layer in question has fields for a species name taxon_name,
latitude lat and longitude long. We would like to be able to make a spatial selection of localities and export
these field values to a text file for the selected record (shown in yellow in the QGIS map area). Here is the action
to achieve this:
bash -c "echo \"%taxon_name %lat %long\" >> /tmp/species_localities.txt"
After selecting a few localities and running the action on each one, opening the output file will show something
like this:
Acacia
Acacia
Acacia
Acacia
mearnsii
mearnsii
mearnsii
mearnsii
-34.0800000000
-34.9000000000
-35.2200000000
-32.2700000000
150.0800000000
150.1200000000
149.9300000000
150.4100000000
As an exercise, we can create an action that does a Google search on the lakes layer. First, we need to determine
the URL required to perform a search on a keyword. This is easily done by just going to Google and doing a
simple search, then grabbing the URL from the address bar in your browser. From this little effort, we see that the
format is http://google.com/search?q=qgis, where QGIS is the search term. Armed with this information, we can
proceed:
1. Make sure the lakes layer is loaded.
2. Open the Layer Properties dialog by double-clicking on the layer in the legend, or right-click and choose
Properties from the pop-up menu.
3. Click on the Actions tab.
4. click
Add a new action
.
5. Enter a name for the action, for example Google Search.
6. For the action, we need to provide the name of the external program to run. In this case, we can use Firefox.
If the program is not in your path, you need to provide the full path.
7. Following the name of the external application, add the URL used for doing a Google search, up to but not
including the search term: http://google.com/search?q=
8. The text in the Action field should now look like this: firefox http://google.com/search?q=
9. Click on the drop-down box containing the field names for the lakes layer. It’s located just to the left of
the [Insert] button.
10. From the drop-down box, select ‘NAMES’ and click [Insert].
11. Your action text now looks like this:
firefox http://google.com/search?q=%NAMES
12. To finalize and add the action, click the [OK] button.
This completes the action, and it is ready to use. The final text of the action should look like this:
firefox http://google.com/search?q=%NAMES
We can now use the action. Close the Layer Properties dialog and zoom in to an area of interest. Make sure the
lakes layer is active and identify a lake. In the result box you’ll now see that our action is visible:
When we click on the action, it brings up Firefox and navigates to the URL http://www.google.com/search?q=
Tustumena. It is also possible to add further attribute fields to the action. Therefore, you can add a + to the end of
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Fig. 12.61: Edit action dialog configured with the example
Fig. 12.62: Select feature and choose action
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the action text, select another field and click on [Insert Field]. In this example, there is just no other field available
that would make sense to search for.
You can define multiple actions for a layer, and each will show up in the Identify Results dialog.
You can also invoke actions from the attribute table by selecting a row and right-clicking, then choosing the action
from the pop-up menu.
There are all kinds of uses for actions. For example, if you have a point layer containing locations of images or
photos along with a file name, you could create an action to launch a viewer to display the image. You could also
use actions to launch web-based reports for an attribute field or combination of fields, specifying them in the same
way we did in our Google search example.
We can also make more complex examples, for instance, using Python actions.
Usually, when we create an action to open a file with an external application, we can use absolute paths, or
eventually relative paths. In the second case, the path is relative to the location of the external program executable
file. But what about if we need to use relative paths, relative to the selected layer (a file-based one, like a shapefile
or SpatiaLite)? The following code will do the trick:
command = "firefox"
imagerelpath = "images_test/test_image.jpg"
layer = qgis.utils.iface.activeLayer()
import os.path
layerpath = layer.source() if layer.providerType() == 'ogr'
else (qgis.core.QgsDataSourceURI(layer.source()).database()
if layer.providerType() == 'spatialite' else None)
path = os.path.dirname(str(layerpath))
image = os.path.join(path,imagerelpath)
import subprocess
subprocess.Popen( [command, image ] )
We just have to remember that the action is one of type Python and the command and imagerelpath variables must
be changed to fit our needs.
But what about if the relative path needs to be relative to the (saved) project file? The code of the Python action
would be:
command = "firefox"
imagerelpath = "images/test_image.jpg"
projectpath = qgis.core.QgsProject.instance().fileName()
import os.path
path = os.path.dirname(str(projectpath)) if projectpath != '' else None
image = os.path.join(path, imagerelpath)
import subprocess
subprocess.Popen( [command, image ] )
Another Python action example is the one that allows us to add new layers to the project. For instance, the
following examples will add to the project respectively a vector and a raster. The names of the files to be added to
the project and the names to be given to the layers are data driven (filename and layername are column names of
the table of attributes of the vector where the action was created):
qgis.utils.iface.addVectorLayer('/yourpath/[% "filename" %].shp',
'[% "layername" %]', 'ogr')
To add a raster (a TIF image in this example), it becomes:
qgis.utils.iface.addRasterLayer('/yourpath/[% "filename" %].tif',
'[% "layername" %]')
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12.2.11 Display Properties
This tab is specifically created for map tips: display a message in the map canvas when hovering over a feature
of the active layer. This message can either be the value of a
Field or a more complex and full
HTML text
mixing fields, expressions and html tags (multiline, fonts, images, hyperlink . . . ).
Map Tips
icon. Map tip is a
To activate Map Tips, select the menu option View → Map Tips or click on the
cross-session feature meaning that once activated, it stays on and apply to any set layer in any project, even in
future QGIS sessions until it’s toggled off.
Figures Display Code and Mapped show an example of HTML code and how it behaves in map canvas.
Fig. 12.63: HTML code for map tip
Fig. 12.64: Map tip made with HTML code
12.2.12 Rendering Properties
Scale dependent visibility
You can set the Maximum (inclusive) and Minimum (exclusive) scale, defining a range of scale in which features
Set to current canvas scale
will be visible. Out of this range, they are hidden. The
button helps you use the current
map canvas scale as boundary of the range visibility. See Scale Dependent Rendering for more information.
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Simplify geometry
QGIS offers support for on-the-fly feature generalisation. This can improve rendering times when drawing
many complex features at small scales. This feature can be enabled or disabled in the layer settings using the
Simplify geometry option. There is also a global setting that enables generalisation by default for newly added
layers (see global simplification for more information).
Fig. 12.65: Layer Geometry Simplification dialog
Note: Feature generalisation may introduce artefacts into your rendered output in some cases. These may include
slivers between polygons and inaccurate rendering when using offset-based symbol layers.
While rendering extremely detailed layers (e.g. polygon layers with a huge number of nodes), this can cause
layout exports in PDF/SVG format to be huge as all nodes are included in the exported file. This can also make
the resultant file very slow to work with/open in other programs.
Checking
Force layer to render as raster forces these layers to be rasterised so that the exported files won’t
have to include all the nodes contained in these layers and the rendering is therefore sped up.
You can also do this by forcing the layout to export as a raster, but that is an all-or-nothing solution, given that the
rasterisation is applied to all layers.
12.2.13 Variables Properties
The Variables tab lists all the variables available at the layer’s level (which includes all global and project’s
variables).
It also allows the user to manage layer-level variables. Click the
button to add a new custom layer-level
variable. Likewise, select a custom layer-level variable from the list and click the
button to remove it.
More information on variables usage in the General Tools Variables section.
12.2.14 Metadata Properties
The Metadata tab provides you with options to create and edit a metadata report on your layer. Information
to fill concern:
• the Data identification: basic attribution of the dataset (parent, identifier, title, abstract, language. . . );
• the Categories the data belongs to. Alongside the ISO categories, you can add custom ones;
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• the Keywords to retrieve the data and associated concepts following a standard based vocabulary;
• the Access to the dataset (licenses, rights, fees, and constraints);
• the Extent of the dataset, either spatial one (CRS, map extent, altitudes) or temporal;
• the Contact of the owner(s) of the dataset;
• the Links to ancillary resources and related information;
• the History of the dataset.
A summary of the filled information is provided in the Validation tab and helps you identify potential issues related
to the form. You can then either fix them or ignore them.
Metadata are currently saved in the project file. It can also be saved as an .XML file alongside file based layers or
in a local .sqlite database for remote layers (e.g. PostGIS).
12.2.15 Legend Properties
The Legend tab provides you with a list of widgets you can embed within the layer tree in the Layers panel.
The idea is to have a way to quickly access some actions that are often used with the layer (setup transparency,
filtering, selection, style or other stuff. . . ).
By default, QGIS provides transparency widget but this can be extended by plugins registering their own widgets
and assign custom actions to layers they manage.
12.2.16 QGIS Server Properties
The QGIS Server tab consists of Description, Attribution, MetadataURL, LegendUrl and Properties sections.
In the Properties section, you get general information about the layer, including specifics about the type and location, number of features, feature type, and editing capabilities. The Extents table provides you with information
on the layer extent and the Layer Spatial Reference System, which is information about the CRS of the layer. This
can provide a quick way to get useful information about the layer.
Additionally, you can add or edit a title and abstract for the layer in the Description section. It’s also possible to
define a Keyword list here. These keyword lists can be used in a metadata catalog. If you want to use a title from
an XML metadata file, you have to fill in a link in the DataUrl field.
Use Attribution to get attribute data from an XML metadata catalog.
In MetadataUrl, you can define the general path to the XML metadata catalog. This information will be saved in
the QGIS project file for subsequent sessions and will be used for QGIS server.
In the LegendUrl section, you can provide the url of a legend image in the url field. You can use the Format
drop-down option to apply the appropriate format of the image. Currently png, jpg and jpeg image formats are
supported.
12.3 Expressions
Based on layer data and prebuilt or user defined functions, Expressions offer a powerful way to manipulate
attribute value, geometry and variables in order to dynamically change the geometry style, the content or position
of the label, the value for diagram, the height of a layout item, select some features, create virtual field . . .
12.3.1 The Expression string builder
Main dialog to build expressions, the Expression string builder is available from many parts in QGIS and, can
particularly be accessed when:
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Fig. 12.66: QGIS Server tab in vector layers properties dialog
• clicking the
button;
• selecting features with the
Select By Expression. . .
• editing attributes with e.g. the
Field calculator
tool;
tool;
• manipulating symbology, label or layout item parameters with the
override setup);
Data defined override
tool (see Data defined
• building a geometry generator symbol layer;
• doing some geoprocessing.
The Expression builder dialog offers access to the:
• Expression tab which, thanks to a list of predefined functions, helps to write and check the expression to
use;
• Function Editor tab which helps to extend the list of functions by creating custom ones.
Some use cases of expressions:
• From Field Calculator, calculate a “pop_density” field using existing “total_pop” and “area_km2” fields:
"total_pop" / "area_km2"
• Update the field “density_level” with categories according to the “pop_density” values:
CASE WHEN "pop_density" < 50 THEN 'Low population density'
WHEN "pop_density" >= 50 and "pop_density" < 150 THEN 'Medium population
˓→density'
WHEN "pop_density" >= 150 THEN 'High population density'
END
• Apply a categorized style to all the features according to whether their average house price is smaller or
higher than 10000C per square metre:
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"price_m2" > 10000
• Using the “Select By Expression. . . ” tool, select all the features representing areas of “High population
density” and whose average house price is higher than 10000C per square metre:
"density_level" = 'High population density' and "price_m2" > 10000
Likewise, the previous expression could also be used to define which features should be labeled or shown
in the map.
Using expressions offers you a lot of possibilities.
Tip: Use named parameters to improve the expression reading
Some functions require many parameters to be set. The expression engine supports the use of named parameters.
This means that instead of writing the cryptic expression clamp( 1, 2, 9), you can use clamp( min:=1,
value:=2, max:=9). This also allows arguments to be switched, e.g. clamp( value:=2, max:=9,
min:=1). Using named parameters helps clarify what the arguments for an expression function refer to, which
is helpful when you are trying to interpret an expression at a later date!
12.3.2 List of functions
The Expression tab provides the main interface to write expressions using functions, layer’s fields and values. It
contains widgets to:
• type expressions using functions and/or fields. At the bottom of the dialog, is displayed the result of the
expression evaluated on the first feature of the layer.
• select the appropriate function among a list, organized in groups. A search box is available to filter the list
and quickly find a particular function or field. Double-clicking on the item’s name adds it to the expression
being written.
• display help for each function selected. When a field is selected, this widget shows a sample of its values.
Double-clicking a value adds it to the expression.
Operators
This group contains operators (e.g., +, -, *). Note that for most of the mathematical functions below, if one of the
inputs is NULL then the result is NULL.
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Fig. 12.67: The Expression tab
Function
a+b
a-b
a*b
a/b
a%b
a^b
a<b
a <= b
a <> b
a=b
a != b
a>b
a >= b
a~b
||
‘\n’
LIKE
ILIKE
a IS b
a OR b
a AND b
NOT
column
name
192
“column name”
‘string’
NULL
Description
Addition of two values (a plus b)
Subtraction of two values (a minus b).
Multiplication of two values (a multiplied by b)
Division of two values (a divided by b)
Remainder of division of a by b (eg, 7 % 2 = 1, or 2 fits into 7 three times with remainder
1)
Power of two values (for example, 2^2=4 or 2^3=8)
Compares two values and evaluates to 1 if the left value is less than the right value (a is
smaller than b)
Compares two values and evaluates to 1 if the left value isless than or equal to the right
value
Compares two values and evaluates to 1 if they are not equal
Compares two values and evaluates to 1 if they are equal
a and b are not equal
Compares two values and evaluates to 1 if the left value is greater than the right value (a
is larger than b)
Compares two values and evaluates to 1 if the left value is greater than or equal to the
right value
a matches the regular expression b
Joins two values together into a string. If one of the values is NULL the result will be
NULL
Inserts a new line in a string
Returns 1 if the first parameter matches the supplied pattern
Returns 1 if the first parameter matches case-insensitive the supplied pattern (ILIKE can
be used instead of LIKE to make the match case-insensitive)
Tests whether two values are identical. Returns 1 if a is the same as b
Returns 1 when condition a or condition b is true
Returns 1 when conditions a and b are true
Negates a condition
Value of the field column name, take care to not
be confused
simplewith
quote,
see below
Chapter
12. with
Working
Vector
Data
a string value, take care to not be confused with double quote, see above
null value
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Note: About fields concatenation
You can concatenate strings using either || or +. The latter also means sum up expression. So if you have an
integer (field or numeric value) this can be error prone. In this case, you should use ||. If you concatenate two
string values, you can use both.
Some examples:
• Joins a string and a value from a column name:
'My feature''s
'My feature''s
"country_name"
"country_name"
id is: ' || "gid"
id is: ' + "gid" => triggers an error as gid is an integer
+ '(' + "country_code" + ')'
|| '(' || "country_code" || ')'
• Test if the “description” attribute field starts with the ‘Hello’ string in the value (note the position of the %
character):
"description" LIKE 'Hello%'
Conditionals
This group contains functions to handle conditional checks in expressions.
Function
CASE WHEN . . . THEN . . .
END
CASE WHEN . . . THEN . . .
ELSE . . . END
coalesce
if
Description
Evaluates an expression and returns a result if true. You can test multiple
conditions
Evaluates an expression and returns a different result whether it’s true or
false. You can test multiple conditions
Returns the first non-NULL value from the expression list
Tests a condition and returns a different result depending on the conditional
check
Some example:
• Send back a value if the first condition is true, else another value:
CASE WHEN "software" LIKE '%QGIS%' THEN 'QGIS' ELSE 'Other' END
Mathematical Functions
This group contains math functions (e.g., square root, sin and cos).
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Function
abs
acos
asin
atan
atan2(y,x)
Description
Returns the absolute value of a number
Returns the inverse cosine of a value in radians
Returns the inverse sine of a value in radians
Returns the inverse tangent of a value in radians
Returns the inverse tangent of y/x by using the signs of the two arguments to determine the
quadrant of the result
azReturns the north-based azimuth as the angle in radians measured clockwise from the vertical on
imuth(a,b) point a to point b
ceil
Rounds a number upwards
clamp
Restricts an input value to a specified range
cos
Returns the cosine of a value in radians
degrees
Converts from radians to degrees
exp
Returns exponential of a value
floor
Rounds a number downwards
ln
Returns the natural logarithm of the passed expression
log
Returns the value of the logarithm of the passed value and base
log10
Returns the value of the base 10 logarithm of the passed expression
max
Returns the largest value in a set of values
min
Returns the smallest value in a set of values
pi
Returns the value of pi for calculations
radians
Converts from degrees to radians
rand
Returns the random integer within the range specified by the minimum and maximum argument
(inclusive)
randf
Returns the random float within the range specified by the minimum and maximum argument
(inclusive)
round
Rounds to number of decimal places
scale_exp
Transforms a given value from an input domain to an output range using an exponential curve
scale_linear Transforms a given value from an input domain to an output range using linear interpolation
sin
Returns the sine of an angle
sqrt
Returns the square root of a value
tan
Returns the tangent of an angle
Aggregates Functions
This group contains functions which aggregate values over layers and fields.
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Function
aggregate
collect
concatenate
count
count_distinct
count_missing
iqr
majority
max_length
maximum
mean
median
min_length
minimum
minority
q1
q3
range
relation_aggregate
stdev
sum
Description
Returns an aggregate value calculated using features from another layer
Returns the multipart geometry of aggregated geometries from an expression
Returns the all aggregated strings from a field or expression joined by a delimiter
Returns the count of matching features
Returns the count of distinct values
Returns the count of missing (null) values
Returns the calculated inter quartile range from a field or expression
Returns the aggregate majority of values (most commonly occurring value) from a field or
expression
Returns the maximum length of strings from a field or expression
Returns the aggregate maximum value from a field or expression
Returns the aggregate mean value from a field or expression
Returns the aggregate median value from a field or expression
Returns the minimum length of strings from a field or expression
Returns the aggregate minimum value from a field or expression
Returns the aggregate minority of values (least commonly occurring value) from a field or
expression
Returns the calculated first quartile from a field or expression
Returns the calculated third quartile from a field or expression
Returns the aggregate range of values (maximum - minimum) from a field or expression
Returns an aggregate value calculated using all matching child features from a layer relation
Returns the aggregate standard deviation value from a field or expression
Returns the aggregate summed value from a field or expression
Examples:
• Return the maximum of the “passengers” field from features in the layer grouped by “station_class” field:
maximum("passengers", group_by:="station_class")
• Calculate the total number of passengers for the stations inside the current atlas feature:
aggregate('rail_stations','sum',"passengers",
intersects(@atlas_geometry, $geometry))
• Return the mean of the “field_from_related_table” field for all matching child features using the
‘my_relation’ relation from the layer:
aggregate_relation('my_relation', 'mean', "field_from_related_table")
or:
aggregate_relation(relation:='my_relation', calculation := 'mean',
expression := "field_from_related_table")
Array Functions
This group contains functions to create and manipulate arrays.
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Function
array
array_append
array_cat
array_contains
array_distinct
array_find
Description
Returns an array containing all the values passed as parameter
Returns an array with the given value added at the end
Returns an array containing all the given arrays concatenated
Returns true if an array contains the given value
Returns an array containing distinct values of the given array
Returns the index (0 for the first one) of a value within an array. Returns -1 if the value is not
found.
Returns the first value of an array
Returns the Nth value (0 for the first one) of an array
Returns an array with the given value added at the given position
array_first
array_get
array_insert
arReturns true if any element of array1 exists in array2
ray_intersect
array_last
Returns the last element of an array
arReturns the number of elements of an array
ray_length
arReturns an array with the given value added at the beginning
ray_prepend
arReturns an array with all the entries of the given value removed
ray_remove_all
arReturns an array with the given index removed
ray_remove_at
arReturns the given array with array values in reversed order
ray_reverse
array_slice Returns the values of the array from the start_pos argument up to and including the end_pos
argument
arConcatenates array elements into a string separated by a delimiter using and optional string for
ray_to_string empty values.
regReturns an array of all strings captured by capturing groups, in the order the groups themselves
exp_matches appear in the supplied regular expression against a string.
string_to_arraySplits string into an array using supplied delimiter and optional string for empty values
Color Functions
This group contains functions for manipulating colors.
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Function
color_cmyk
Description
Returns a string representation of a color based on its cyan, magenta, yellow and black components
color_cmyka Returns a string representation of a color based on its cyan, magenta, yellow, black and alpha
(transparency) components
color_hsl
Returns a string representation of a color based on its hue, saturation, and lightness attributes
color_hsla
Returns a string representation of a color based on its hue, saturation, lightness and alpha (transparency) attributes
color_hsv
Returns a string representation of a color based on its hue, saturation, and value attributes
color_hsva Returns a string representation of a color based on its hue, saturation, value and alpha (transparency) attributes
color_part
Returns a specific component from a color string, eg the red component or alpha component
color_rgb
Returns a string representation of a color based on its red, green, and blue components
color_rgba Returns a string representation of a color based on its red, green, blue, and alpha (transparency)
components
creReturns a gradient ramp from a map of color strings and steps
ate_ramp
darker
Returns a darker (or lighter) color string
lighter
Returns a lighter (or darker) color string
project_color Returns a color from the project’s color scheme
ramp_color Returns a string representing a color from a color ramp
set_color_partSets a specific color component for a color string, eg the red component or alpha component
Composition Functions
This group contains functions to manipulate print layout items properties.
Function
item_variables
Description
Returns a map of variables from a layout item inside this composition
Some example:
• Get the scale of the ‘Map 0’ in the current print layout:
map_get( item_variables('Map 0'), 'map_scale')
Conversions Functions
This group contains functions to convert one data type to another (e.g., string to integer, integer to string).
Function
to_date
to_datetime
to_int
to_interval
to_real
to_string
to_time
Description
Converts a string into a date object
Converts a string into a datetime object
Converts a string to integer number
Converts a string to an interval type (can be used to take days, hours, months, etc. of a date)
Converts a string to a real number
Converts number to string
Converts a string into a time object
Custom Functions
This group contains functions created by the user. See Function Editor for more details.
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Date and Time Functions
This group contains functions for handling date and time data.
Function
age
day
day_of_week
epoch
hour
minute
month
now
second
week
year
Description
Returns as an interval the difference between two dates or datetimes
Extracts the day from a date or datetime, or the number of days from an interval
Returns a number corresponding to the day of the week for a specified date or datetime
Returns the interval in milliseconds between the unix epoch and a given date value
Extracts the hour from a datetime or time, or the number of hours from an interval
Extracts the minute from a datetime or time, or the number of minutes from an interval
Extracts the month part from a date or datetime, or the number of months from an interval
Returns current date and time
Extracts the second from a datetime or time, or the number of seconds from an interval
Extracts the week number from a date or datetime, or the number of weeks from an interval
Extracts the year part from a date or datetime, or the number of years from an interval
This group also shares several functions with the Conversions Functions ( to_date, to_time, to_datetime,
to_interval) and String Functions (format_date) groups.
Some examples:
• Get today’s month and year in the “month_number/year” format:
format_date(now(),'MM/yyyy')
-- Returns '03/2017'
Besides these functions, subtracting dates, datetimes or times using the - (minus) operator will return an interval.
Adding or subtracting an interval to dates, datetimes or times, using the + (plus) and - (minus) operators, will
return a datetime.
• Get the number of days until QGIS 3.0 release:
to_date('2017-09-29') - to_date(now())
-- Returns <interval: 203 days>
• The same with time:
to_datetime('2017-09-29 12:00:00') - to_datetime(now())
-- Returns <interval: 202.49 days>
• Get the datetime of 100 days from now:
now() + to_interval('100 days')
-- Returns <datetime: 2017-06-18 01:00:00>
Note: Storing date and datetime and intervals on fields
The ability to store date, time and datetime values directly on fields may depend on the data source’s provider (e.g.,
shapefiles accept date format, but not datetime or time format). The following are some suggestions to overcame
this limitation.
date, Datetime and time can be stored in text type fields after using the to_format() function.
Intervals can be stored in integer or decimal type fields after using one of the date extraction functions (e.g.,
day() to get the interval expressed in days)
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Fields and Values
Contains a list of fields from the layer.
Generally, you can use the various fields, values and functions to construct the calculation expression, or you can
just type it into the box.
To display the values of a field, you just click on the appropriate field and choose between Load top 10 unique
values and Load all unique values. On the right side, the Field Values list opens with the unique values. At the
top of the list, a search box helps filtering the values. To add a value to the expression you are writing, double
click its name in the list.
Sample values can also be accessed via right-click. Select the field name from the list, then right-click to access a
context menu with options to load sample values from the selected field.
Fields name should be double-quoted in the expression. Values or string should be simple-quoted.
Fuzzy Matching Functions
This group contains functions for fuzzy comparisons between values.
Function
hamming_distance
levensheim
Description
Returns the number of characters at corresponding positions within the input strings where
the characters are different
Returns the minimum number of character edits (insertions, deletions or substitutions) required to change one string to another. Measure the similarity between two strings
longest_common_substring
Returns the longest common substring between two strings
soundex
Returns the Soundex representation of a string
General Functions
This group contains general assorted functions.
Function
env
Description
Gets an environment variable and returns its content as a string. If the variable is not found, NULL
will be returned.
eval
Evaluates an expression which is passed in a string. Useful to expand dynamic parameters passed
as context variables or fields
is_layer_visible
Returns true if a specified layer is visible
layer_property
Returns a property of a layer or a value of its metadata. It can be layer name, crs, geometry type,
feature count. . .
raster_statistic
Returns statistics from a raster layer
var
Returns the value stored within a specified variable. See variable functions below
with_variable
Creates and sets a variable for any expression code that will be provided as a third argument. Useful
to avoid repetition in expressions where the same value needs to be used more than once.
Geometry Functions
This group contains functions that operate on geometry objects (e.g., length, area).
Function
$area
$geometry
$length
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Description
Returns the area size of the current feature
Returns the geometry of the current feature (can be used for processing with other functions)
Returns the length of the current line feature
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Function
$perimeter
$x
$x_at(n)
$y
$y_at(n)
angle_at_vertex
area
azimuth
boundary
bounds
bounds_height
bounds_width
buffer
centroid
closest_point
combine
contains(a,b)
convex_hull
crosses
difference(a,b)
disjoint
distance
distance_to_vertex
end_point
extend
exterior_ring
extrude(geom,x,y)
geom_from_gml
geom_from_wkt
geom_to_wkt
geometry
geometry_n
interior_ring_n
intersection
intersects
intersects_bbox
is_closed
length
line_interpolate_angle
line_interpolate_point
line_locate_point
line_merge
m
make_line
make_point(x,y,z,m)
make_point_m(x,y,m)
make_polygon
nodes_to_points
num_geometries
num_interior_rings
num_points
num_rings
offset_curve
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Description
Returns the perimeter of the current polygon feature
Returns the x coordinate of the current feature
Returns the x coordinate of the nth node of the current feature’s geometry
Returns the y coordinate of the current feature
Returns the y coordinate of the nth node of the current feature’s geometry
Returns the bisector angle (average angle) to the geometry for a specified vertex on a linestring geometr
Returns the area of a geometry polygon feature. Calculations are in the Spatial Reference System of this
Returns the north-based azimuth as the angle in radians measured clockwise from the vertical on point_
Returns the closure of the combinatorial boundary of the geometry (ie the topological boundary of the g
Returns a geometry which represents the bounding box of an input geometry. Calculations are in the Sp
Returns the height of the bounding box of a geometry. Calculations are in the Spatial Reference System
Returns the width of the bounding box of a geometry. Calculations are in the Spatial Reference System
Returns a geometry that represents all points whose distance from this geometry is less than or equal to
Returns the geometric center of a geometry
Returns the point on a geometry that is closest to a second geometry
Returns the combination of two geometries
Returns 1 (true) if and only if no points of b lie in the exterior of a, and at least one point of the interior
Returns the convex hull of a geometry (this represents the minimum convex geometry that encloses all g
Returns 1 (true) if the supplied geometries have some, but not all, interior points in common
Returns a geometry that represents that part of geometry a that does not intersect with geometry b
Returns 1 (true) if the geometries do not share any space together
Returns the minimum distance (based on Spatial Reference System) between two geometries in projecte
Returns the distance along the geometry to a specified vertex
Returns the last node from a geometry
Extends the start and end of a linestring geometry by a specified amount
Returns a line string representing the exterior ring of a polygon geometry, or null if the geometry is not
Returns an extruded version of the input (Multi-) Curve or (Multi-)Linestring geometry with an extensio
Returns a geometry created from a GML representation of geometry
Returns a geometry created from a well-known text (WKT) representation
Returns the well-known text (WKT) representation of the geometry without SRID metadata
Returns a feature’s geometry
Returns the nth geometry from a geometry collection, or null if the input geometry is not a collection
Returns the geometry of the nth interior ring from a polygon geometry, or null if the geometry is not a p
Returns a geometry that represents the shared portion of two geometries
Tests whether a geometry intersects another. Returns 1 (true) if the geometries spatially intersect (share
Tests whether a geometry’s bounding box overlaps another geometry’s bounding box. Returns 1 (true) i
Returns true if a line string is closed (start and end points are coincident), false if a line string is not clos
Returns length of a line geometry feature (or length of a string)
Returns the angle parallel to the geometry at a specified distance along a linestring geometry. Angles are
Returns the point interpolated by a specified distance along a linestring geometry.
Returns the distance along a linestring corresponding to the closest position the linestring comes to a sp
Returns a (Multi-)LineString geometry, where any connected LineStrings from the input geometry have
Returns the m value of a point geometry
Creates a line geometry from a series of point geometries
Returns a point geometry from x and y (and optional z or m) values
Returns a point geometry from x and y coordinates and m values
Creates a polygon geometry from an outer ring and optional series of inner ring geometries
Returns a multipoint geometry consisting of every node in the input geometry
Returns the number of geometries in a geometry collection, or null if the input geometry is not a collect
Returns the number of interior rings in a polygon or geometry collection, or null if the input geometry is
Returns the number of vertices in a geometry
Returns the number of rings (including exterior rings) in a polygon or geometry collection, or null if the
Returns a geometry formed by offsetting a linestring geometry to the side. Distances are in the Spatial R
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Function
order_parts
overlaps
perimeter
point_n
point_on_surface
pole_of_inaccessibility
project
relate
reverse
segments_to_lines
shortest_line
simplify
simplify_vw
single_sided_buffer
smooth
start_point
sym_difference
touches
transform
translate
union
within (a,b)
x
x_min
x_max
y
y_min
y_max
z
Table 12.1 – continued from previo
Description
Orders the parts of a MultiGeometry by a given criteria
Tests whether a geometry overlaps another. Returns 1 (true) if the geometries share space, are of the sam
Returns the perimeter of a geometry polygon feature. Calculations are in the Spatial Reference System o
Returns a specific node from a geometry
Returns a point guaranteed to lie on the surface of a geometry
Calculates the approximate pole of inaccessibility for a surface, which is the most distant internal point
Returns a point projected from a start point using a distance and bearing (azimuth) in radians
Tests or returns the Dimensional Extended 9 Intersection Model (DE-9IM) representation of the relation
Reverses the direction of a line string by reversing the order of its vertices
Returns a multi line geometry consisting of a line for every segment in the input geometry
Returns the shortest line joining two geometries. The resultant line will start at geometry 1 and end at ge
Simplifies a geometry by removing nodes using a distance based threshold
Simplifies a geometry by removing nodes using an area based threshold
Returns a geometry formed by buffering out just one side of a linestring geometry. Distances are in the S
Smooths a geometry by adding extra nodes which round off corners in the geometry
Returns the first node from a geometry
Returns a geometry that represents the portions of two geometries that do not intersect
Tests whether a geometry touches another. Returns 1 (true) if the geometries have at least one point in c
Returns the geometry transformed from the source CRS to the destination CRS
Returns a translated version of a geometry. Calculations are in the Spatial Reference System of this geo
Returns a geometry that represents the point set union of the geometries
Tests whether a geometry is within another. Returns 1 (true) if geometry a is completely inside geometr
Returns the x coordinate of a point geometry, or the x coordinate of the centroid for a non-point geomet
Returns the minimum x coordinate of a geometry. Calculations are in the Spatial Reference System of th
Returns the maximum x coordinate of a geometry. Calculations are in the Spatial Reference System of t
Returns the y coordinate of a point geometry, or the y coordinate of the centroid for a non-point geomet
Returns the minimum y coordinate of a geometry. Calculations are in the Spatial Reference System of th
Returns the maximum y coordinate of a geometry. Calculations are in the Spatial Reference System of t
Returns the z coordinate of a point geometry
Some examples:
• You can manipulate the current geometry with the variable $geometry to create a buffer or get the point on
surface:
buffer( $geometry, 10 )
point_on_surface( $geometry )
• Return the x coordinate of the current feature’s centroid:
x( $geometry )
• Send back a value according to feature’s area:
CASE WHEN $area > 10 000 THEN 'Larger' ELSE 'Smaller' END
Map Functions
This group contains functions to create or manipulate keys and values of maps.
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FuncDescription
tion
map
Returns a map containing all the keys and values passed as pair of parameters
map_akeys Returns all the keys of a map as an array
map_avals Returns all the values of a map as an array
map_concatReturns a map containing all the entries of the given maps. If two maps contain the same key, the
value of the second map is taken.
map_delete Returns a map with the given key and its corresponding value deleted
map_exist Returns true if the given key exists in the map
map_get Returns the value of a map, given it’s key
map_insert Returns a map with an added key/value
Record Functions
This group contains functions that operate on record identifiers.
Function
$currentfeature
$id
$map
Description
Returns the current feature being evaluated. This can be used with the ‘attribute’ function to
evaluate attribute values from the current feature.
Returns the feature id of the current row
Returns the id of the current map item if the map is being drawn in a composition, or “canvas”
if the map is being drawn within the main QGIS window
$rownum
Returns the number of the current row
$scale
Returns the current scale of the map canvas
attribute
Returns the value of a specified attribute from a feature
get_feature Returns the first feature of a layer matching a given attribute value
get_feature_by_id
Returns the feature of a layer matching the given feature ID
is_selected Returns if a feature is selected
num_selected Returns the number of selected features on a given layer
uuid
Generates a Universally Unique Identifier (UUID) for each row. Each UUID is 38 characters
long.
Some examples:
• Return the first feature in layer “LayerA” whose field “id” has the same value as the field “name” of the
current feature (a kind of jointure):
get_feature( 'layerA', 'id', attribute( $currentfeature, 'name') )
• Calculate the area of the joined feature from the previous example:
area( geometry( get_feature( 'layerA', 'id', attribute( $currentfeature, 'name
˓→') ) ) )
String Functions
This group contains functions that operate on strings (e.g., that replace, convert to upper case).
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Function
char
concat
format
format_date
format_number
left(string,
n)
length
lower
lpad
regexp_match
regexp_replace
regexp_substr
replace
right(string,
n)
rpad
strpos
substr
title
trim
upper
wordwrap
Description
Returns the character associated with a unicode code
Concatenates several strings to one
Formats a string using supplied arguments
Formats a date type or string into a custom string format
Returns a number formatted with the locale separator for thousands (also truncates the number
to the number of supplied places)
Returns a substring that contains the n leftmost characters of the string
Returns length of a string (or length of a line geometry feature)
converts a string to lower case
Returns a string with supplied width padded using the fill character
Returns the first matching position matching a regular expression within a string, or 0 if the
substring is not found
Returns a string with the supplied regular expression replaced
Returns the portion of a string which matches a supplied regular expression
Returns a string with the supplied string, array, or map of strings replaced by a string, an array
of strings or paired values
Returns a substring that contains the n rightmost characters of the string
Returns a string with supplied width padded using the fill character
Returns the first matching position of a substring within another string, or 0 if the substring is
not found
Returns a part of a string
Converts all words of a string to title case (all words lower case with leading capital letter)
Removes all leading and trailing white space (spaces, tabs, etc.) from a string
Converts string a to upper case
Returns a string wrapped to a maximum/ minimum number of characters
Recent Functions
This group contains recently used functions. Any expression used in the Expression dialog is added to the list,
sorted from the more recent to the less one. This helps to quickly retrieve any previous expression.
Variables Functions
This group contains dynamic variables related to the application, the project file and other settings. It means that
some functions may not be available according to the context:
• from the
Select by expression
• from the
Field calculator
dialog
dialog
• from the layer properties dialog
• from the print layout
To use these functions in an expression, they should be preceded by @ character (e.g, @row_number). Are
concerned:
Function
atlas_feature
Description
Returns the current atlas feature (as feature object)
Continu
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Function
atlas_featureid
atlas_featurenumber
atlas_filename
atlas_geometry
atlas_pagename
atlas_totalfeatures
cluster_color
cluster_size
geometry_part_count
geometry_part_num
geometry_point_count
geometry_point_num
grid_axis
grid_number
item_id
item_uuid
layer
layer_id
layer_name
layout_dpi
layout_numpages
layout_page
layout_pageheight
layout_pagewidth
map_crs
map_crs_definition
map_extent
map_extent_center
map_extent_height
map_extent_width
map_id
map_rotation
map_scale
map_units
project_crs
project_crs_definition
project_filename
project_folder
project_path
project_title
qgis_locale
qgis_os_name
qgis_platform
qgis_release_name
qgis_short_version
qgis_version
qgis_version_no
symbol_angle
symbol_color
user_account_name
user_full_name
row_number
value
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Description
Returns the current atlas feature ID
Returns the number of pages in composition
Returns the current atlas file name
Returns the current atlas feature geometry
Returns the current atlas page name
Returns the total number of features in atlas
Returns the color of symbols within a cluster, or NULL if symbols have mixed colors
Returns the number of symbols contained within a cluster
Returns the number of parts in rendered feature’s geometry
Returns the current geometry part number for feature being rendered
Returns the number of points in the rendered geometry’s part
Returns the current point number in the rendered geometry’s part
Returns the current grid annotation axis (eg, ‘x’ for longitude, ‘y’ for latitude)
Returns the current grid annotation value
Returns the layout item user ID (not necessarily unique)
Returns the layout item unique ID
Returns the current layer
Returns the ID of current layer
Returns the name of current layer
Returns the composition resolution (DPI)
Returns the number of pages in the composition
Returns the current page of the layout item
Returns the composition height in mm
Returns the composition width in mm
Returns the Coordinate reference system of the current map
Returns the full definition of the Coordinate reference system of the current map
Returns the geometry representing the current extent of the map
Returns the point feature at the center of the map
Returns the current height of the map
Returns the current width of the map
Returns the ID of current map destination. This will be ‘canvas’ for canvas renders, and the item ID for l
Returns the current rotation of the map
Returns the current scale of the map
Returns the units of map measurements
Returns the Coordinate reference system of the project
Returns the full definition of the Coordinate reference system of the project
Returns the filename of current project
Returns the folder for current project
Returns the full path (including file name) of current project
Returns the title of current project
Returns the current language of QGIS
Returns the current Operating system name, eg ‘windows’, ‘linux’ or ‘osx’
Returns QGIS platform, eg ‘desktop’ or ‘server’
Returns current QGIS release name
Returns current QGIS version short string
Returns current QGIS version string
Returns current QGIS version number
Returns the angle of the symbol used to render the feature (valid for marker symbols only)
Returns the color of the symbol used to render the feature
Returns the current user’s operating system account name
Returns the current user’s operating system user name
Stores the number of the current row
Returns the current value
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12.3.3 Function Editor
With the Function Editor, you are able to define your own Python custom functions in a comfortable way.
Fig. 12.68: The Function Editor tab
The function editor will create new Python files in .qgis2\python\expressions folder and will auto load
all functions defined when starting QGIS. Be aware that new functions are only saved in the expressions
folder and not in the project file. If you have a project that uses one of your custom functions you will need to also
share the .py file in the expressions folder.
Here’s a short example on how to create your own functions:
@qgsfunction(args='auto', group='Custom')
def myfunc(value1, value2, feature, parent):
pass
The short example creates a function myfunc that will give you a function with two values. When using the
args='auto' function argument the number of function arguments required will be calculated by the number
of arguments the function has been defined with in Python (minus 2 - feature, and parent).
This function then can be used with the following expression:
myfunc('test1', 'test2')
Your function will be implemented in the Custom functions group of the Expression tab after using the Run Script
button.
Further information about creating Python code can be found in the PyQGIS-Developer-Cookbook.
The function editor is not only limited to working with the field calculator, it can be found whenever you work
with expressions.
Docs for ‘QGIS testing’. Visit http://docs.qgis.org/2.18 for QGIS 2.18 docs and translations.
12.4 Working with the Attribute Table
The attribute table displays information on features of a selected layer. Each row in the table represents a feature
(with or without geometry), and each column contains a particular piece of information about the feature. Features
in the table can be searched, selected, moved or even edited.
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12.4.1 Foreword: Spatial and non-spatial tables
QGIS allows you to load spatial and non-spatial layers. This currently includes tables supported by OGR and
delimited text, as well as the PostgreSQL, MSSQL, SpatiaLite, DB2 and Oracle provider. All loaded layers are
listed in the Layers Panel. Whether a layer is spatially enabled or not determines whether you can interact with it
on the map.
Non-spatial tables can be browsed and edited using the attribute table view. Furthermore, they can be used for
field lookups. For example, you can use columns of a non-spatial table to define attribute values, or a range of
values that are allowed, to be added to a specific vector layer during digitizing. Have a closer look at the edit
widget in section Source Fields Properties to find out more.
12.4.2 Introducing the attribute table interface
To open the attribute table for a vector layer, activate the layer by clicking on it in the Layers Panel. Then, from
the main Layer menu, choose
Open Attribute Table. It is also possible to right-click on the layer and choose
Open Attribute Table from the drop-down menu, or to click on the
Open Attribute Table button in the
Attributes toolbar. If you prefer shortcuts, F6 will open the attribute table. Shift+F6 will open the attribute
table filtered to selected features and Ctrl+F6 will open the attribute table filtered to visible features.
This will open a new window that displays the feature attributes for the layer (figure_attributes_table). According
to the setting in Settings → Options → Data sources menu, the attribute table will open in a docked window or
a regular window. The total number of features in the layer and the number of currently selected/filtered features
are shown in the attribute table title, as well as if the layer is spatially limited.
Fig. 12.69: Attribute Table for regions layer
The buttons at the top of the attribute table window provide the following functionality:
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Icon
Label
Purpose
Default Shortcut
Toggle editing mode
Enable editing functionalities
Ctrl+E
Toggle multi edit mode
Update multiple fields of many features
Save Edits
Save current modifications
Ctrl+S
Reload the table
Add feature
Add new geometryless feature
Delete selected features
Remove selected features from the layer
Select features using an Expression
Select All
Select all features in the layer
Ctrl+A
Invert selection
Invert the current selection in the layer
Ctrl+R
Deselect all
Deselect all features in the current layer
Ctrl+Shift+A
Filter/Select features using form
Move selected to top
Ctrl+F
Move selected rows to the top of the table
Pan map to the selected rows
Ctrl+P
Zoom map to the selected rows
Ctrl+J
Copy selected rows to clipboard
Ctrl+C
Paste features from clipboard
Insert new features from copied ones
Ctrl+V
New field
Add a new field to the data source
Ctrl+W
Delete field
Remove a field from the data source
Ctrl+L
Open field calculator
Update field for many features in a row
Ctrl+I
Conditional formatting
Enable table formatting
Table Attribute 1: Available Tools
Note: Depending on the format of the data and the OGR library built with your QGIS version, some tools may
not be available.
Below these buttons is the Quick Field Calculation bar (enabled only in edit mode), which allows to quickly apply
calculations to all or part of the features in the layer. This bar uses the same expressions as the
(see Editing attribute values).
Field Calculator
Table view vs Form view
QGIS provides two view modes to easily manipulate data in the attribute table:
Table view, displaying values of multiple features in a tabular mode, each row representing a feature
• the
and each column a field;
• and the
Form view which shows identifiers of features in a first panel and displays only the attributes
of the clicked identifier in the second one. Form view uses the layer fields configuration (see Source Fields
Properties).
You can switch from one mode to the other by clicking the convenient icon at the bottom right of the dialog.
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You can also specify the Default view mode at the opening of the attribute table in Settings → Options → Data
Sources menu. It can be ‘Remember last view’, ‘Table view’ or ‘Form view’.
Fig. 12.70: Attribute table in form view (top) vs table view (bottom)
Configuring the columns
Right-click in a column header when in table view to have access to tools that help you configure what can be
displayed in the attribute table and how.
Hiding and organizing columns and enabling actions
By right-clicking in a column header, you can choose to hide it from the attribute table. To change several columns
behavior at once, unhide a column or change the order of the columns, choose Organize columns . . . . In the new
dialog, you can:
• check/uncheck columns you want to show or hide
• drag-and-drop items to reorder the columns in the attribute table. Note that this change is for the table
rendering and does not alter the fields order in the layer datasource
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• enable a new virtual Actions column that displays in each row a drop-down box or button list of actions for
each row, see Actions Properties for more information about actions.
Resizing columns widths
Columns width can be set through a right-click on the column header and select either:
• Set width. . . to enter the desired value. By default, the current value is displayed in the widget
• Autosize to resize at the best fit the column.
It can also be changed by dragging the boundary on the right of the column heading. The new size of the column
is maintained for the layer, and restored at the next opening of the attribute table.
Sorting columns
The table can be sorted by any column, by clicking on the column header. A small arrow indicates the sort
order (downward pointing means descending values from the top row down, upward pointing means ascending
values from the top row down). You can also choose to sort the rows with the sort option of the column header
context menu and write an expression, e.g. to sort the row using multiple columns you can write concat(col0,
col1).
In form view, features identifier can be sorted using the
Sort by preview expression option.
Tip: Sorting based on columns of different types
Trying to sort an attribute table based on columns of string and numeric types may lead to unexpected result because of the concat("USE", "ID") expression returning string values (ie, 'Borough105' <
'Borough6'). You can workaround this by using eg concat("USE", lpad("ID", 3, 0)) which returns 'Borough105' > 'Borough006'.
Formatting of table cells using conditions
Conditional formatting settings can be used to highlight in the attribute table features you may want to put a
particular focus on, using custom conditions on feature’s:
• geometry (e.g., identifying multi-parts features, small area ones or in a defined map extent. . . );
• or field value (e.g., comparing values to a threshold, identifying empty cells. . . )
You can enable the conditional formatting panel clicking on
view (not available in form view).
at the top right of the attributes window in table
The new panel allows user to add new rules to format rendering of
a form to define:
Field or
Full row. Adding new rule opens
• the name of the rule;
• a condition using any of the expression builder functions;
• the formatting: it can be choosen from a list of predefined formats or created based on properties like:
– background and text colors;
– use of icon;
– bold, italic, underline, or strikeout;
– font.
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Fig. 12.71: Conditional Formatting of an attribute table
12.4.3 Interacting with features in an attribute table
Selecting features
In table view, each row in the attribute table displays the attributes of a unique feature in the layer. Selecting a row
selects the feature and likewise, selecting a feature in the map canvas (in case of geometry enabled layer) selects
the row in the attribute table. If the set of features selected in the map canvas (or attribute table) is changed, then
the selection is also updated in the attribute table (or map canvas) accordingly.
Rows can be selected by clicking on the row number on the left side of the row. Multiple rows can be marked by
holding the Ctrl key. A continuous selection can be made by holding the Shift key and clicking on several
row headers on the left side of the rows. All rows between the current cursor position and the clicked row are
selected. Moving the cursor position in the attribute table, by clicking a cell in the table, does not change the row
selection. Changing the selection in the main canvas does not move the cursor position in the attribute table.
In form view of the attribute table, features are by default identified in the left panel by the value of their displayed
field (see Display Properties). This identifier can be replaced using the drop-down list at the top of the panel,
either by selecting an existing field or using a custom expression. You can also choose to sort the list of features
from the drop-down menu.
Click a value in the left panel to display the feature’s attributes in the right one. To select a feature, you need to
click inside the square symbol at the left of the identifier. By default, the symbol turns into yellow. Like in the
table view, you can perform multiple feature selection using the keyboard combinations previously exposed.
Beyond selecting features with the mouse, you can perform automatic selection based on feature’s attribute using
tools available in the attribute table toolbar, such as (see section Automatic selection and following one for more
information and use case):
•
Select By Expression. . .
•
Select Features By Value. . .
•
Deselect Features from All Layers
•
Select All Features
•
Invert Feature Selection.
It is also possible to select features using the Filtering and selecting features using forms.
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Filtering features
Once you have selected features in the attribute table, you may want to display only these records in the table.
This can be easily done using the Show Selected Features item from the drop-down list at the bottom left of the
attribute table dialog. This list offers the following filters:
• Show All Features
• Show Selected Features
• Show Features visible on map
• Show Edited and New Features
• Field Filter - allows the user to filter based on value of a field: choose a column from a list, type a value and
press Enter to filter. Then, only the matching features are shown in the attribute table.
• Advanced filter (Expression) - Opens the expression builder dialog. Within it, you can create complex
expressions to match table rows. For example, you can filter the table using more than one field. See
Expressions for more information.
It is also possible to filter features using the Filtering and selecting features using forms.
Note: Filtering records out of the attribute table does not filter features out of the layer; they are simply momentaneously hidden from the table and can be accessed from the map canvas or by removing the filter. For filters that
do hide features from the layer, use the Query Builder.
Tip: Update datasource filtering with Show Features Visible on Map
When for performance reasons, features shown in attribute table are spatially limited to the canvas extent at its
opening (see Data Source Options for a how-to), selecting Show Features Visible on Map on a new canvas extent
updates the spatial restriction.
Filtering and selecting features using forms
Filter/Select features using form
Clicking the
or pressing Ctrl+F the attribute table dialog will switch to form view
and all widgets are replaced with their search variant.
From this point onwards, this tool functionality is similar to the one described in the Select Features By Value,
where you can find descriptions of all operators and selecting modes.
Moreover, in the attribute table case, there is also a Filter features button that allows filtering features instead of
selecting them (by creating an Advanced Filter (Expression) for the user).
If there are already filtered features, you can refine the filter using the drop-down list next to the Filter features
button. The options are:
• Filter within (“AND”)
• Extend filter (“OR”)
To clear the filter, either select Show all features option mentioned in Filtering features, or click the clear the
expression and click [Apply].
12.4.4 Using action on features
Users have several possibilities to manipulate feature with the contextual menu like:
• Select all (Ctrl+A) the features
• Copy the content of a cell in the clipboard with Copy cell content.
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Fig. 12.72: Attribute table filtered by the filter form
• Zoom to feature without having to select it beforehand
• Open form. It toggles attribute table into form view with a focus on the clicked feature
Fig. 12.73: Copy cell content button
If you want to use attribute data in external programs (such as Excel, LibreOffice, QGIS or a custom web applicaCopy selected rows to clipboard
tion), select one or more row(s) and use the
button or press Ctrl+C. In Settings →
Options → Data Sources menu you can define the format to paste to with Copy features as dropdown list:
• Plain text, no geometry,
• Plain text, WKT geometry,
• GeoJSON
You can also display a list of actions in this contextual menu. This is enabled in the Layer properties → Actions
tab. See Actions Properties for more information on actions.
Saving selected features as new layer
The selected features can be saved as any OGR-supported vector format and also transformed into another coordinate reference system (CRS). In the contextual menu of the layer, from the Layers Panel, click on Save as to
define the name of the output file, its format and CRS (see section Creating new layers from an existing layer).
To save the selection ensure that the
creation options within the dialog.
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12.4.5 Editing attribute values
Editing attribute values can be done by:
• typing the new value directly in the cell, whether the attribute table is in table or form view. Changes are
hence done cell by cell, feature by feature;
• using the field calculator: update in a row a field that may already exist or to be created but for multiple
features; it can be used to create virtual fields.
• using the quick field calculation bar: same as above but for only existing field
• or using the multi edit mode: update in a row multiple fields for multiple features.
Using the Field Calculator
Field Calculator
button in the attribute table allows you to perform calculations on the basis of existing
The
attribute values or defined functions, for instance, to calculate length or area of geometry features. The results can
be written to a new attribute field, a virtual field, or they can be used to update values in an existing field.
The field calculator is available on any layer that supports edit. When you click on the field calculator icon the
dialog opens (see figure_field_calculator). If the layer is not in edit mode, a warning is displayed and using the
field calculator will cause the layer to be put in edit mode before the calculation is made.
Based on the Expression Builder dialog, the field calculator dialog offers a complete interface to define an expression and apply it to an existing or a newly created field. To use the field calculator dialog, you first must select
whether you want to only update selected features, create a new attribute field where the results of the calculation
will be added or update an existing field.
Fig. 12.74: Field Calculator
If you choose to add a new field, you need to enter a field name, a field type (integer, real, date or string) and if
needed, the total field length and the field precision. For example, if you choose a field length of 10 and a field
precision of 3, it means you have 6 digits before the dot, then the dot and another 3 digits for the precision.
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A short example illustrates how field calculator works when using the Expression tab. We want to calculate the
length in km of the railroads layer from the QGIS sample dataset:
Open Attribute Table
1. Load the shapefile railroads.shp in QGIS and press
2. Click on
3. Select the
Toggle editing mode
and open the
Field Calculator
.
dialog.
Create a new field checkbox to save the calculations into a new field.
4. Add length as Output field name and real as Output field type, and define Output field length to be 10
and Precision, 3.
5. Now double click on function $length in the Geometry group to add it into the Field calculator expression
box.
6. Complete the expression by typing / 1000 in the Field calculator expression box and click [Ok].
7. You can now find a new field length in the attribute table.
Creating a Virtual Field
A virtual field is a field based on an expression calculated on the fly, meaning that its value is automatically updated
as soon as the underlying parameter changes. The expression is set once; you no longer need to recalculate the
field each time underlying values change. For example, you may want to use a virtual field if you need area to
be evaluated as you digitize features or to automatically calculate a duration between dates that may change (e.g.,
using now() function).
Note: Use of Virtual Fields
• Virtual fields are not permanent in the layer attributes, meaning that they’re only saved and available in the
project file they’ve been created.
• A field can be set virtual only at its creation and the expression used can’t be changed later: you’ll need to
delete and recreate that field.
Using the Quick Field Calculation Bar
While Field calculator is always available, the quick field calculation bar on top of the attribute table is only visible
if the layer is in edit mode. Thanks to the expression engine, it offers a quicker access to edit an already existing
field.
In quick field calculation bar, you simply need to:
• select the existing field name in the drop-down list
• fill the textbox with an expression you directly write or build using the
expression button
• and click on [Update All], [Update Selected] or [Update Filtered] button according to your need.
Editing multiple fields
Unlike the previous tools, multi edit mode allows multiple attributes of different features to be edited simultaneously. When the layer is toggled to edit, multi edit capabilities are accessible:
• using the
Toggle multi edit mode
• or selecting Edit →
button from the toolbar inside the attribute table dialog,
Modify attributes of selected features menu.
In order to edit multiple fields in a row:
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1. select the features you want to edit;
2. from the attribute table toolbar, click the
selection could also be made at this step;
button. This will toggle the dialog to its form view. Feature
3. at the right side of the attribute table, fields (and values) of selected features are shown. New widgets appear
next to each field allowing for display of the current multi edit state:
•
the field contains different values for selected features. It’s shown empty and each feature will
keep its original value. You can reset the value of the field from the drop-down list of the widget.
•
all selected features have the same value for this field and the value displayed in the form will be
kept.
•
the field has been edited and the entered value will be applied to all the selected features. A
message appears at the top of the dialog, inviting you to either apply or reset your modification.
Clicking any of these widgets allows you to either set the current value for the field or reset to original value,
meaning that you can roll back changes on a field-by-field basis.
4. make the changes to the fields you want and click on Apply changes in the upper message text or any other
feature in the left panel.
Changes will apply to all selected features. If no feature is selected, the whole table is updated with your changes.
Modifications are made as a single edit command. So pressing
selected features at once.
Undo
will rollback the attribute changes for all
Note: Unlike the tool from the attribute table, hitting the Edit → Modify Attributes of Selected Features option
provides you with a modal dialog to fill attributes changes. Hence, features selection is required before execution.
Fig. 12.75: Editing fields of multiple features
Note: Multi edit mode is only available for auto generated and drag and drop forms (see Customize a form for
your data); it is not supported by custom ui forms.
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12.4.6 Creating one or many to many relations
Relations are a technique often used in databases. The concept is that features (rows) of different layers (tables)
can belong to each other.
Introducing 1-N relations
As an example you have a layer with all regions of alaska (polygon) which provides some attributes about its name
and region type and a unique id (which acts as primary key).
Then you get another point layer or table with information about airports that are located in the regions and you
also want to keep track of these. If you want to add them to the regions layer, you need to create a one to many
relation using foreign keys, because there are several airports in most regions.
Fig. 12.76: Alaska region with airports
Layers in 1-N relations
QGIS makes no difference between a table and a vector layer. Basically, a vector layer is a table with a geometry.
So you can add your table as a vector layer. To demonstrate the 1-n relation, you can load the regions shapefile
and the airports shapefile which has a foreign key field (fk_region) to the layer regions. This means, that
each airport belongs to exactly one region while each region can have any number of airports (a typical one to
many relation).
Foreign keys in 1-N relations
In addition to the already existing attributes in the airports attribute table, you’ll need another field fk_region
which acts as a foreign key (if you have a database, you will probably want to define a constraint on it).
This field fk_region will always contain an id of a region. It can be seen like a pointer to the region it belongs
to. And you can design a custom edit form for editing and QGIS takes care of the setup. It works with different
providers (so you can also use it with shape and csv files) and all you have to do is to tell QGIS the relations
between your tables.
Defining 1-N relations (Relation Manager)
The first thing we are going to do is to let QGIS know about the relations between the layers. This is done in
Project → Project Properties. . . . Open the Relations tab and click on [Add Relation].
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• name is going to be used as a title. It should be a human readable string, describing, what the relation is
used for. We will just call say Airports in this case.
• referencing layer also considered as child layer, is the one with the foreign key field on it. In our case, this
is the airports layer
• referencing field will say, which field points to the other layer so this is fk_region in this case
• referenced layer also considered as parent layer, is the one with the primary key, pointed to, so here it is
the regions layer
• referenced field is the primary key of the referenced layer so it is ID
• id will be used for internal purposes and has to be unique. You may need it to build custom forms. If you
leave it empty, one will be generated for you but you can assign one yourself to get one that is easier to
handle.
Fig. 12.77: Relation Manager
Forms for 1-N relations
Now that QGIS knows about the relation, it will be used to improve the forms it generates. As we did not change
the default form method (autogenerated) it will just add a new widget in our form. So let’s select the layer region
in the legend and use the identify tool. Depending on your settings, the form might open directly or you will have
to choose to open it in the identification dialog under actions.
As you can see, the airports assigned to this particular region are all shown in a table. And there are also some
buttons available. Let’s review them shortly
• The
button is for toggling the edit mode. Be aware that it toggles the edit mode of the airport layer,
although we are in the feature form of a feature from the region layer. But the table is representing features
of the airport layer.
button will add a new feature to the airport layer. And it will assign the new airport to the current
• The
region by default.
• The
button will delete the selected airport permanently.
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Fig. 12.78: Identification dialog regions with relation to airports
• The
symbol will open a new dialog where you can select any existing airport which will then be assigned
to the current region. This may be handy if you created the airport on the wrong region by accident.
• The
symbol will unlink the selected airport from the current region, leaving them unassigned (the
foreign key is set to NULL) effectively.
• The two buttons to the right switch between table view and form view where the later let’s you view all the
airports in their respective form.
If you work on the airport table, a new widget type is available which lets you embed the feature form of the
referenced region on the feature form of the airports. It can be used when you open the layer properties of the
airports table, switch to the Fields menu and change the widget type of the foreign key field ‘fk_region’ to Relation
Reference.
If you look at the feature dialog now, you will see, that the form of the region is embedded inside the airports form
and will even have a combobox, which allows you to assign the current airport to another region.
Fig. 12.79: Identification dialog airport with relation to regions
Introducing many-to-many (N-M) relations
N-M relations are many-to-many relation between two tables. For instance, the airports and airlines
layers: an airport receives several airline companies and an airline company flies to several airports.
In such case, we need a pivot table to list all airlines for all airports. In QGIS, you should setup two one-to-many
relations as explained above:
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• a relation between airlines table and the pivot table;
• and a second one between airports table and the pivot table.
When we add a new child (i.e. a company to an airport), QGIS will add a new row in the pivot table and in the
airlines table. If we link a company to an airport, QGIS will only add a row in the pivot table.
In case you want to remove a link, an airline or an airport, QGIS won’t remove the row in the pivot table. The
database administrator should add a ON DELETE CASCADE instruction in the foreign key constraint:
ALTER TABLE location.airlines
ADD CONSTRAINT location_airlines_airports_id_fkey
FOREIGN KEY (id)
REFERENCES location.airports(id)
ON DELETE CASCADE;
Note: Combining N-M relation with automatic transaction group
You should enable the transaction mode in Project Properties → Data Sources → when working on such context.
QGIS should be able to add or update row(s) in all tables (airlines, airports and the pivot tables).
Finally, adding such relations in a form is done in the same way that for a one-to-many relation. The Relations
panel in the Fields properties of the vector layer will let the user add the relation in the form. It will appear as a
Many to many relation.
12.5 Editing
QGIS supports various capabilities for editing OGR, SpatiaLite, PostGIS, MSSQL Spatial and Oracle Spatial
vector layers and tables.
Note: The procedure for editing GRASS layers is different - see section Digitizing and editing a GRASS vector
layer for details.
Tip: Concurrent Edits
This version of QGIS does not track if somebody else is editing the same feature at the same time as you are. The
last person to save its edits wins.
12.5.1 Setting the Snapping Tolerance and Search Radius
For an optimal and accurate edit of the vector layer geometries, we need to set an appropriate value of snapping
tolerance and search radius for features vertices.
Snapping tolerance
Snapping tolerance is the distance QGIS uses to search for the closest vertex and/or segment you are trying to
connect to when you set a new vertex or move an existing vertex. If you aren’t within the snapping tolerance,
QGIS will leave the vertex where you release the mouse button, instead of snapping it to an existing vertex and/or
segment. The snapping tolerance setting affects all tools that work with tolerance.
1. A general, project-wide snapping tolerance can be defined by choosing Settings → Options. . . , Digitizing
tab. You can select between ‘To vertex’, ‘To segment’ or ‘To vertex and segment’ as default snap mode.
You can also define a default snapping tolerance and a search radius for vertex edits. The tolerance can be
set either in map units or in pixels. The advantage of choosing pixels is that the snapping tolerance doesn’t
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have to be changed after zoom operations. In our small digitizing project (working with the Alaska dataset),
we define the snapping units in feet. Your results may vary, but something on the order of 300 ft at a scale
of 1:10000 should be a reasonable setting.
2. A layer-based snapping tolerance that overrides the global snapping options can be defined by choosing
Project → Snapping options. It helps to enable and adjust snapping mode and tolerance on a layer basis
(see figure_edit_snapping). This dialog offers three different modes to select the layer(s) to snap to:
• Current layer: only the active layer is used, a convenient way to ensure topology within the layer being
edited
• All layers: a quick and simple setting for all visible layers in the project so that the pointer snaps to all
vertices and/or segments. In most cases it is sufficient to use this snapping mode.
• Advanced: if you need to edit a layer and snap its vertices to another layer, ensure the target layer
is checked and increase the snapping tolerance to a greater value. Furthermore, snapping will never
occur to a layer that is not checked in the snapping options dialog, regardless of the global snapping
tolerance. So be sure to mark the checkbox for those layers that you need to snap to.
Fig. 12.80: Edit snapping options on a layer basis (Advanced mode)
Tip: Control the list of layers to snap
The Snapping Options dialog is by default populated with parameters (mode, tolerance, units) set in the global
Digitizing tab. To avoid layers being checked by default in the Advanced mode and hence set snappable, define
the Default Snap mode to Off.
Snapping tolerance can be set in pixels or map units (the units of the map view). While in the Advanced
layer selection mode, it is possible to use a snapping tolerance that refers to layer units, the units of the
reprojected layer when ‘on-the-fly’ CRS transformation is on.
Search radius
Search radius is the distance QGIS uses to search for the closest vertex you are trying to select when you click
on the map. If you aren’t within the search radius, QGIS won’t find and select any vertex for editing. The search
radius for vertex edits can be defined under Settings → Options → Digitizing tab. This is the same place where
you define the general, project-wide snapping tolerance.
Snap tolerance and search radius are set in map units or pixels, so you may find you need to experiment to
get them set right. If you specify too big of a tolerance, QGIS may snap to the wrong vertex, especially if you are
dealing with a large number of vertices in close proximity. Set search radius too small, and it won’t find anything
to move.
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12.5.2 Topological editing
Besides layer-based snapping options, you can also define topological functionalities in the Snapping options. . .
dialog in the Settings (or File) menu. Here, you can define
layers, activate the
Enable topological editing, and/or for polygon
Avoid Intersections option.
Enable topological editing
Enable topological editing is for editing and maintaining common boundaries in features mosaics.
The option
QGIS ‘detects’ shared boundary by the features, so you only have to move a common vertex/segment once, and
QGIS will take care of updating the neighboring features.
Avoid intersections of new polygons
Avoid intersections prevents you to draw new features that overlap an
A second topological option called
existing one. It is for quicker digitizing of adjacent polygons. If you already have one polygon, it is possible
with this option to digitize the second one such that both intersect, and QGIS then cuts the second polygon to
the boundary of the existing one. The advantage is that you don’t have to digitize all vertices of the common
boundary.
Note: If the new geometry is totally covered by existing ones, it gets cleared and the new feature will have no
geometry when allowed by the provider, otherwise saving modifications will make QGIS pop-up an error message.
Warning: Use cautiously the Avoid Intersections option
Because the option cuts or clears geometry of any overlaping feature from any polygon layer, do not forget to
uncheck this option once you no longer need it otherwise, you can get unexpected geometries.
Enable snapping on intersections
Another option is to use
Enable snapping on intersection. It allows you to snap on an intersection of background layers, even if there’s no vertex on the intersection.
Geometry Checker
A core plugin can help the user to find the geometry invalidity. You can find more information on this plugin at
Geometry Checker Plugin.
12.5.3 Digitizing an existing layer
By default, QGIS loads layers read-only. This is a safeguard to avoid accidentally editing a layer if there is a slip
of the mouse. However, you can choose to edit any layer as long as the data provider supports it (see Exploring
Data Formats and Fields), and the underlying data source is writable (i.e., its files are not read-only).
Tip: Restrict edit permission on layers within a project
From the Project → Project properties → Identify tab, You can choose to set any layer read-only regardless
the provider permission. This can be a handy way, in a multi-users environment to avoid unauthorized users to
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mistakenly edit layers (e.g., shapefile), hence potentially corrupt data. Note that this setting only applies inside
the current project.
In general, tools for editing vector layers are divided into a digitizing and an advanced digitizing toolbar, described
in section Advanced digitizing. You can select and unselect both under View → Toolbars →. Using the basic
digitizing tools, you can perform the following functions:
Icon
Purpose
Icon
Purpose
Current edits
Toggle editing
Add Feature: Capture Point
Add Feature: Capture Line
Add Feature: Capture Polygon
Move Feature
Add Circular String
Add Circular String By Radius
Vertex tool
Delete Selected
Cut Features
Copy Features
Paste Features
Save layer edits
Table Editing: Vector layer basic editing toolbar
Note that while using any of the digitizing tools, you can still zoom or pan in the map canvas without losing the
focus on the tool.
Toggle editing
All editing sessions start by choosing the
option found in the context menu of a given layer, from
the attribute table dialog, the digitizing toolbar or the Edit menu.
Once the layer is in edit mode, additional tool buttons on the editing toolbar will become available and markers
will appear at the vertices of all features unless Show markers only for selected features option under Settings →
Options. . . → Digitizing menu is checked.
Tip: Save Regularly
Remember to
Save Layer Edits
regularly. This will also check that your data source can accept all the changes.
Adding Features
Add Feature
Add Feature
,
or
You can use the
and polygon) into the current layer.
The next buttons
circular geometry.
Add circular string
or
Add Feature
icons on the toolbar to add new feature (point, line
Add circular string by radius
allow users to add line or polygon features with a
To create features with these tools, you first digitize the geometry then enter its attributes. To digitize the geometry,
left-click on the map area to create the first point of your new feature.
For linear or curved geometries, keep on left-clicking for each additional point you wish to capture or use automatic tracing capability to accelerate the digitization. You can switch back and forth between linear Add feature tool and curved Add circular string. . . tools to create compound curved geometry. Pressing Delete or
Backspace key reverts the last node you add. When you have finished adding points, right-click anywhere on
the map area to confirm you have finished entering the geometry of that feature.
Note: Curved geometries are stored as such only in compatible data provider
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Although QGIS allows to digitize curved geometries within any editable data format, you need to be using a data
provider (e.g. PostGIS, GML or WFS) that supports curves to have features stored as curved, otherwise QGIS
segmentizes the circular arcs. The memory layer provider also supports curves.
Tip: Customize the digitizing rubber band
While capturing polygon, the by-default red rubber band can hide underlying features or places you’d like to
capture a point. This can be fixed by setting a lower opacity (or alpha channel) to the rubber band’s Fill Color in
Settings → Options → Digitizing menu. You can also avoid the use of the rubber band by checking Don’t update
rubber band during node editing.
The attribute window will appear, allowing you to enter the information for the new feature. Figure_edit_values
shows setting attributes for a fictitious new river in Alaska. However, in the Digitizing menu under the Settings →
Options menu, you can also activate:
•
Suppress attributes pop-up windows after each created feature to avoid the form opening
• or
Reuse last entered attribute values to have fields automatically filled at the opening of the form and
just have to type changing values.
Fig. 12.81: Enter Attribute Values Dialog after digitizing a new vector feature
With the
Move Feature(s)
icon on the toolbar, you can move existing features.
Vertex tool
Note: QGIS 3 major changes
In QGIS 3, the node tool has been fully redesigned and renamed. It was previously working with “click and drag”
ergonomy, and now uses a “click - click” workflow. This allows major improvements like taking profit of the
advanced digitizing panel with the vertex tool while digitizing or editing objects of multiple layers at the same
time.
Vertex tool
For any editable vector layer, the
provides manipulation capabilities of feature vertices similar to CAD
programs. It is possible to simply select multiple vertices at once and to move, add or delete them altogether. The
vertex tool also works with ‘on the fly’ projection turned on and supports the topological editing feature. This tool
is selection persistent, so when some operation is done, selection stays active for this feature and tool.
Options → Digitizing → Search Radius:
to a number
It is important to set the property Settings →
greater than zero. Otherwise, QGIS will not be able to tell which vertex is being edited and will display a warning.
Tip: Vertex Markers
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The current version of QGIS supports three kinds of vertex markers: ‘Semi-transparent circle’, ‘Cross’ and ‘None’.
To change the marker style, choose
Options from the Settings menu, click on the Digitizing tab and select the
appropriate entry.
Basic operations
Start by activating the
Vertex tool
. Red circles will appear when hovering vertices.
• Selecting vertices: You can select vertices by clicking on them one at a time holding Shift key pressed,
or by clicking and dragging a rectangle around some vertices. When a vertex is selected, its color changes
to blue. To add more vertices to the current selection, hold down the Shift key while clicking. To remove
vertices from the selection, hold down Ctrl.
• Batch vertex selection mode: The batch selection mode can be activated by pressing Shift + R. Select
a first node with one single click, and then hover without clicking another vertex. This will dynamically
select all the nodes in between using the shortest path (for polygons).
Fig. 12.82: Batch vertex selection using Shift + R
Press Ctrl will invert the selection, selecting the longest path along the feature boundary. Ending your
node selection with a second click, or pressing Esc will escape the batch mode.
• Adding vertices: To add a vertex, a virtual new node appears on the segment center. Simply grab it to add a
new vertex. Double click on any location of the boundary also creates a new node. For lines, a virtual node
is also proposed at both extremities of a line to extend it.
• Deleting vertices: Select the vertices and click the Delete key. Deleting all the vertices of a feature
generates, if compatible with the datasource, a geometryless feature. Note that this doesn’t delete the
complete feature, just the geometry part; To delete a complete feature use the
Delete Selected
tool.
• Moving vertices: Select all the vertices you want to move, click on a selected vertex or edge, and click
again on the desired new location. All the selected vertices will move together. If snapping is enabled, the
whole selection can jump to the nearest vertex or line. You can use Advanced Digitizing Panel constraints
for distance, angles, exact X Y location before the second click.
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Fig. 12.83: Virtual nodes for adding vertices
Each change made with the vertex is stored as a separate entry in the Undo dialog. Remember that all operations
support topological editing when this is turned on. On-the-fly projection is also supported, and the node tool
provides tooltips to identify a vertex by hovering the pointer over it.
Tip: Move features with vertex tool
Vertex tool
is a way of moving a whole feature, select ALL the vertices of the feature, click a vertex,
Using the
drag and snap it to a target vertex: the whole feature is moved and snapped to the other feature. In QGIS 2, the
Move Feature tool didn’t support snapping. This is now fixed since it takes advantage of the new click-click
ergonomy, allowing interactive constraints and snapping of features while moving.
The Vertex Editor Panel
When using the Vertex tool on a feature, it is possible to right click to open the Vertex Editor panel listing all
the vertices of the feature with their x, y (z, m if applicable) coordinates and r (for the radius, in case of circular
geometry). Simply select a row in the table does select the corresponding vertex in the map canvas, and vice versa.
Simply change a coordinate in the table and your vertex position is updated. You can also select multiple rows
and delete them altogether.
Note: Changed behavior in QGIS3
In QGIS 2.x, the panel was opening each time the vertex tool was used which was slow and confusing when
editing big features. Now, just invoke it with a right - click.
Fig. 12.84: Button to open the vertex editor panel via right-click
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Fig. 12.85: Vertex editor panel showing selected nodes
Cutting, Copying and Pasting Features
Selected features can be cut, copied and pasted between layers in the same QGIS project, as long as destination
layers are set to
Toggle editing
beforehand.
Tip: Transform polygon into line and vice-versa using copy/paste
Copy a line feature and paste it in a polygon layer: QGIS pastes in the target layer a polygon whose boundary
corresponds to the closed geometry of the line feature. This is a quick way to generate different geometries of the
same data.
Features can also be pasted to external applications as text. That is, the features are represented in CSV format,
with the geometry data appearing in the OGC Well-Known Text (WKT) format. WKT and GeoJSON features
from outside QGIS can also be pasted to a layer within QGIS.
When would the copy and paste function come in handy? Well, it turns out that you can edit more than one layer
at a time and copy/paste features between layers. Why would we want to do this? Say we need to do some work
on a new layer but only need one or two lakes, not the 5,000 on our big_lakes layer. We can create a new layer
and use copy/paste to plop the needed lakes into it.
As an example, we will copy some lakes to a new layer:
1. Load the layer you want to copy from (source layer)
2. Load or create the layer you want to copy to (target layer)
3. Start editing for target layer
4. Make the source layer active by clicking on it in the legend
5. Use the
6. Click on the
Select Features by area or single click
Copy Features
tool to select the feature(s) on the source layer
tool
7. Make the destination layer active by clicking on it in the legend
8. Click on the
Paste Features
tool
9. Stop editing and save the changes
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What happens if the source and target layers have different schemas (field names and types are not the same)?
QGIS populates what matches and ignores the rest. If you don’t care about the attributes being copied to the target
layer, it doesn’t matter how you design the fields and data types. If you want to make sure everything - the feature
and its attributes - gets copied, make sure the schemas match.
Note: Congruency of Pasted Features
If your source and destination layers use the same projection, then the pasted features will have geometry identical
to the source layer. However, if the destination layer is a different projection, then QGIS cannot guarantee the geometry is identical. This is simply because there are small rounding-off errors involved when converting between
projections.
Tip: Copy string attribute into another
If you have created a new column in your attribute table with type ‘string’ and want to paste values from another
attribute column that has a greater length the length of the column size will be extended to the same amount. This
is because the GDAL Shapefile driver starting with GDAL/OGR 1.10 knows to auto-extend string and integer
fields to dynamically accommodate for the length of the data to be inserted.
Deleting Selected Features
If we want to delete an entire feature (attribute and geometry), we can do that by first selecting the geometry using
the regular
Select Features by area or single click
tool. Selection can also be done from the attribute table. Once you
have the selection set, press Delete or Backspace key or use the
Multiple selected features can be deleted at once.
The
Cut Features
Delete Selected
tool to delete the features.
tool on the digitizing toolbar can also be used to delete features. This effectively deletes the
feature but also places it on a “spatial clipboard”. So, we cut the feature to delete. We could then use the
Paste Features
tool to put it back, giving us a one-level undo capability. Cut, copy, and paste work on the currently
selected features, meaning we can operate on more than one at a time.
Saving Edited Layers
When a layer is in editing mode, any changes remain in the memory of QGIS. Therefore, they are not committed/saved immediately to the data source or disk. If you want to save edits to the current layer but want to continue
editing without leaving the editing mode, you can click the
Save Layer Edits
button. When you turn editing mode
Toggle editing
off with
(or quit QGIS for that matter), you are also asked if you want to save your changes or
discard them.
If the changes cannot be saved (e.g., disk full, or the attributes have values that are out of range), the QGIS
in-memory state is preserved. This allows you to adjust your edits and try again.
Tip: Data Integrity
It is always a good idea to back up your data source before you start editing. While the authors of QGIS have
made every effort to preserve the integrity of your data, we offer no warranty in this regard.
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Saving multiple layers at once
This feature allows the digitization of multiple layers. Choose
Save for Selected Layers to save all changes
you made in multiple layers. You also have the opportunity to
Rollback for Selected Layers, so that the
digitization may be withdrawn for all selected layers. If you want to stop editing the selected layers,
for Selected Layer(s) is an easy way.
Cancel
The same functions are available for editing all layers of the project.
Tip: Use transaction group to edit, save or rollback multiple layers changes at once
When working with layers from the same PostGreSQL database, activate the Automatically create transaction
groups where possible option in Project → Project Properties → Data Sources to sync their behavior (enter or
exit the edit mode, save or rollback changes at the same time).
12.5.4 Advanced digitizing
Icon
Purpose
Icon
Purpose
Enable Advanced Digitizing Tools
Enable Tracing
Undo
Redo
Rotate Feature(s)
Simplify Feature
Add Ring
Add Part
Fill Ring
Delete Ring
Delete Part
Offset Curve
Reshape Features
Split Parts
Split Features
Merge Attributes of Selected Features
Merge Selected Features
Rotate Point Symbols
Offset Point Symbols
Table Advanced Editing: Vector layer advanced editing toolbar
Undo and Redo
Undo
Redo
The
and
tools allows you to undo or redo vector editing operations. There is also a dockable
widget, which shows all operations in the undo/redo history (see Figure_edit_undo). This widget is not displayed
by default; it can be displayed by right-clicking on the toolbar and activating the Undo/Redo Panel checkbox. The
Undo/Redo capability is however active, even if the widget is not displayed.
When Undo is hit or Ctrl+Z (or Cmd+Z) pressed, the state of all features and attributes are reverted to the
state before the reverted operation happened. Changes other than normal vector editing operations (for example,
changes done by a plugin) may or may not be reverted, depending on how the changes were performed.
To use the undo/redo history widget, simply click to select an operation in the history list. All features will be
reverted to the state they were in after the selected operation.
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Fig. 12.86: Redo and Undo digitizing steps
Rotate Feature(s)
Rotate Feature(s)
Rotate Feature(s)
to rotate one or multiple features in the map canvas. Press the
icon and
Use
then click on the feature to rotate. Either click on the map to place the rotated feature or enter an angle in the user
input widget. If you want to rotate several features, they shall be selected first.
If you enable the map tool with feature(s) selected, its (their) centroid appears and will be the rotation anchor
point. If you want to move the anchor point, hold the Ctrl button and click on the map to place it.
If you hold Shift before clicking on the map, the rotation will be done in 45 degree steps, which can be modified
afterwards in the user input widget.
To abort feature rotation, you need to click on
Rotate Feature(s)
icon.
Simplify Feature
Simplify Feature
The
tool allows you to reduce the number of vertices of a feature, as long as the geometry
remains valid. With the tool you can also simplify many features at once or multi-part features.
First, click on the feature or drag a rectangle over the features. A dialog where you can define a tolerance in map
units, layer units or pixels pops up and a colored and simplified copy of the feature(s), using the given
tolerance, appears over them. QGIS calculates the amount of vertices that can be deleted while maintaining the
geometry. The higher the tolerance is the more vertices can be deleted. When the expected geometry fits your
needs just click the [OK] button. The tolerance you used will be saved when leaving a project or when leaving an
edit session. So you can go back to the same tolerance the next time when simplifying a feature.
To abort feature simplification, you need to click on
Simplify Feature
icon.
Note: Unlike the feature simplification option in Settings → Options → Rendering menu which simplifies the
geometry just for rendering, the
Simplify Feature
tool permanently modifies feature’s geometry in data source.
Add Part
Add Part
You can
to a selected feature generating a multipoint, multiline or multipolygon feature. The new part
must be digitized outside the existing one which should be selected beforehand.
The
Add Part
can also be used to add a geometry to a geometryless feature. First, select the feature in the
attribute table and digitize the new geometry with the
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Add Part
tool.
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Delete Part
Delete Part
tool allows you to delete parts from multifeatures (e.g., to delete polygons from a multi-polygon
The
feature). This tool works with all multi-part geometries: point, line and polygon. Furthermore, it can be used to
totally remove the geometric component of a feature. To delete a part, simply click within the target part.
Add Ring
Add Ring
You can create ring polygons using the
icon in the toolbar. This means that inside an existing area, it
is possible to digitize further polygons that will occur as a ‘hole’, so only the area between the boundaries of the
outer and inner polygons remains as a ring polygon.
Fill Ring
You can use the
Fill Ring
function to add a ring to a polygon and add a new feature to the layer at the same time.
Using this tool, you simply have to digitize a polygon within an existing one. Thus you need not first use the
Add Ring
icon and then the
Add feature
function anymore.
Delete Ring
Delete Ring
tool allows you to delete rings within an existing polygon, by clicking inside the hole. This
The
tool only works with polygon and multi-polygon features. It doesn’t change anything when it is used on the outer
ring of the polygon.
Reshape Features
Reshape Features
You can reshape line and polygon features using the
tool on the toolbar. For lines, it replaces the
line part from the first to the last intersection with the original line.
Fig. 12.87: Reshape line
Tip: Extend linestring geometries with reshape tool
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Reshape Features
Use the
tool to extend existing linestring geometries: snap to the first or last vertex of the line
and draw a new one. Validate and the feature’s geometry becomes the combination of the two lines.
For polygons, it will reshape the polygon’s boundary. For it to work, the reshape tool’s line must cross the
polygon’s boundary at least twice. To draw the line, click on the map canvas to add vertexes. To finish it, just
right-click. Like with the lines, only the segment between the first and the last intersections is considered. The
reshape line’s segments that are inside the polygon will result in cropping it, where the ones outside the polygon
will extend it.
Fig. 12.88: Reshape polygon
With polygons, reshaping can sometimes lead to unintended results. It is mainly useful to replace smaller parts
of a polygon, not for major overhauls, and the reshape line is not allowed to cross several polygon rings, as this
would generate an invalid polygon.
Note: The reshape tool may alter the starting position of a polygon ring or a closed line. So, the point that is
represented ‘twice’ will not be the same any more. This may not be a problem for most applications, but it is
something to consider.
Offset Curves
Offset Curve
The
tool creates parallel shifts of line layers. The tool can be applied to the edited layer (the
geometries are modified) or also to background layers (in which case it creates copies of the lines / rings and adds
them to the edited layer). It is thus ideally suited for the creation of distance line layers. The User Input dialog
pops-up, showing the displacement distance.
Offset Curve
tool. Then click
To create a shift of a line layer, you must first go into editing mode and activate the
on a feature to shift it. Move the mouse and click where wanted or enter the desired distance in the user input
widget. Your changes may then be saved with the
Save Layer Edits
tool.
QGIS options dialog (Digitizing tab then Curve offset tools section) allows you to configure some parameters
like Join style, Quadrant segments, Miter limit.
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Split Features
You can split features using the
split.
Split Features
icon on the toolbar. Just draw a line across the feature you want to
Split parts
In QGIS it is possible to split the parts of a multi part feature so that the number of parts is increased. Just draw a
line across the part you want to split using the
Split Parts
icon.
Tip: Split a polyline feature in one-click
A single click on a snapped vertex of a line feature with the
it split into new features or parts.
Split Features
or
Split Parts
tool is enough to have
Merge selected features
Merge Selected Features
The
tool allows you to create a new feature by merging existing ones: their geometries are merged to generate a new one. If features don’t have common boundaries, a multipolygon/multipolyline/multipoint feature is created.
Merge Selected Features
button. In the new dialog, you can select at the
First, select several features. Then press the
top of the dialog which value to apply to each field of the new feature. That value can be:
• picked from the attributes of the initial features,
• an aggregation of the initial features attributes (Minimum, Maximum, Median, Sum, Count Concatenation. . . depending on the type of the field. see Statistical Summary Panel for the full list of functions),
• skipped, meaning that the field will be empty,
• or manually entered, at the bottom of the rows.
Merge attributes of selected features
Merge Attributes of Selected Features
The
tool allows you to apply same attributes to features without merging their
boundaries. The dialog is the same as the Merge Selected Features tool’s except that unlike that tool,
selected objects are kept with their geometry while some of their attributes are made identical.
Rotate Point Symbols
The
Rotate Point Symbols
allows you to change the rotation of point symbols in the map canvas. First of all, you
must apply to the symbol a data-defined rotation: in the Layer Properties → Style dialog, click the
Datadefined override widget near the Rotation option of the highest level (preferably) of the symbol layers and choose
a field in the Field Type combobox. Values of this field are hence used to rotate each feature’s symbol accordingly.
Note: As a global option, setting the rotation field at the first level of the symbol applies it to all the underlying
levels while setting it at a lower level will rotate only this symbol layer (unless you have a single symbol layer).
Rotate Point Symbols
To change the rotation of a symbol, click on a point feature in the map canvas with the
and
move the mouse around, holding the left button pressed. A red arrow with the rotation value will be visualized
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Fig. 12.89: Rotate Point Symbols
(see Figure_rotate_point). When you release the left mouse button again, the symbol is defined with this new
rotation and the rotation field is updated in the layer’s attribute table.
Tip: If you hold the Ctrl key pressed, the rotation will be done in 15 degree steps.
Offset Point Symbols
The
Offset Point Symbols
allows you to interactively change the rendered position of point symbols in the map
Rotate Point Symbols
tool except that it requires you to connect a field to the
canvas. This tool behaves like the
data-defined Offset (X,Y) property of the symbol, field which will then be populated with the offset coordinates
while moving the symbol in the map canvas.
Offset Point Symbols
Note: The
tool doesn’t move the point feature itself; you should use the
Move Feature
tool for this purpose.
Vertex tool
or
Warning: Ensure to assign the same field to all symbol layers
If at least two layers of the symbol have different fields assigned to their data-defined property (e.g. rotation),
the corresponding tool will consider that no field is assigned to the symbol property and won’t perform the
action.
Automatic Tracing
Usually, when using capturing map tools (add feature, add part, add ring, reshape and split), you need to click
each vertex of the feature.
Tracing
Using the automatic tracing mode you can speed up the digitization process. Enable the
tool by pushing
the icon or pressing t key and snap to a vertex or segment of a feature you want to trace along. Move the mouse
over another vertex or segment you’d like to snap and instead of an usual straight line, the digitizing rubber band
represents a path from the last point you snapped to the current position. QGIS actually uses the underlying
features topology to build the shortest path between the two points. Click and QGIS places the intermediate
vertices following the path. You no longer need to manually place all the vertices during digitization.
Tracing requires snapping to be activated in traceable layers to build the path. You should also snap to an existing
vertex or segment while digitizing and ensure that the two nodes are topologically connectable following existing
features, otherwise QGIS is unable to connect them and thus traces a single straight line.
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Note: Adjust map scale or snapping settings for an optimal tracing
If there are too many features in map display, tracing is disabled to avoid potentially long tracing structure preparation and large memory overhead. After zooming in or disabling some layers the tracing is enabled again.
Tip: Quickly enable or disable automatic tracing by pressing t key
By pressing t key, tracing can be enabled/disabled anytime even while digitizing one feature, so it is possible to
digitize some parts of the feature with tracing enabled and other parts with tracing disabled. Tools behave as usual
when tracing is disabled.
12.5.5 The Advanced Digitizing panel
When capturing, reshaping, splitting new or existing geometries you also have the possibility to use the Advanced
Digitizing panel. You can digitize lines exactly parallel or perpendicular to a particular angle or lock lines to
specific angles. Furthermore, you can enter coordinates directly so that you can make a precise definition of your
new geometry.
Fig. 12.90: The Advanced Digitizing panel
Note: The tools are not enabled if the map view is in geographic coordinates.
The Advanced Digitizing panel can be open either with a right-click on the toolbar and choose Advanced Digitizing panel or in View → Panels → Advanced Digitizing Panel. Once the panel is visible, click the
enable advanced digitizing tool
button to activate the Advanced Digitizing tool.
Concepts
The aim of the Advanced Digitizing tool is to lock coordinates, lengths, and angles when moving the mouse during
the digitalizing in the map canvas.
You can also create constraints with relative or absolute reference. Relative reference means that the next vertex
constraints’ values will be relative to the previous vertex or segment.
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Snapping Settings
button to set the Advanced Digitizing Tool snapping settings. You can make the tool snap to common
Click the
angles. The options are:
• Do not snap to common angles
• Snap to 30º angles
• Snap to 45º angles
• Snap to 90º angles
You can also control the snapping to features. The options are:
• Do not snap to vertices or segments
• Snap according to project configuration
• Snap to all layers
Keyboard shortcuts
To speed up the use of Advanced Digitizing Panel, there are a couple of keyboard shorcuts available:
Key
d
a
x
y
c
p
Simple
Ctrl + or Alt +
Set distance
Lock distance
Set angle
Lock angle
Set x coordinate Lock x coordinate
Set y coordinate Lock y coordinate
Toggle construction mode
Toggle perpendicular and parallel modes
Shift +
Toggle relative angle to last segment
Toggle relative x to last vertex
Toggle relative y to last vertex
Absolute reference digitizing
When drawing a new geometry from scratch, it is very useful to have the possibility to start digitizing vertexes at
given coordinates.
For example, to add a new feature to a polygonal layer, click the
coordinates where you want to start editing the feature, then:
button. You can choose the X and Y
• Click the x text box (or use the x keyboard shortcuts).
• Type the X coordinate value you want and press Enter or click the
mouse to the X axis on the map canvas.
button to their right to lock the
• Click the y text box (or use the y keyboard shortcuts).
• Type the Y coordinate value you want and press Enter or click the
mouse to the Y axis on the map canvas.
button to their right to lock the
Two blue dotted lines and a green cross identify the exact coordinates you entered. Start digitizing by clicking on
the map canvas; the mouse position is locked at the green cross.
You can continue digitizing by free hand, adding a new pair of coordinates, or you can type the segment’s length
(distance) and angle.
If you want to draw a segment of a given length, click the d (distance) text box (keyboard shortcut d), type the
distance value (in map units) and press Enter or click the
button on the right to lock the mouse in the map
canvas to the length of the segment. In the map canvas, the clicked point is surrounded by a circle whose radius is
the value entered in the distance text box.
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Fig. 12.91: Start drawing at given coordinates
Fig. 12.92: Fixed length segment
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Finally, you can also choose the angle of the segment. As described before , click the a (angle) text box (keyboard
shortcut a), type the angle value (in degrees), and press Enter or click the
this way the segment will follow the desired angle:
buttons on the right to lock it. In
Fig. 12.93: Fixed angle segment
Relative reference digitizing
Instead of using absolute values of angles or coordinates, you can also use values relative to the last digitized
vertex or segment.
For angles, you can click the
button on the left of the a text box (or press Shift + a) to toggle relative
angles to the previous segment. With that option on, angles are measured between the last segment and the mouse
pointer.
For coordinates, click the
buttons to the left of the x or y text boxes (or press Shift + x or Shift + y) to
toggle relative coordinates to the previous vertex. With these options on, coordinates measurement will consider
the last vertex to be the x and y axes origin.
Continuous lock
Both in absolute or relative reference digitizing, angle, distance, x and y constraints can be locked continuously by
clicking the
Continuous lock buttons. Using continuous lock allows you to digitize several points or vertexes
using the same constraints.
Parallel and perpendiculars line
Perpendicular
All the tools described above can be combined with the
and
drawing segments perfectly perpendicular or parallel to another segment.
Parallel
tools. These two tools allow
Perpendicular
To draw a perpendicular segment, during the editing click the
icon (keyboard shortcut p) to activate it. Before drawing the perpendicular line, click on the segment of an existing feature that you want to be
perpendicular to (the line of the existing feature will be colored in light orange); you should see a blue dotted line
where your feature will be snapped:
Parallel
To draw a parallel feature, the steps are the same: click on the
icon (keyboard shortcut p twice), click
on the segment you want to use as reference and start drawing your feature:
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Fig. 12.94: Perpendicular digitizing
Fig. 12.95: Parallel digitizing
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These two tools just find the right angle of the perpendicular and parallel angle and lock this parameter during
your editing.
Construction mode
Construction
icon or with the c keyboard
You can enable and disable construction mode by clicking on the
shortcut. While in construction mode, clicking the map canvas won’t add new vertexes, but will capture the
clicks’ positions so that you can use them as reference points to then lock distance, angle or x and y relative
values.
As an example, the construction mode can be used to draw some point at an exact distance from an existing point.
With an existing point in the map canvas and the snapping mode correctly activated, you can easily draw other
points at given distances and angles from it. In addition to the
mode by clicking the
Construction
button, you have to activate also the construction
icon or with the c keyboard shortcut.
Click next to the point from which you want to calculate the distance and click on the d box (d shortcut) type the
desired distance and press Enter to lock the mouse position in the map canvas:
Fig. 12.96: Distance from point
Before adding the new point, press c to exit the construction mode. Now, you can click on the map canvas, and
the point will be placed at the distance entered.
You can also use the angle constraint to, for example, create another point at the same distance of the original one,
Construction
but at a particular angle from the newly added point. Click the
icon or with the c keyboard shortcut
to enter construction mode. Click the recently added point, and then the other one to set a direction segment. Then,
click on the d text box (d shortcut) type the desired distance and press Enter. Click the a text box (a shortcut)
type the angle you want and press Enter. The mouse position will be locked both in distance and angle.
Before adding the new point, press c to exit the construction mode. Now, you can click on the map canvas, and
the point will be placed at the distance and angle entered. Repeating the process, several points can be added.
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Fig. 12.97: Distance and angle from points
Fig. 12.98: Points at given distance and angle
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CHAPTER
13
Working with Raster Data
13.1 Raster Properties Dialog
To view and set the properties for a raster layer, double click on the layer name in the map legend, or right click on
the layer name and choose Properties from the context menu. This will open the Raster Layer Properties dialog
(see figure_raster_properties).
There are several tabs in the dialog:
• Source
• Style
• Transparency
• Histogram
• Rendering
• Pyramids
• Legend
• QGIS Server
Tip: Live update rendering
The Layer Styling Panel provides you with some of the common features of the Layer properties dialog and is a
good modeless widget that you can use to speed up the configuration of the layer styles and automatically view
your changes in the map canvas.
Note: Because properties (symbology, label, actions, default values, forms. . . ) of embedded layers (see Nesting
Projects) are pulled from the original project file and to avoid changes that may break this behavior, the layer
properties dialog is made unavailable for these layers.
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Fig. 13.1: Raster Layers Properties Dialog
Layer Info
The Source tab displays basic information about the selected raster, including the layer source path, the display
name in the legend (which can be modified), and the number of columns, rows and no-data values of the raster.
Coordinate Reference System
Displays the layer’s Coordinate Reference System (CRS) as a PROJ.4 string. You can change the layer’s CRS,
Select CRS
selecting a recently used one in the drop-down list or clicking on
button (see Coordinate Reference
System Selector). Use this process only if the CRS applied to the layer is a wrong one or if none was applied.
If you wish to reproject your data into another CRS, rather use layer reprojection algorithms from Processing or
Save it into another layer.
13.1.2 Style Properties
Band rendering
QGIS offers four different Render types. The renderer chosen is dependent on the data type.
1. Multiband color - if the file comes as a multiband with several bands (e.g., used with a satellite image with
several bands)
2. Paletted - if a single band file comes with an indexed palette (e.g., used with a digital topographic map)
3. Singleband gray - (one band of) the image will be rendered as gray; QGIS will choose this renderer if the
file has neither multibands nor an indexed palette nor a continuous palette (e.g., used with a shaded relief
map)
4. Singleband pseudocolor - this renderer is possible for files with a continuous palette, or color map (e.g.,
used with an elevation map)
Multiband color
With the multiband color renderer, three selected bands from the image will be rendered, each band representing
the red, green or blue component that will be used to create a color image. You can choose several Contrast
enhancement methods: ‘No enhancement’, ‘Stretch to MinMax’, ‘Stretch and clip to MinMax’ and ‘Clip to min
max’.
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Fig. 13.2: Raster Style - Multiband color rendering
This selection offers you a wide range of options to modify the appearance of your raster layer. First of all, you
have to get the data range from your image. This can be done by choosing the Extent and pressing [Load]. QGIS
can
Estimate (faster) the Min and Max values of the bands or use the
Actual (slower) Accuracy.
Now you can scale the colors with the help of the Load min/max values section. A lot of images have a few very
low and high data. These outliers can be eliminated using the
Cumulative count cut setting. The standard data
range is set from 2% to 98% of the data values and can be adapted manually. With this setting, the gray character
of the image can disappear. With the scaling option
Min/max, QGIS creates a color table with all of the data
included in the original image (e.g., QGIS creates a color table with 256 values, given the fact that you have 8
bit bands). You can also calculate your color table using the
Mean +/- standard deviation x
. Then,
only the values within the standard deviation or within multiple standard deviations are considered for the color
table. This is useful when you have one or two cells with abnormally high values in a raster grid that are having a
negative impact on the rendering of the raster.
All calculations can also be made for the
Current extent.
Tip: Viewing a Single Band of a Multiband Raster
If you want to view a single band of a multiband image (for example, Red), you might think you would set the
Green and Blue bands to “Not Set”. But this is not the correct way. To display the Red band, set the image type to
‘Singleband gray’, then select Red as the band to use for Gray.
Paletted
This is the standard render option for singleband files that already include a color table, where each pixel value
is assigned to a certain color. In that case, the palette is rendered automatically. If you want to change colors
assigned to certain values, just double-click on the color and the Select color dialog appears. Also, in QGIS it’s
possible to assign a label to the color values. The label appears in the legend of the raster layer then.
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Fig. 13.3: Raster Style - Paletted Rendering
Contrast enhancement
Note: When adding GRASS rasters, the option Contrast enhancement will always be set automatically to stretch
to min max, regardless of if this is set to another value in the QGIS general options.
Singleband gray
This renderer allows you to render a single band layer with a Color gradient: ‘Black to white’ or ‘White to black’.
You can define a Min and a Max value by choosing the Extent first and then pressing [Load]. QGIS can
Estimate (faster) the Min and Max values of the bands or use the
Actual (slower) Accuracy.
Fig. 13.4: Raster Style - Singleband gray rendering
With the Load min/max values section, scaling of the color table is possible. Outliers can be eliminated using the
Cumulative count cut setting. The standard data range is set from 2% to 98% of the data values and can be
adapted manually. With this setting, the gray character of the image can disappear. Further settings can be made
with
Min/max and
Mean +/- standard deviation x
. While the first one creates a color table with
all of the data included in the original image, the second creates a color table that only considers values within
the standard deviation or within multiple standard deviations. This is useful when you have one or two cells with
abnormally high values in a raster grid that are having a negative impact on the rendering of the raster.
Singleband pseudocolor
This is a render option for single-band files, including a continuous palette. You can also create individual color
maps for the single bands here.
Three types of color interpolation are available:
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Fig. 13.5: Raster Style - Singleband pseudocolor rendering
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1. Discrete
2. Linear
3. Exact
In the left block, the button
Add values manually
adds a value to the individual color table. The button
Remove selected row
Sort colormap items
deletes a value from the individual color table, and the
button sorts the color
table according to the pixel values in the value column. Double clicking on the value column lets you insert a
specific value. Double clicking on the color column opens the dialog Change color, where you can select a color
to apply on that value. Further, you can also add labels for each color, but this value won’t be displayed when you
use the identify feature tool. You can also click on the button
Load color map from band
, which tries to load the table
Load color map from file
or
from the band (if it has any). And you can use the buttons
an existing color table or to save the defined color table for other sessions.
Export color map to file
to load
In the right block, Generate new color map allows you to create newly categorized color maps. For the Classification mode
‘Equal interval’, you only need to select the number of classes
Classify. You can invert the colors of the color map by clicking the
and press the button
Invert checkbox. In the case of the Mode
‘Continuous’, QGIS creates classes automatically depending on the Min and Max. Defining Min/Max values can be done with the help of the Load min/max values section. A lot of images have a few very low and high
data. These outliers can be eliminated using the
Cumulative count cut setting. The standard data range is set
from 2% to 98% of the data values and can be adapted manually. With this setting, the gray character of the image
can disappear. With the scaling option
Min/max, QGIS creates a color table with all of the data included in the
original image (e.g., QGIS creates a color table with 256 values, given the fact that you have 8 bit bands). You can
also calculate your color table using the
Mean +/- standard deviation x
. Then, only the values within
the standard deviation or within multiple standard deviations are considered for the color table.
Color rendering
For every Band rendering, a Color rendering is possible.
You can also achieve special rendering effects for your raster file(s) using one of the blending modes (see Blending
Modes).
Further settings can be made in modifying the Brightness, the Saturation and the Contrast. You can also use a
Grayscale option, where you can choose between ‘By lightness’, ‘By luminosity’ and ‘By average’. For one hue
in the color table, you can modify the ‘Strength’.
Resampling
The Resampling option makes its appearance when you zoom in and out of an image. Resampling modes can
optimize the appearance of the map. They calculate a new gray value matrix through a geometric transformation.
Fig. 13.6: Raster Style - Color rendering and Resampling settings
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When applying the ‘Nearest neighbour’ method, the map can have a pixelated structure when zooming in. This
appearance can be improved by using the ‘Bilinear’ or ‘Cubic’ method, which cause sharp features to be blurred.
The effect is a smoother image. This method can be applied, for instance, to digital topographic raster maps.
At the bottom of the Style tab, you can see a thumbnail of the layer, its legend symbol, and the palette.
13.1.3 Transparency Properties
QGIS has the ability to display each raster layer at a different transparency level. Use the transparency slider
to indicate to what extent the underlying layers (if any) should be visible though the current
raster layer. This is very useful if you like to overlay more than one raster layer (e.g., a shaded relief map overlayed
by a classified raster map). This will make the look of the map more three dimensional.
Additionally, you can enter a raster value that should be treated as NODATA in the Additional no data value option.
An even more flexible way to customize the transparency can be done in the Custom transparency options section.
The transparency of every pixel can be set here.
As an example, we want to set the water of our example raster file landcover.img to a transparency of 20%.
The following steps are necessary:
1. Load the raster file landcover.img.
2. Open the Properties dialog by double-clicking on the raster name in the legend, or by right-clicking and
choosing Properties from the pop-up menu.
3. Select the Transparency tab.
4. From the Transparency band drop-down menu, choose ‘None’.
5. Click the
Add values manually
button. A new row will appear in the pixel list.
6. Enter the raster value in the ‘From’ and ‘To’ column (we use 0 here), and adjust the transparency to 20%.
7. Press the [Apply] button and have a look at the map.
You can repeat steps 5 and 6 to adjust more values with custom transparency.
As you can see, it is quite easy to set custom transparency, but it can be quite a lot of work. Therefore, you
Export to file
can use the button
to save your transparency list to a file. The button
transparency settings and applies them to the current raster layer.
Import from file
loads your
13.1.4 Histogram Properties
The Histogram tab allows you to view the distribution of the bands or colors in your raster. The histogram is
generated automatically when you open the Histogram tab. All existing bands will be displayed together. You
can save the histogram as an image with the
button. With the Visibility option in the
Prefs/Actions menu,
Show selected band.
you can display histograms of the individual bands. You will need to select the option
The Min/max options allow you to ‘Always show min/max markers’, to ‘Zoom to min/max’ and to ‘Update style
to min/max’. With the Actions option, you can ‘Reset’ and ‘Recompute histogram’ after you have chosen the
Min/max options.
13.1.5 Rendering
Scale dependent visibility
You can set the Maximum (inclusive) and Minimum (exclusive) scale, defining a range of scale in which the layer
Set to current canvas scale
button helps you use the current map
will be visible. Out of this range, it’s hidden. The
canvas scale as boundary of the range visibility. See Scale Dependent Rendering for more information.
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Fig. 13.7: Raster Histogram
13.1.6 Pyramids Properties
Large resolution raster layers can slow navigation in QGIS. By creating lower resolution copies of the data (pyramids), performance can be considerably improved, as QGIS selects the most suitable resolution to use depending
on the level of zoom.
You must have write access in the directory where the original data is stored to build pyramids.
From the Resolutions list, select resolutions for which you want to create pyramid by clicking on them.
If you choose Internal (if possible) from the Overview format drop-down menu, QGIS tries to build pyramids
internally.
Note: Please note that building pyramids may alter the original data file, and once created they cannot be removed.
If you wish to preserve a ‘non-pyramided’ version of your raster, make a backup copy prior to building pyramids.
If you choose External and External (Erdas Imagine) the pyramids will be created in a file next to the original
raster with the same name and a .ovr extension.
Several Resampling methods can be used to calculate the pyramids:
• Nearest Neighbour
• Average
• Gauss
• Cubic
• Mode
• None
Finally, click [Build pyramids] to start the process.
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Fig. 13.8: Raster Pyramids
13.1.7 Legend Properties
The Legend tab provides you with a list of widgets you can embed within the layer tree in the Layers panel. The
idea is to have a way to quickly access some actions that are often used with the layer (setup transparency, filtering,
selection, style or other stuff. . . ).
By default, QGIS provides transparency widget but this can be extended by plugins registering their own widgets
and assign custom actions to layers they manage.
13.1.8 QGIS Server Properties
The QGIS Server tab displays a wealth of information about the raster layer, including statistics about each band
in the current raster layer. From this tab, entries may be made for the Description, Attribution, MetadataUrl and
Properties. In Properties, statistics are gathered on a ‘need to know’ basis, so it may well be that a given layer’s
statistics have not yet been collected.
13.2 Raster Analysis
13.2.1 Raster Calculator
The Raster Calculator in the Raster menu allows you to perform calculations on the basis of existing raster pixel
values (see figure_raster_calculator). The results are written to a new raster layer with a GDAL-supported format.
The Raster bands list contains all loaded raster layers that can be used. To add a raster to the raster calculator
expression field, double click its name in the Fields list. You can then use the operators to construct calculation
expressions, or you can just type them into the box.
In the Result layer section, you will need to define an output layer. You can then define the extent of the calculation
area based on an input raster layer, or based on X,Y coordinates and on columns and rows, to set the resolution of
the output layer. If the input layer has a different resolution, the values will be resampled with the nearest neighbor
algorithm.
The Operators section contains all available operators. To add an operator to the raster calculator expression box,
click the appropriate button. Mathematical calculations (+, -, *, . . . ) and trigonometric functions (sin, cos,
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Fig. 13.9: QGIS Server in Raster Properties
Fig. 13.10: Raster Calculator
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tan, . . . ) are available. Conditional expressions (=, !=, <, >=, . . . ) return either 0 for false or 1 for true, and
therefore can be used with other operators and functions. Stay tuned for more operators to come!
Add result to project checkbox, the result layer will automatically be added to the legend area and
With the
can be visualized.
Examples
Convert elevation values from meters to feet
Creating an elevation raster in feet from a raster in meters, you need to use the conversion factor for meters to feet:
3.28. The expression is:
"elevation@1" * 3.28
Using a mask
If you want to mask out parts of a raster – say, for instance, because you are only interested in elevations above 0
meters – you can use the following expression to create a mask and apply the result to a raster in one step.
("elevation@1" >= 0) * "elevation@1"
In other words, for every cell greater than or equal to 0 the conditional expression evaluates to 1, which keeps the
original value by multiplying it by 1. Otherwise the conditional expression evaluates to 0, which sets the raster
value to 0. This creates the mask on the fly.
If you want to classify a raster – say, for instance into two elevation classes, you can use the following expression
to create a raster with two values 1 and 2 in one step.
("elevation@1" < 50) * 1 + ("elevation@1" >= 50) * 2
In other words, for every cell less than 50 set its value to 1. For every cell greater than or equal 50 set its value to
2.
13.2.2 Raster Alignment
This tool is able to take several rasters as input and to align them perfectly, that means:
• reproject to the same CRS,
• resample to the same cell size and offset in the grid,
• clip to a region of interest,
• rescale values when required.
All rasters will be saved in another files.
Add new raster
First, open the tools from Raster → Align Raster. . . and click on the
button to choose one existing
raster in QGIS. Select an output file to save the raster after the alignment, the resampling method and if the tools
need to Rescale values according to the cell size. The resampling method can be (see figure_raster_align_edit):
• Nearest Neighbor
• Bilinear (2x2 kernel)
• Cubic (4x4 kernel): Cubic Convolution Approximation
• Cubic B-Spline (4x4 kernel): Cubic B-Spline Approximation
• Lanczos (6x6 kernel): Lanczos windowed sinc interpolation
• Average: computes the average of all non-NODATA contributing pixels
• Mode: selects the value which appears most often of all the sampled points
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• Maximum, Minimum, Mediane, First Quartile (Q1) or Third Quartile (Q3) of all non-NODATA contributing pixels
Note: Methods like maximum, minimum, mediane, first and third quartiles are available only if QGIS is built
with GDAL >= 2.0.
Fig. 13.11: Select Raster Resampling Options
Edit file settings
Remove an existing file
In the main Align raster dialog, you can still
or
from the list of raster layers.
You can also choose one or more other options (see figure_raster_align):
• Select the Reference Layer,
• Transform into a new CRS,
• Setup a different Cell size,
• Setup a different Grid Offset,
• Clip to Extent: it can be user-defined or based on a layer or the map view
• Output Size,
• Add aligned raster to the map canvas.
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Fig. 13.12: Raster Alignment
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CHAPTER
14
Laying out the maps
With the Map Layout you can create nice maps and atlasses that can be printed or saved as image, PDF or SVG
file. This is a powerful way to share geographical information produced with QGIS that can be included in reports
or published.
14.1 Overview of the Print Layout
The print layout provides growing layout and printing capabilities. It allows you to add elements such as the QGIS
map canvas, text labels, images, legends, scale bars, basic shapes, arrows, attribute tables and HTML frames. You
can size, group, align, position and rotate each element and adjust their properties to create your layout. The layout
can be printed or exported to image formats, PostScript, PDF or to SVG (export to SVG is not working properly
with some recent Qt4 versions; you should try and check individually on your system). You can save the layout as
a template and load it again in another session. Finally, generating several maps based on a template can be done
through the atlas generator.
14.1.1 Sample Session
Before you start to work with the print layout, you need to load some raster or vector layers in the QGIS map
canvas and adapt their properties to suit your own convenience. After everything is rendered and symbolized
New Print Layout
to your liking, click the
icon in the toolbar or choose File → New Print Layout. You will be
prompted to choose a title for the new layout.
To demonstrate how to create a map please follow the next instructions.
Add new map
1. On the left side, select the
toolbar button and draw a rectangle on the canvas holding down
the left mouse button. Inside the drawn rectangle the QGIS map view to the canvas.
Add new scalebar
2. Select the
toolbar button and click with the left mouse button on the print layout canvas.
A scalebar will be added to the canvas.
Add new legend
3. Select the
toolbar button and draw a rectangle on the canvas holding down the left mouse
button. Inside the drawn rectangle the legend will be drawn.
4. Select the
Select/Move item
icon to select the map on the canvas and move it a bit.
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5. While the map item is still selected you can also change the size of the map item. Click while holding down
the left mouse button, in a white little rectangle in one of the corners of the map item and drag it to a new
location to change its size.
6. Click the Item Properties panel on the left down side and find the setting for the orientation. Change the
value of the setting Map orientation to ‘15.00° ‘. You should see the orientation of the map item change.
7. Now, you can print or export your print layout to image formats, PDF or to SVG with the export tools in
Layout menu.
8. Finally, you can save your print layout within the project file with the
Save Project
button.
You can add multiple elements to the print layout. It is also possible to have more than one map view or legend or
scale bar in the print layout canvas, on one or several pages. Each element has its own properties and, in the case
of the map, its own extent. If you want to remove any elements from the layout canvas you can do that with the
Delete or the Backspace key.
14.1.2 The Layout Manager
The Layout Manager is the main window to manage print layouts in the project. It helps you add new print layout,
Layout Manager
duplicate an existing one, rename or delete it. To open the layout manager dialog, click on the
button in the toolbar or choose Layout → Layout Manager. It can also be reached from the main window of QGIS
with Project → Layout Manager. . . .
Fig. 14.1: The Print Layout Manager
The layout manager lists in its upper part all the available print layouts in the project. The bottom part shows tools
that help to:
• show the selected print layout(s): you can open multiple print layouts in one-click
• duplicate the selected print layout (available only if one print layout is selected): it creates a new layout
using the selected one as template. You’ll be prompted to choose a new title for the new layout
• rename the layout (available only if one print layout is selected): You’ll be prompted to choose a new title
for the layout. Note that you can also rename the print layout by double-clicking on its title in the upper part
• remove the layout: the selected print layout(s) will be deleted from the project.
With the layout manager, it’s also possible to create new print layouts as an empty layout or from a saved template. By default, QGIS will look for templates in user profile folder (~/.qgis2/composer_templates)
or application’s one (ApplicationFolder/composer_templates). QGIS will retrieve all the available
templates and propose them in the combobox. The selected template will be used to create a new print layout
when clicking Add button. You can also save layout templates in another folder. Choosing specific in the template
list offers the ability to select such template and use it to create a new print layout.
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14.1.3 Menus, tools and panels of the print layout
Opening the print layout provides you with a blank canvas that represents the paper surface when using the print
option. Initially you find buttons on the left beside the canvas to add print layout items: the current QGIS map
canvas, text labels, images, legends, scale bars, basic shapes, arrows, attribute tables and HTML frames. In this
toolbar you also find buttons to navigate, zoom in on an area and pan the view on the layout a well as buttons to
select any layout item and to move the contents of the map item.
figure_layout_overview shows the initial view of the print layout before any elements are added.
Fig. 14.2: Print Layout
On the right beside the canvas you find two set of panels. The upper one holds the panels Items and Command
History and the lower holds the panels Composition, Item properties and Atlas generation.
• The Items panel provides a list of all the print layout items added to the canvas.
• The Command history panel displays a history of all changes applied to the layout. With a mouse click, it
is possible to undo and redo layout steps back and forth to a certain status.
• The Composition panel allows you to set paper size, orientation, the page background, number of pages and
print quality for the output file in dpi. Furthermore, you can also activate the
Print as raster checkbox.
This means all items will be converted to raster before printing or saving as PostScript or PDF. In this panel,
you can also customize settings for grid and smart guides.
Select/Move item
• The Item Properties panel displays the properties for the selected item. Click the
icon
to select an item (e.g., legend, scale bar or label) on the canvas. Then click the Item Properties panel and
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customize the settings for the selected item (see Layout Items for detailed information on each item settings).
• The Atlas generation panel allows you to enable the generation of an atlas for the current layout and gives
access to its parameters (see Generate an Atlas for detailed information on atlas generation usage).
In the bottom part of the print layout window, you can find a status bar with mouse position, current page number,
a combo box to set the zoom level, the number of selected items if applicable and, in the case of atlas generation,
the number of features.
In the upper part of the print layout window, you can find menus and other toolbars. All print layout tools are
available in menus and as icons in a toolbar. See a list of tools in table_layout_tools.
The toolbars and the panels can be switched off and on using the right mouse button over any toolbar or through
View → Toolbars or View → Panels.
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Tools
Icon
Purpose
Icon
Purpose
Save Project
New Layout
Duplicate Layout
Layout Manager
Load from template
Save as template
Print or export as PostScript
Export to an image format
Export print composition to SVG
Export as PDF
Revert last change
Restore last change
Zoom to full extent
Zoom to 100%
Zoom in
Zoom out
Refresh View
Pan
Zoom to specific region
Select/Move item in print composition
Move content within an item
Add new map from QGIS map canvas
Add image to print composition
Add label to print composition
Add new legend to print composition
Add scale bar to print composition
Add basic shape to print composition
Add arrow to print composition
Add attribute table to print composition
Add an HTML frame
Add nodes shape to print composition
Edit a nodes shape
Group items of print composition
Ungroup items of print composition
Lock Selected Items
Unlock All items
Raise selected items
Lower selected items
Move selected items to top
Move selected items to bottom
Align selected items left
Align selected items right
Align selected items center
Align selected items center vertical
Align selected items top
Align selected items bottom
Preview Atlas
First Feature
Previous Feature
Next Feature
Last feature
Print Atlas
Export Atlas as Image
Atlas Settings
Table Layout 1: Print Layout Tools
Layout Menu
With the Layout → Save Project action, you can save the project file directly from the print layout window. The
Layout menu also provides actions to:
• Create a new and blank print layout with
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New Layout. . .
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•
Duplicate Layout. . . : Create a new print layout by duplicating the current one
• Open the
Layout Manager. . .
• Layouts → : Open an existing print layout
Once the layout is designed, with
Save as template and
Add items from template icons, you can save the
current state of a print layout session as a .qpt template and load its item again in another session.
In the Layout menu, there are also powerful ways to share geographical information produced with QGIS that can
be included in reports or published. These tools are
as SVG. . . and
Export as Image. . . ,
Export as PDF. . . ,
Export
Print. . . .
Settings Menu
From Settings → Layout Options. . . you can set some options that will be used as default on any layout during
your work.
• Layout defaults let you specify the default font to use.
• With Grid appearance, you can set the grid style and its color. There are three types of grid: Dots, Solid
lines and Crosses.
• Grid and guide defaults defines spacing, offset and tolerance of the grid.
Edit Menu
Copy/Cut and Paste Items
The print layout includes actions to use the common Copy/Cut/Paste functionality for the items in the layout. As
usual first you need to select the items using one of the options seen above; at this point the actions can be found in
the Edit menu. When using the Paste action, the elements will be pasted according to the current mouse position.
Using the Edit → Paste in Place action or pressing Ctrl+Shift+V will paste the items into the current page,
at the same position they were in their initial page. It ensures to copy/paste items at the same place, from page to
page.
Note: HTML items can not be copied in this way. As a workaround, use the [Add Frame] button in the Item
Properties panel.
View Menu
Navigation Tools
To navigate in the canvas layout, the print layout provides some general tools:
•
Zoom In
•
Zoom Out
•
Zoom Full
•
Zoom to 100%
•
Refresh view
(if you find the view in an inconsistent state)
• Show Grid behind items.
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• Snap Grid to snap items on the grid.
• Show Guides to help user to align items. These are red line that you can click in the rule (above or at the
left side of the layout) and drag and drop to the desired location.
• Snap Guides: allows user to snap items to the guides,
• Smart Guides: uses other layout items as guides to dynamically snap to as user moves or reshapes an item.
• Clear Guides to remove all current guides.
• Show Bounding box around the items.
• Show Rules around the layout.
• Show Pages or set up pages to transparent. Often layout is used to create non-print layouts, e.g. for inclusion
in presentations or other documents, and it’s desirable to export the composition using a totally transparent
background. It’s sometimes referred to as “infinite canvas” in other editing packages.
• Toggle Full Screen makes the layout window to full screen.
• Hide Panels hides/shows the right panel
• Panels lists all panels available to hide/show them.
• Toolbars same as above for toolbars.
You can change the zoom level also using the mouse wheel or the combo box in the status bar. If you need to
switch to pan mode while working in the layout area, you can hold the Spacebar or the mouse wheel. With
Ctrl+Spacebar, you can temporarily switch to Zoom In mode, and with Ctrl+Shift+Spacebar, to Zoom
Out mode.
Hide and Show Panels
To maximise the space available to interact with a composition you can use View →
Hide panels or press F10.
Note: It’s also possible to switch to a full screen mode to have more space to interact by pressing F11 or using
View →
Toggle full screen.
Composition Panel
In the Composition panel, you can define the global settings of the current composition.
General settings
Composition can be divided into several pages. For instance, a first page can show a map canvas, and a second
page can show the attribute table associated with a layer, while a third one shows an HTML frame linking to your
organization website. Set the Number of pages to the desired value. You can also custom the Page Background
with the color or the symbol you want.
The reference map select the map item to be used as the composition’s master map. The layout will use this map
in any properties and variable calculating units or scale. This includes exporting the composition to georeferenced
formats.
Page size and resize
In the Page size group, you can choose one of the Presets formats for your paper sheet, or enter your custom width,
height and units. You can also choose the page Orientation to use.
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Fig. 14.3: Composition Settings in the Print Layout
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The Page size options apply to all the pages in the composition. However, you can modify the values using the
data defined override options (see Data Defined Override Buttons).
A custom page size can also be set, using the Resize page tool. This creates an unique page composition, resizes
the page to fit the current contents of the composition (with optional margins).
Export settings
You can define a resolution to use for all exported maps in Export resolution. This setting can, however, be
overridden each time you are exporting a map.
When checked,
PDF.
print as raster means all elements will be rasterized before printing or saving as PostScript or
While exporting to an image file format, you can choose to generate a world file by checking
Save world file
using the map selected in
Reference map in the general settings group. The world file is created beside the
exported map, has the same name and contains information to georeference it easily.
Fig. 14.4: Export Settings in the Print Layout
Grid and guides
You can put some reference marks on your composition paper sheet to help you place some items. These marks
can be:
• simple lines (called Guides) put at the position you want. To do that, ensure that Show Rulers and Show
Guides in View menu are checked. Then, click and drag from within the ruler to the paper sheet. A vertical
or horizontal line is added to the paper and you can set its position following the coordinates displayed at
the left bottom of the print layout dialog.
• or regular Grid.
Whether grids or guides should be shown is set in View menu. There, you can also decide if they might be used to
snap layout items. The Grid and guides section lets you customize grid settings like Grid spacing, Grid offset and
Snap tolerance to your need. The tolerance is the maximum distance below which an item is snapped to a grid or
a guide.
Fig. 14.5: Snapping to Grids in the Print Layout
In the Options → Layout menu in QGIS main canvas, you can also set the spacing, offset and snap tolerance of
the grid as much as its style and color. These options are applied by default to any new print layout.
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Variables
The Variables lists all the variables available at the composition’s level (which includes all global and project’s
variables).
It also allows the user to manage composition-level variables. Click the
button to add a new custom
composition-level variable. Likewise, select a custom composition-level variable from the list and click the
button to remove it.
More information on variables usage in the General Tools Variables section.
Fig. 14.6: Variables Editor in the Print Layout
Command History Panel: Revert and Restore actions
During the layout process, it is possible to revert and restore changes. This can be done with the revert and restore
tools:
•
Revert last change
•
Restore last change
This can also be done by mouse click within the Command history panel (see figure_layout). The History panel
lists the last actions done within the print layout. Just select the point you want to revert to and once you do new
action all the actions done after the selected one will be removed.
Fig. 14.7: Command History in the Print Layout
Items Panel
The Items panel offers some options to manage selection and visibility of items. All the items added to the print
layout canvas are shown in a list and selecting an item makes the corresponding row selected in the list as well as
selecting a row does select the corresponding item in the print layout canvas. This is thus a handy way to select an
item placed behind another one. Note that a selected row is shown as bold.
For any selected item, you can :
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•
set it visible or not,
•
lock or unlock its position,
• order its Z position. You can move up and down each item in the list with a click and drag. The upper item
in the list will be brought to the foreground in the print layout canvas. By default, a newly created item is
placed in the foreground.
• change the name by double-clicking the text.
column. Locked
Once you have found the correct position for an item, you can lock it by ticking the box in
items are not selectable on the canvas. Locked items can be unlocked by selecting the item in the Items panel and
unchecking the tickbox or you can use the icons on the toolbar.
14.2 Layout Items
14.2.1 Layout Items Common Options
Layout items have a set of common properties you will find at the bottom of the Item Properties panel: Position
and size, Rotation, Frame, Background, Item ID, Variables and Rendering (See figure_layout_common).
Fig. 14.8: Common Item Properties Dialogs
• The Position and size dialog lets you define the size and position of the frame which contains the item. You
can also choose which Reference point will be set at the X and Y coordinates previously defined.
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• The Rotation sets the rotation of the element (in degrees).
• The
Frame shows or hides the frame around the item. Click on the [Color] and [Thickness] buttons to
adjust those properties.
• Use the Background color menu for setting a background color. Click on the [Color. . . ] button to display a
dialog where you can pick a color or choose from a custom setting. Transparency can be adjusted through
atlering the alpha field settings.
• Use the Item ID to create a relationship to other print layout items. This is used with QGIS server and other
potential web clients. You can set an ID on an item (for example, a map or a label), and then the web client
can send data to set a property (e.g., label text) for that specific item. The GetProjectSettings command will
list the items and IDs which are available in a layout.
• Rendering mode helps you set whether and how the item can be displayed.
Note:
• If you checked
Use live-updating color chooser dialogs in the QGIS general options, the color button
will update as soon as you choose a new color from Color Dialog windows. If not, you need to close the
Color Dialog.
Data defined override
• The
icon next to a field means that you can associate the field with data in the map
item or use expressions. These are particularly helpful with atlas generation (See Data Defined Override
Buttons).
Rendering mode
QGIS allows advanced rendering for layout items just like vector and raster layers.
Fig. 14.9: Rendering mode
• Blending mode: With this tool you can achieve effects which would otherwise only be achieved using
graphic rendering software. The pixels of your overlaying and underlaying items can be mixed according
to the mode set (see Blending Modes for description of each effect).
• Transparency
: You can make the underlying item in the layout visible with this tool.
Use the slider to adapt the visibility of your item to your needs. You can also make a precise definition of
the percentage of visibility in the menu beside the slider.
•
Exclude item from exports: You can decide to make an item invisible in all exports. After activating this
checkbox, the item will not be included in export to PDF, print etc..
Size and position
Each item inside the print layout can be moved and resized to create a perfect layout.For both operations the
Select/Move item
tool and to click on the item; you can then move it using the mouse
first step is to activate the
while holding the left button. If you need to constrain the movements to the horizontal or the vertical axis, just
hold the Shift button on the keyboard while moving the mouse. If you need better precision, you can move
a selected item using the Arrow keys on the keyboard; if the movement is too slow, you can speed up it by
holding Shift.
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A selected item will show squares on its boundaries; moving one of them with the mouse, will resize the item
in the corresponding direction. While resizing, holding Shift will maintain the aspect ratio. Holding Alt will
resize from the item center.
The correct position for an item can be obtained using the grid snapping or smart guides. Guides are set by clicking
and dragging within the ruler area. To move a guide, click on the ruler, level with the guide and drag it to a new
position. To delete a guide move it off the canvas. If you need to disable the snap on the fly, hold Ctrl while
moving the mouse.
Select/Move item
button. Just hold the Shift button and click on all the
You can choose multiple items with the
items you need. You can then resize/move this group like a single item.
Once you have found the correct position for an item, you can lock it by using the items on the toolbar or ticking
the box next to the item in the Items panel. Locked items are not selectable on the canvas.
Locked items can be unlocked by selecting the item in the Items panel and unchecking the tickbox or you can use
the icons on the toolbar.
To unselect an item, just click on it holding the Shift button.
Inside the Edit menu, you can find actions to select all the items, to clear all selections or to invert the current
selection.
Alignment
Raise selected items
pull-down menu. Choose
Raising or lowering the visual hierarchy for elements are inside the
an element on the print layout canvas and select the matching functionality to raise or lower the selected element
compared to the other elements. This order is shown in the Items panel. You can also raise or lower objects in the
Items panel by clicking and dragging an object’s label in this list.
Fig. 14.10: Alignment helper lines in the print layout
Align selected items
There are several alignment options available within the
pull-down menu (see figure_layout_common_align). To use an alignment function, you first select the elements then click on the matching
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alignment icon. All selected elements will then be aligned to their common bounding box. When moving items
on the layout canvas, alignment helper lines appear when borders, centers or corners are aligned.
Variables
The Variables lists all the variables available at the layout item’s level (which includes all global, project and
composition’s variables). Map items also include Map settings variables that provide easy access to values like
the map’s scale, extent, and so on.
button to add a new custom variable.
In Variables, it’s also possible to manage item-level variables. Click the
Likewise, select any custom item-level variable from the list and click the
button to remove it.
More information on variables usage in the General Tools Variables section.
14.2.2 The Map Item
Add new map
Click on the
toolbar button in the print layout toolbar to add the QGIS map canvas. Now, drag a
rectangle onto the layout canvas with the left mouse button to add the map. To display the current map, you can
choose between three different modes in the map Item Properties panel:
• Rectangle is the default setting. It only displays an empty box with a message ‘Map will be printed here’.
• Cache renders the map in the current screen resolution. If you zoom the layout window in or out, the map
is not rendered again but the image will be scaled.
• Render means that if you zoom the layout window in or out, the map will be rendered again, but for space
reasons, only up to a maximum resolution.
Cache is the default preview mode for newly added print layout maps.
Select/Move item
button, selecting the element, and dragging one
You can resize the map item by clicking on the
of the blue handles in the corner of the map. This button also helps to move the map to another place. Select the
item and while holding the left mouse button, move to the new place and release the mouse button. After you
have found the right place for an item, you can lock the item position within the print layout canvas. Select the
map item and use the toolbar
Lock Selected Items
or the Items panel to Lock the item. A locked item can only be
selected using the Items panel. Once selected you can use the Items panel to unlock individual items. The
Unlock All Items
icon will unlock all locked layout items. With the map selected, you can now adapt more properties
in the map Item Properties panel.
To move layers within the map element, select the map element, click the
layers within the map item frame with the left mouse button.
Move item content
icon and move the
Main properties
The Main properties dialog of the map Item Properties panel provides the following functionalities (see figure_layout_map):
• The Preview drop-down menu allows you to select one of the preview modes ‘Rectangle’, ‘Cache’ and
‘Render’, as described above. If you change the view on the QGIS map canvas by changing vector or raster
properties, you can update the print layout view by selecting the map element and clicking the [Update
preview] button.
• The field Scale
manually sets the map item scale.
• The field Map rotation
allows you to rotate the map element content clockwise in degrees. The
rotation of the map canvas can be imitated here.
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Fig. 14.11: Map Item Properties Panel
•
Draw map canvas items lets you show annotations that may be placed on the map canvas in the main
QGIS window.
Layers
The Layers dialog of the map item panel provides the following functionality (see figure_layout_map_layers):
Fig. 14.12: Map Layers Dialog
If you want to keep the map item consistent with an existing map theme, use
Follow map theme and select
the desired theme. (See Configuring map themes to find out how to configure map themes.) Any changes applied
to the theme in QGIS’ main window (using the replace theme function) will automatically affect the map item. If
a map theme is selected, the Lock styles for layers option will be disabled because Follow map theme also updates
the style (symbology, labels, diagrams) of the layers.
To lock the layers shown in a map item to the current map canvas check
14.2. Layout Items
Lock layers. After this option
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is enabled, any changes on the layers’ visibility in QGIS’ main window won’t affect the layout’s map item.
Nevertheless, style and labels of locked layers are still refreshed according to QGIS’ main window. You can
prevent this by using Lock styles for layers.
button, you can lock the map item’s layers to one of the map themes you have prepared (see
Using the
Configuring map themes). Clicking the
button will show the list of all themes. Select the theme you want to
display. The map canvas will lock the theme layers automatically by enabling the
the theme by unchecking the
Lock layers and press the
Lock layers. You can release
button in the print layout’s Navigation toolbar.
Note that, unlike the Follow map theme, using the Lock layers option enabled and set to a theme, the map item
layers won’t be updated if the theme is changed (using the replace theme function) in QGIS’ main window.
Locked layers in the map can also be data-defined, using the
icon beside the option. When used, this overrides
the selection set in the drop-down list. You need to pass a list of layers separated by | character. The following
example locks the map item to use only layers layer 1 and layer 2:
concat ('layer 1', '|', 'layer 2')
Extents
The Extents dialog of the map item panel provides the following functionalities (see figure_layout_map_extents):
Fig. 14.13: Map Extents Dialog
The Map extents area allows you to specify the map extent using X and Y min/max values and by clicking the [Set
to map canvas extent] button. This button sets the map extent of the layout map item to the extent of the current
map view in the main QGIS application. The button [View extent in map canvas] does exactly the opposite; it
updates the extent of the map view in the QGIS application to the extent of the layout map item.
If you change the view on the QGIS map canvas by changing vector or raster properties, you can update the print
layout view by selecting the map element in the print layout and clicking the [Update preview] button in the map
Item Properties panel (see figure_layout_map).
Grids
The Grids dialog of the map Item Properties panel provides the possibility to add several grids to a map item.
• With the
and
buttons you can add or remove a selected grid.
• With the
and
buttons you can move a grid in the list and set the drawing priority.
When you double-click the added grid you can give it another name.
After you have added a grid, you can activate the checkbox
Draw grid to overlay a grid onto the map element.
Expand this option to provide a lot of configuration options, see Figure_layout_map_grid_draw.
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Fig. 14.14: Map Grids Dialog
Fig. 14.15: Draw Grid Dialog
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As grid type, you can specify to use a ‘Solid’, ‘Cross’, ‘Markers’ or ‘Frame and annotations only’. ‘Frame and
annotations only’ is especially useful when working with rotated maps or reprojected grids. In the divisions section
of the Grid Frame Dialog mentioned below you then have a corresponding setting. Symbology of the grid and
its rendering mode can be chosen. See Rendering mode. Furthermore, you can define an interval in the X and Y
directions, an X and Y offset, and the width used for the cross or line grid type.
Fig. 14.16: Grid Frame Dialog
• There are different options to style the frame that holds the map. Following options are available: No Frame,
Zebra, Interior ticks, Exterior ticks, Interior and Exterior ticks and Lineborder.
• With ‘Latitude/Y only’ and ‘Longitude/X only’ setting in the divisions section you have the possibility to
prevent a mix of latitude/y and longitude/x coordinates showing on a side when working with rotated maps
or reprojected grids.
• Advanced rendering mode is also available for grids.
• The
Draw coordinates checkbox allows you to add coordinates to the map frame. You can choose the
annotation numeric format, the options range from decimal to degrees, minute and seconds, with or without
suffix, aligned or not and a custom format using the expression dialog. You can choose which annotation
to show. The options are: show all, latitude only, longitude only, or disable(none). This is useful when the
map is rotated. The annotation can be drawn inside or outside the map frame. The annotation direction can
be defined as horizontal, vertical ascending or vertical descending. Finally, you can define the annotation
font, the annotation font color, the annotation distance from the map frame and the precision of the drawn
coordinates.
Overviews
The Overviews dialog of the map Item Properties panel provides the following functionalities:
You can choose to create an overview map, which shows the extents of the other map(s) that are available in the
layout. First you need to create the map(s) you want to include in the overview map and the map you want to use
as the overview map, just like a normal map.
Then expand Overviews option and press the green plus icon-button to add an overview. Initially this overview is
named ‘Overview 1’ (see Figure_layout_map_overview). You can change the name when you double-click on the
overview item in the list named ‘Overview 1’ and change it to another name.
• With the plus and minus button you can add or remove an overview.
• With the up and down button you can move an overview in the list and set the drawing priority.
When you select the overview item in the list you can customize it.
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Fig. 14.17: Grid Draw Coordinates dialog
Fig. 14.18: Map Overviews Dialog
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• The
frame.
Draw “<name_overview>” overview needs to be activated to draw the extent of selected map
• The Map frame combo list can be used to select the map item whose extents will be drawn on the present
map item.
• The Frame Style allows you to change the style of the overview frame.
• The Blending mode allows you to set different transparency blend modes.
• The
Invert overview creates a mask around the extents when activated: the referenced map extents are
shown clearly, whereas everything else is blended with the frame color.
Center on overview puts the extent of the overview frame in the center of the overview map. You
• The
can only activate one overview item to center, when you have added several overviews.
14.2.3 The Label Item
Add label
To add a label, click the
icon, place the element with the left mouse button on the print layout canvas
and position and customize its appearance in the label Item Properties panel.
The Item Properties panel of a label item provides the following functionality for the label item (see figure_layout_label):
Fig. 14.19: Label Item Properties Panel
Main properties
• The main properties dialog is where the text (HTML or not) or the expression needed to fill the label is
added to the layout canvas.
Render as HTML. You can now insert a URL, a
• Labels can be interpreted as HTML code: check
clickable image that links to a web page or something more complex.
• You can also insert an expression. Click on [Insert an expression] to open a new dialog. Build an expression by clicking the functions available in the left side of the panel. Two special categories can be
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useful, particularly associated with the atlas functionality: geometry and records functions. At the bottom,
a preview of the expression is shown.
Appearance
• Define Font by clicking on the [Font. . . ] button or a Font color selecting a color using the color selection
tool.
• You can specify different horizontal and vertical margins in mm. This is the margin from the edge of the
layout item. The label can be positioned outside the bounds of the label e.g. to align label items with other
items. In this case you have to use negative values for the margin.
• Using the Alignment is another way to position your label. Note that when e.g. using the Horizontal
Center Position the Horizontal margin feature is disabled.
alignment in
14.2.4 The Legend Item
Add new legend
icon, place the element with the left mouse button on the print
To add a map legend, click the
layout canvas and position and customize the appearance in the legend Item Properties panel.
The Item properties panel of a legend item provides the following functionalities (see figure_layout_legend):
Fig. 14.20: Legend Item Properties Panel
Main properties
The Main properties dialog of the legend Item Properties panel provides the following functionalities (see figure_layout_legend_ppt):
In Main properties you can:
• change the title of the legend;
• set the title alignment to Left, Center or Right;
• choose which Map item the current legend will refer to in the select list;
• wrap the text of the legend title on a given character;
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Fig. 14.21: Legend Main properties Dialog
• use
Resize to fit contents to control whether or not a legend should be automatically resized to fit its
contents. If unchecked, then the legend will never resize and instead just stick to whatever size the user has
set. Any content which doesn’t fit the size is cropped out.
Legend items
The Legend items dialog of the legend Item Properties panel provides the following functionalities (see figure_layout_legend_items):
Fig. 14.22: Legend Items Dialog
• The legend will be updated automatically if
Auto-update is checked. When Auto-update is unchecked
this will give you more control over the legend items. The icons below the legend items list will be activated.
• The legend items window lists all legend items and allows you to change item order, group layers, remove
and restore items in the list, edit layer names and add a filter.
– The item order can be changed using the
and
buttons or with ‘drag-and-drop’ functionality.
The order can not be changed for WMS legend graphics.
– Use the
button to add a legend group.
– Use the
button to add layers and
button to remove groups, layers or symbol classes.
button is used to edit the layer, group name or title. First you need to select the legend item.
– The
Double-clicking the item also opens the text box to rename it.
– The
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– With the
Filter legend by map content
button, only the legend items visible in the map will be listed in the
legend. This tool remains available when
Auto-update is active.
Filter legend by expression
helps you filter which of the legend items of a layer will be displayed, i.e.
– The
using a layer that has different legend items (e.g., from a rule-based or categorized symbology), you
can specify a boolean expression to remove from the legend tree, styles that have no feature satisfying
a condition. Note that the features are nevertheless kept and shown in the layout map item.
While the default behavior of the legend item is to mimic the Layers panel tree, displaying the same groups,
layers and classes of symbology, right-click any item offers you options to hide layer’s name or raise it as a
group or subgroup. In case you have made some changes to a layer, you can revert them by choosing Reset
to defaults from the contextual menu.
After changing the symbology in the QGIS main window, you can click on [Update All] to adapt the
changes in the legend element of the print layout.
• While generating an atlas with polygon features, you can filter out legend items that lie outside the current
atlas feature. To do that, check the
Only show items inside current atlas feature option.
Fonts, Columns, Symbol
The Fonts, Columns and Symbol dialogs of the legend Item Properties panel provide the following functionalities
(see figure_layout_legend_fonts):
Fig. 14.23: Legend Fonts, Columns and Symbol Dialogs
• You can change the font of the legend title, group, subgroup and item (layer) in the legend item. Click on a
category button to open a Select font dialog.
• You provide the labels with a Color using the advanced color picker, however the selected color will be
given to all font items in the legend..
• Legend items can be arranged over several columns. Set the number of columns in the Count
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field.
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–
Equal column widths sets how legend columns should be adjusted.
– The
Split layers option allows a categorized or a graduated layer legend to be divided between
columns.
• You can also change the width and height of the legend symbol, set a color and a thickness in case of raster
layer symbol.
WMS LegendGraphic and Spacing
The WMS LegendGraphic and Spacing dialogs of the legend Item Properties panel provide the following functionalities (see figure_layout_legend_wms):
Fig. 14.24: WMS LegendGraphic and Spacing Dialogs
When you have added a WMS layer and you insert a legend item, a request will be sent to the WMS server to
provide a WMS legend. This Legend will only be shown if the WMS server provides the GetLegendGraphic
capability. The WMS legend content will be provided as a raster image.
WMS LegendGraphic is used to be able to adjust the Legend width and the Legend height of the WMS legend
raster image.
Spacing around title, group, subgroup, symbol, icon label, box space or column space can be customized through
this dialog.
14.2.5 The Scale Bar Item
Add new scalebar
To add a scale bar, click the
icon, place the element with the left mouse button on the print layout
canvas and position and customize the appearance in the scale bar Item Properties panel.
The Item properties of a scale bar item tab provides the following functionalities (see figure_layout_scalebar):
Main properties
The Main properties dialog of the scale bar Item Properties panel provides the following functionalities (see
figure_layout_scalebar_ppt):
• First, choose the map the scale bar will be attached to.
• Then, choose the style of the scale bar. Six styles are available:
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Fig. 14.25: Scale Bar Item Properties Panel
Fig. 14.26: Scale Bar Main properties Dialog
– Single box and Double box styles, which contain one or two lines of boxes alternating colors.
– Middle, Up or Down line ticks.
– Numeric, where the scale ratio is printed (i.e., 1:50000).
Units and Segments
The Units and Segments dialogs of the scale bar Item Properties panel provide the following functionalities (see
figure_layout_scalebar_units):
In these two dialogs, you can set how the scale bar will be represented.
• Select the units you want to use with Scalebar units. There are four possible choices: Map Units, the
default one and Meters, Feet or Nautical Miles which may force unit conversions.
• The Label unit multiplier specifies how many scalebar units per labeled unit. Eg, if your scalebar units are
set to “meters”, a multiplier of 1000 will result in the scale bar labels in “kilometers”.
• The Label for units field defines the text used to describe the units of the scale bar, eg “m” or “km”. This
should be matched to reflect the multiplier above.
• You can define how many Segments will be drawn on the left and on the right side of the scale bar.
• You can set how long each segment will be (fixed width), or limit the scale bar size in mm with Fit segment
width option. In the latter case, each time the map scale changes, the scale bar is resized (and its label
updated) to fit the range set.
• Height is used to define the height of the bar.
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Fig. 14.27: Scale Bar Units and Segments Dialogs
Display
The Display dialog of the scale bar Item Properties panel provides the following functionalities (see figure_layout_scalebar_display):
Fig. 14.28: Scale Bar Display
You can define how the scale bar will be displayed in its frame.
• Box margin : space between text and frame borders
• Labels margin : space between text and scale bar drawing
• Line width : line width of the scale bar drawing
• Join style : Corners at the end of scalebar in style Bevel, Rounded or Square (only available for Scale bar
style Single Box & Double Box)
• Cap style : End of all lines in style Square, Round or Flat (only available for Scale bar style Line Ticks Up,
Down and Middle)
• Alignment : Puts text on the left, middle or right side of the frame (works only for Scale bar style Numeric)
Fonts and colors
The Fonts and colors dialog of the scale bar Item Properties panel provides the following functionalities (see
figure_layout_scalebar_fonts):
You can define the fonts and colors used for the scale bar.
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Fig. 14.29: Scale Bar Fonts and colors Dialogs
• Use the [Font] button to set the font of scale bar label
• Font color: set the font color
• Fill color: set the first fill color
• Secondary fill color: set the second fill color
• Stroke color: set the color of the lines of the Scale Bar
Fill colors are only used for scale box styles Single Box and Double Box. To select a color you can use the list
option using the dropdown arrow to open a simple color selection option or the more advanced color selection
option, that is started when you click in the colored box in the dialog.
14.2.6 The Attribute Table Item
Add attribute table
It is possible to add parts of a vector attribute table to the Print Layout canvas: Click the
icon,
click and drag with the left mouse button on the Print Layout canvas to place and size the item. You can better
position and customize its appearance in the Item Properties panel.
The Item properties panel of an attribute table provides the following functionalities (see figure_layout_table):
Fig. 14.30: Attribute table Item Properties Panel
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Main properties
The Main properties dialog of the attribute table provides the following functionalities (see figure_layout_table_ppt):
Fig. 14.31: Attribute table Main properties Dialog
• For Source you can normally select only Layer features.
• With Layer you can choose from the vector layers loaded in the project.
• In case you activated the
Source possible:
Generate an atlas option in the Atlas generation panel, there are two additional
– Current atlas feature (see figure_layout_table_atlas): you won’t see any option to choose the layer,
and the table item will only show a row with the attributes from the current feature of the atlas coverage
layer.
– and Relation children (see figure_layout_table_relation): an option with the relation names will show
up. This feature can only be used if you have defined a relation using your atlas coverage layer as
parent, and the table will show the children rows of the atlas coverage layer’s current feature (for
further information about the atlas generation, see Generate an Atlas).
• The button [Refresh table data] can be used to refresh the table when the actual contents of the table has
changed.
Fig. 14.32: Attribute table Main properties for ‘Current atlas feature’
Fig. 14.33: Attribute table Main properties for ‘Relation children’
• The button [Attributes. . . ] starts the Select attributes menu, see figure_layout_table_select, that can be used
to change the visible contents of the table. After making changes use the [OK] button to apply changes to
the table. The upper part of the window shows the list of the attributes to display and the lower part helps to
set the way the data is sorted.
In the Columns section you can:
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Fig. 14.34: Attribute table Select attributes Dialog
– Move attributes up or down the list by selecting the rows and then using the the
to shift the rows. Multiple rows can be selected and moved at any one time.
and
buttons
– Add an attribute with the
button. This will add an empty row at the bottom of the table where you
can select a field to be the attribute value or create an attribute via a regular expression.
– Remove an attribute with the
button. Multiple rows can be selected and removed at any one time.
– Reset the attribute table back to its default state with the [Reset] button.
– Clear the table using the [Clear] button. This is useful when you have a large table but only want to
show a small number of attributes. Instead of manually removing each row, it may be quicker to clear
the table and add the rows needed.
– Cell headings can be altered by adding the custom text in the Heading column.
– Cell alignment can be managed with the Alignment column which will dictate the texts position within
the table cell.
– Cell width can be manually managed by adding custom values to the width column.
In the Sorting section you can:
– Add an attribute to sort the table with. Select an attribute and set the sorting order to Ascending or
Descending and press the
button. A new line is added to the sort order list.
– select a row in the list and use the
and
buttons to change the sort priority on attribute level.
Selecting a cell in the Sort Order column helps you change the sorting order of the attribute field.
– use the
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button to remove an attribute from the sort order list.
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Feature filtering
The Feature filtering dialog of the attribute table provides the following functionalities (see figure_layout_table_filter):
Fig. 14.35: Attribute table Feature filtering Dialog
You can:
• Define the Maximum rows to be displayed.
• Activate
Remove duplicate rows from table to show unique records only.
• Activate
Show only visible features within a map and select the corresponding Linked map whose visible
features attributes will be displayed.
• Activate
Show only features intersecting Atlas feature is only available when
Generate an atlas is
activated. When activated it will show a table with only the features which intersect the current atlas feature.
• Activate
Filter with and provide a filter by typing in the input line or insert a regular expression using
the given
expression button. A few examples of filtering statements you can use when you have loaded
the airports layer from the Sample dataset:
– ELEV > 500
– NAME = 'ANIAK'
– NAME NOT LIKE 'AN%'
– regexp_match( attribute( $currentfeature, 'USE' ) , '[i]')
The last regular expression will include only the airports that have a letter ‘i’ in the attribute field ‘USE’.
Appearance
The Appearance dialog of the attribute table provides the following functionalities (see figure_layout_table_appearance):
Show empty rows to fill the attribute table with empty cells. This option can also be used to provide
• Click
additional empty cells when you have a result to show!
• With Cell margins you can define the margin around text in each cell of the table.
• With Display header you can select from a list one of ‘On first frame’, ‘On all frames’ default option, or
‘No header’.
• The option Empty table controls what will be displayed when the result selection is empty.
– Draw headers only, will only draw the header except if you have chosen ‘No header’ for Display
header.
– Hide entire table, will only draw the background of the table. You can activate
background if frame is empty in Frames to completely hide the table.
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Fig. 14.36: Attribute table appearance Dialog
– Show set message, will draw the header and adds a cell spanning all columns and display a message
like ‘No result’ that can be provided in the option Message to display
• The option Message to display is only activated when you have selected Show set message for Empty table.
The message provided will be shown in the table in the first row, when the result is an empty table.
• With Background color you can set the background color of the table. The Advanced customization option
helps you define different background colors for each cell (see figure_layout_table_background)
Fig. 14.37: Attribute table Advanced Background Dialog
• With the Wrap text on option, you can define a character on which the cell content will be wraped each time
it is met
• With Oversized text you define the behaviour when the width set for a column is smaller than its content’s
length. It can be Wrap text or Truncate text.
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Show grid
The Show grid dialog of the attribute table provides the following functionalities (see figure_layout_table_grid):
Fig. 14.38: Attribute table Show grid Dialog
• Activate
Show grid when you want to display the grid, the outlines of the table cells.
• With Line width you can set the thickness of the lines used in the grid.
• The Color of the grid can be set using the color selection dialog.
Fonts and text styling
The Fonts and text styling dialog of the attribute table provides the following functionalities (see figure_layout_table_fonts):
Fig. 14.39: Attribute table Fonts and text styling Dialog
• You can define Font and Color for Table heading and Table contents.
• For Table heading you can additionally set the Alignment to Follow column alignment or override this
setting by choosing Left, Center or Right. The column alignment is set using the Select Attributes dialog
(see figure_layout_table_select ).
Frames
The Frames dialog of the attribute table provides the following functionalities (see figure_layout_table_frames):
Fig. 14.40: Attribute table Frames Dialog
• With Resize mode you can select how to render the attribute table contents:
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– Use existing frames displays the result in the first frame and added frames only.
– Extend to next page will create as many frames (and corresponding pages) as necessary to display the
full selection of attribute table. Each frame can be moved around on the layout. If you resize a frame,
the resulting table will be divided up between the other frames. The last frame will be trimmed to fit
the table.
– Repeat until finished will also create as many frames as the Extend to next page option, except all
frames will have the same size.
• Use the [Add Frame] button to add another frame with the same size as selected frame. The result of the
table that will not fit in the first frame will continue in the next frame when you use the Resize mode Use
existing frames.
Don’t export page if frame is empty prevents the page to be exported when the table frame has
• Activate
no contents. This means all other layout items, maps, scalebars, legends etc. will not be visible in the result.
• Activate
Don’t draw background if frame is empty prevents the background to be drawn when the table
frame has no contents.
14.2.7 The Image Item
Add image
icon and drag a rectangle onto the layout canvas with the left mouse button.
To add an image, click the
You can then position and customize its appearance in the image Item Properties panel.
The image Item Properties tab provides the following functionalities (see figure_layout_image):
Fig. 14.41: Image Item Properties panel
You first have to select the image you want to display. There are several ways to set the image source in the Main
properties area.
1. Use the browse button
of image source to select a file on your computer using the browse dialog. The
browser will start in the SVG-libraries provided with QGIS. Besides SVG, you can also select other image
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formats like .png or .jpg.
2. You can enter the source directly in the image source text field. You can even provide a remote URL-address
to an image.
3. From the Search directories area you can also select an image from loading previews . . . to set the image
source.
4. Use the data defined button
to set the image source from a record or using a regular expression.
With the Resize mode option, you can set how the image is displayed when the frame is changed, or choose to
resize the frame of the image item so it matches the original size of the image.
You can select one of the following modes:
• Zoom: Enlarges the image to the frame while maintaining aspect ratio of picture.
• Stretch: Stretches image to fit inside the frame, ignores aspect ratio.
• Clip: Use this mode for raster images only, it sets the size of the image to original image size without scaling
and the frame is used to clip the image, so only the part of the image inside the frame is visible.
• Zoom and resize frame: Enlarges image to fit frame, then resizes frame to fit resultant image.
• Resize frame to image size: Sets size of frame to match original size of image without scaling.
Selected resize mode can disable the item options ‘Placement’ and ‘Image rotation’. The Image rotation is active
for the resize mode ‘Zoom’ and ‘Clip’.
With Placement you can select the position of the image inside its frame. The Search directories area allows you
to add and remove directories with images in SVG format to the picture database. A preview of the pictures found
in the selected directories is shown in a pane and can be used to select and set the image source. It is possible to
change SVG fill/outline color and outline width when using parameterized SVG files such as those included with
QGIS. If you add a SVG file you should add the following tags in order to add support for transparency:
• fill-opacity=”param(fill-opacity)”
• stroke-opacity=”param(outline-opacity)”
You can read this blog post to see an example.
Sync with map checkbox synchronizes the
Images can be rotated with the Image rotation field. Activating the
rotation of the image (i.e., a rotated north arrow) with the rotation applied to the selected map item.
It is also possible to select a north arrow directly. If you first select a north arrow image from Search directories
and then use the browse button
of the field Image source, you can now select one of the north arrow from
the list as displayed in figure_layout_image_north.
Note: Many of the north arrows do not have an ‘N’ added in the north arrow, this is done on purpose for languages
that do not use an ‘N’ for North, so they can use another letter.
14.2.8 The HTML Frame Item
It is possible to add a frame that displays the contents of a website or even create and style your own HTML page
and display it!
Add HTML frame
Click the
icon, place the element by dragging a rectangle holding down the left mouse button on the Print Layout canvas and position and customize the appearance in the Item Properties panel (see
figure_layout_html).
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Fig. 14.42: North arrows available for selection in provided SVG library
Fig. 14.43: HTML Frame, the Item Properties Panel
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HTML Source
As an HTML source, you can either set a URL and activate the URL radiobutton or enter the HTML source
directly in the textbox provided and activate the Source radiobutton.
The HTML Source dialog of the HTML frame Item Properties panel provides the following functionalities (see
figure_layout_html_ppt):
Fig. 14.44: HTML frame, the HTML Source properties
• In URL you can enter the URL of a webpage you copied from your Internet browser or select an HTML file
using the browse button
. There is also the option to use the Data defined override button, to provide
an URL from the contents of an attribute field of a table or using a regular expression.
• In Source you can enter text in the textbox with some HTML tags or provide a full HTML page.
• The [insert an expression] button can be used to insert an expression like [%Year($now)%] in the
Source textbox to display the current year. This button is only activated when radiobutton Source is selected.
After inserting the expression click somewhere in the textbox before refreshing the HTML frame, otherwise
you will lose the expression.
• Activate
Evaluate QGIS expressions in HTML code to see the result of the expression you have included,
otherwise you will see the expression instead.
• Use the [Refresh HTML] button to refresh the HTML frame(s) to see the result of changes.
Frames
The Frames dialog of the HTML frame Item Properties panel provides the following functionalities (see figure_layout_html_frames):
Fig. 14.45: HTML frame, the Frames properties
• With Resize mode you can select how to render the HTML contents:
– Use existing frames displays the result in the first frame and added frames only.
– Extend to next page will create as many frames (and corresponding pages) as necessary to render the
height of the web page. Each frame can be moved around on the layout. If you resize a frame, the
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webpage will be divided up between the other frames. The last frame will be trimmed to fit the web
page.
– Repeat on every page will repeat the upper left of the web page on every page in frames of the same
size.
– Repeat until finished will also create as many frames as the Extend to next page option, except all
frames will have the same size.
• Use the [Add Frame] button to add another frame with the same size as selected frame. If the HTML page
that will not fit in the first frame it will continue in the next frame when you use Resize mode or Use existing
frames.
Don’t export page if frame is empty prevents the map layout from being exported when the
• Activate
frame has no HTML contents. This means all other layout items, maps, scalebars, legends etc. will not be
visible in the result.
• Activate
is empty.
Don’t draw background if frame is empty prevents the HTML frame being drawn if the frame
Use smart page breaks and User style sheet
The Use smart page breaks dialog and Use style sheet dialog of the HTML frame Item Properties panel provides
the following functionalities (see figure_layout_html_breaks):
Fig. 14.46: HTML frame, Use smart page breaks and User stylesheet properties
• Activate
Use smart page breaks to prevent the html frame contents from breaking mid-way a line of text
so it continues nice and smooth in the next frame.
• Set the Maximum distance allowed when calculating where to place page breaks in the html. This distance
is the maximum amount of empty space allowed at the bottom of a frame after calculating the optimum
break location. Setting a larger value will result in better choice of page break location, but more wasted
space at the bottom of frames. This is only used when Use smart page breaks is activated.
• Activate
User stylesheet to apply HTML styles that often is provided in cascading style sheets. An
example of style code is provide below to set the color of <h1> header tag to green and set the font and
fontsize of text included in paragraph tags <p>.
h1 {color: #00ff00;
}
p {font-family: "Times New Roman", Times, serif;
font-size: 20px;
}
• Use the [Update HTML] button to see the result of the stylesheet settings.
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14.2.9 Shape Items
The Arrow Item
Add Arrow
To add an arrow, click the
icon, place the element holding down the left mouse button and drag a
line to draw the arrow on the print layout canvas and position and customize the appearance in the scale bar Item
Properties panel.
When you also hold down the Shift key while placing the arrow, it is placed in an angle of exactly 45° .
The arrow item can be used to add a line or a simple arrow that can be used, for example, to show the relation
between other print layout items. To create a north arrow, the image item should be considered first. QGIS has
a set of North arrows in SVG format. Furthermore you can connect an image item with a map so it can rotate
automatically with the map (see The Image Item).
Fig. 14.47: Arrow Item Properties Panel
Item Properties
The Arrow item properties panel allows you to configure an arrow item.
The [Line style. . . ] button can be used to set the line style using the line style symbol editor.
In Arrows markers you can select one of three radio buttons.
• Default: To draw a regular arrow, gives you options to style the arrow head
• None: To draw a line without arrow head
• SVG Marker: To draw a line with an SVG Start marker and/or End marker
For Default Arrow marker you can use following options to style the arrow head.
• Arrow outline color: Set the outline color of the arrow head
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• Arrow fill color: Set the fill color of the arrow head
• Arrow outline width: Set the outline width of the arrow head
• Arrow head width: Set the size of the arrow head
For SVG Marker you can use following options.
• Start marker: Choose an SVG image to draw at the beginning of the line
• End marker: Choose an SVG image to draw at the end of the line
• Arrow head width: Set the size of Start and/or End marker
SVG images are automatically rotated with the line. Outline and fill colors of QGIS predefined SVG images can
be changed using the corresponding options. Custom SVG may require some tags following this instruction.
The Basic Shape Items
Add basic shape
icon, place the element holding down
To add a basic shape (ellipse, rectangle, triangle), click the
the left mouse. Customize the appearance in the Item Properties panel.
When you also hold down the Shift key while placing the basic shape you can create a perfect square, circle or
triangle.
Fig. 14.48: Shape Item Properties Panel
The Shape item properties panel allows you to select if you want to draw an ellipse, rectangle or triangle inside
the given frame.
You can set the style of the shape using the advanced symbol style dialog with which you can define its outline
and fill color, fill pattern, use markers. . .
For the rectangle shape, you can set the value of the corner radius to round of the corners.
Note: Unlike other items, you can not style the frame or the background color of the frame.
The Node-Based Shape Items
While arrow and basic shape items offer you simple and predefined geometric item to use, a node-based shape
(polygon or polyline) helps you create a custom and more advanced geometric item. You can add as many lines
or sides as you want to the item and independently and directly interact with each of its vertices.
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Add nodes item
To add a node-based shape, click the
icon. Then perform left clicks to add nodes to your current
shape. When you’re done, a simple right click terminates the shape. Customize the appearance in the Item
Properties panel.
Fig. 14.49: Nodes Shape Item Properties Panel
You can set the style of the shape using the advanced symbol style dialog available thanks to the [Change. . . ]
button in Main properties.
Edit Nodes Item
. Within this mode, you can select
A specific tool is provided to edit node-based shapes through
a node by clicking on it (a marker is displayed on the selected node). A selected node can be moved either by
dragging it or by using the arrow keys. Moreover, in this mode, you are able to add nodes to an existing shape.
You just have to do a left click near a segment and if you are not too far from the shape, a node is added. Finally,
you can remove the currently selected node by hitting the DEL key.
14.3 Creating an Output
figure_layout_output shows the print layout with an example print layout, including each type of map item described in the previous section.
Before printing a layout you have the possibility to view your composition without bounding boxes. This can be
enabled by deactivating View →
Show bounding boxes or pressing the shortcut Ctrl+Shift+B.
The print layout allows you to create several output formats, and it is possible to define the resolution (print
quality) and paper size:
Print
• The
icon allows you to print the layout to a connected printer or a PostScript file, depending on
installed printer drivers.
• The
JPG,. . .
Export as image
icon exports the layout canvas in several image formats, such as PNG, BPM, TIF,
• The
Export as SVG
icon saves the print layout canvas as an SVG (Scalable Vector Graphic).
• The
Export as PDF
icon saves the defined print layout canvas directly as a PDF.
14.3.1 Export as Image
Export as image
Clicking the
icon will ask you to enter the filename to use to export composition: in the case of
multi-page composition, each page will be exported to a file with the given name appended with the page number.
You can then override the print resolution and the exported image dimensions (set in Composition panel).
By checking
Crop to content option, the image output by the layout includes the minimal area enclosing all
the items (map, legend, scale bar, shapes, label, image. . . ) of each page of the composition:
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Fig. 14.50: Print Layout with map view, legend, image, scale bar, coordinates, text and HTML frame added
• If the composition includes a single page, then the output is resized to include EVERYTHING on the
composition. The page can then be reduced or extended to all items depending on their position (on, above,
below, left or right of the page).
• In case of a multi-page composition, each page will be resized to include items in its area (left and right
sides for all pages, plus top for the first page and bottom for the last page). Each resized page is exported to
a separate file.
The Crop to content dialog also allows to add some margins around the cropped bounds.
If you need to export your layout as a georeferenced image (e.g., to share with other projects), you need to enable
this feature under the Composition Panel.
If the output format is a TIFF format, all you need to do is making sure to select the correct map item to use in
Reference map, and the output will always be a GeoTIFF. For other image formats, you also need to check
the
Save world file option. With this option, the ‘Export as image’ action will create a world file along with
the exported image.
Note: Exporting large rasters can sometimes fail, even if there seems to be enough memory. This is a problem
with the underlying Qt management of rasters.
14.3.2 Export as SVG
With
Export as SVG
, you also need to fill the filename (used as a basename for all files in case of multi-page
composition) and then can apply
14.3. Creating an Output
Crop to content option.
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Fig. 14.51: Image Export Options, output is resized to items extent
The SVG export options dialog also allows to:
• export map layers as svg groups:
• render map labels as outlines
Fig. 14.52: SVG Export Options
Note: Currently, the SVG output is very basic. This is not a QGIS problem, but a problem with the underlying
Qt library. This will hopefully be sorted out in future versions.
14.3.3 Export as PDF
The
Export as PDF
exports all the composition into a single PDF file.
If you applied to your composition or any shown layer an advanced effect such as blend modes, transparency or
symbol effects, these cannot be printed as vectors, and the effects may be lost. Checking Print as a raster in the
Composition Panel helps to keep the effects but rasterize the composition. Note that the Force layer to render as
raster in the Rendering tab of Layer Properties dialog is a layer-level alternative that avoids global composition
rasterization.
If you need to export your layout as a georeferenced PDF, in the Composition Panel, make sure to select the
correct map item to use in
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14.3.4 Generate an Atlas
The print layout includes generation functions that allow you to create map books in an automated way. The
concept is to use a coverage layer, which contains geometries and fields. For each geometry in the coverage layer,
a new output will be generated where the content of some canvas maps will be moved to highlight the current
geometry. Fields associated with this geometry can be used within text labels.
Every page will be generated with each feature. To enable the generation of an atlas and access generation
parameters, refer to the Atlas generation panel.This panel contains the following widgets (see figure_layout_atlas):
Fig. 14.53: Atlas Generation Panel
•
Generate an atlas, which enables or disables the atlas generation.
• A Coverage layer
which to iterate over.
combo box that allows you to choose the (vector) layer containing the features on
• An optional
Hidden coverage layer that, if checked, will hide the coverage layer (but not the other ones)
during the generation.
• An optional Page name combo box to give a more explicit name to each feature page(s) when previewing
atlas. You can select an attribute of the coverage layer or set an expression. If this option is empty, QGIS
will use an internal ID, according to the filter and/or the sort order applied to the layer.
• An optional Filter with text area that allows you to specify an expression for filtering features from the
coverage layer. If the expression is not empty, only features that evaluate to True will be selected. The
button on the right allows you to display the expression builder.
• An optional
Sort by that, if checked, allows you to sort features of the coverage layer. The associated
combo box allows you to choose which column will be used as the sorting key. Sort order (either ascending
or descending) is set by a two-state button that displays an up or a down arrow.
You also have options to set the output of the atlas:
• An Output filename expression textbox that is used to generate a filename for each geometry if needed. It is
based on expressions. This field is meaningful only for rendering to multiple files.
• A
Single file export when possible that allows you to force the generation of a single file if this is possible
with the chosen output format (PDF, for instance). If this field is checked, the value of the Output filename
expression field is meaningless.
You can use multiple map items with the atlas generation; each map will be rendered according to the coverage
features. To enable atlas generation for a specific map item, you need to check
item properties of the map item. Once checked, you can set:
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Controlled by Atlas under the
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• A
Margin around feature that allows you to select the amount of space added around each geometry
within the allocated map. Its value is meaningful only when using the auto-scaling mode.
Predefined scale (best fit). It will use the best fitting option from the list of predefined scales in your
• A
project properties settings (see Project → Project Properties → General → Project Scales to configure
these predefined scales).
• A
Fixed scale that allows you to toggle between auto-scale and fixed-scale mode. In fixed-scale mode,
the map will only be translated for each geometry to be centered. In auto-scale mode, the map’s extents are
computed in such a way that each geometry will appear in its entirety.
Labels
In order to adapt labels to the feature the atlas plugin iterates over, you can include expressions. What you should
take care of is to place expression part (including functions, fields or variables) between [% and %]. For example,
for a city layer with fields CITY_NAME and ZIPCODE, you could insert this:
The area of [% upper(CITY_NAME) || ',' || ZIPCODE || ' is '
format_number($area/1000000,2) %] km2
or, another combination:
The area of [% upper(CITY_NAME)%],[%ZIPCODE%] is
[%format_number($area/1000000,2) %] km2
The
information
[% upper(CITY_NAME) || ',' || ZIPCODE || ' is '
format_number($area/1000000,2) %] is an expression used inside the label. Both expressions
would result in the generated atlas as:
The area of PARIS,75001 is 1.94 km2
Data Defined Override Buttons
Data Defined Override
There are several places where you can use a
button to override the selected setting. These
options are particularly useful with Atlas Generation.
For the following examples the Regions layer of the QGIS sample dataset is used and selected for Atlas Generation.
We also assume the paper format A4 (210X297) is selected in the Composition panel for field Presets.
With a Data Defined Override button you can dynamically set the paper orientation. When the height (northsouth) of the extents of a region is greater than its width (east-west), you rather want to use portrait instead of
landscape orientation to optimize the use of paper.
In the Composition you can set the field Orientation and select Landscape or Portrait. We want to set the orientabutton of field Orientation,
tion dynamically using an expression depending on the region geometry. Press the
select Edit. . . so the Expression string builder dialog opens. Enter the following expression:
CASE WHEN bounds_width($atlasgeometry) > bounds_height($atlasgeometry)
THEN 'Landscape' ELSE 'Portrait' END
Now the paper orients itself automatically. For each Region you need to reposition the location of the layout
item as well. For the map item you can use the
expression:
button of field Width to set it dynamically using following
(CASE WHEN bounds_width($atlasgeometry) > bounds_height($atlasgeometry)
THEN 297 ELSE 210 END) - 20
Use the
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(CASE WHEN bounds_width($atlasgeometry) > bounds_height($atlasgeometry)
THEN 210 ELSE 297 END) - 20
When you want to give a title above the map in the center of the page, insert a label item above the map. First use
the item properties of the label item to set the horizontal alignment to
Center. Next activate from Reference
point the upper middle checkbox. You can provide the following expression for field X :
(CASE WHEN bounds_width($atlasgeometry) > bounds_height($atlasgeometry)
THEN 297 ELSE 210 END) / 2
For all other layout items you can set the position in a similar way so they are correctly positioned when the page
is automatically rotated in portrait or landscape.
Information provided is derived from the excellent blog (in English and Portuguese) on the Data Defined Override
options Multiple_format_map_series_using_QGIS_2.6 .
This is just one example of how you can use the Data Defined Override option.
Preview and generate
Fig. 14.54: Atlas Preview toolbar
Once the atlas settings have been configured and layout items (map, table, image. . . ) linked to it, you can create a
preview of all the pages by clicking Atlas → Preview Atlas or
the same toolbar to navigate through all the features:
•
First feature
•
Previous feature
•
Next feature
•
Last feature
Preview Atlas
icon. You can then use the arrows in
You can also use the combo box to directly select and preview a specific feature. The combo box shows atlas
features name according to the expression set in the atlas Page name option.
As for simple compositions, an atlas can be generated in different ways (see Creating an Output for more information). Instead of Layout menu, rather use tools from Atlas menu or Atlas toolbar.
This means that you can directly print your compositions with Atlas → Print Atlas. You can also create a PDF
using Atlas → Export Atlas as PDF. . . : The user will be asked for a directory to save all the generated PDF files,
except if the
filename.
Single file export when possible has been selected. In that case, you’ll be prompted to give a
With Atlas → Export Atlas as Images. . . or Atlas → Export Atlas as SVG. . . tool, you’re also prompted to select
a folder. Each page of each atlas feature composition is exported to an image or SVG file.
Tip: Print a specific atlas feature
If you want to print or export the composition of only one feature of the atlas, simply start the preview, select the
desired feature in the drop-down list and click on Layout → Print (or Export. . . to any supported file format).
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15
Working with OGC Data
15.1 QGIS as OGC Data Client
The Open Geospatial Consortium (OGC) is an international organization with membership of more than 300
commercial, governmental, nonprofit and research organizations worldwide. Its members develop and implement
standards for geospatial content and services, GIS data processing and exchange.
Describing a basic data model for geographic features, an increasing number of specifications are developed
by OGC to serve specific needs for interoperable location and geospatial technology, including GIS. Further
information can be found at http://www.opengeospatial.org/.
Important OGC specifications supported by QGIS are:
• WMS — Web Map Service (WMS/WMTS Client)
• WMTS — Web Map Tile Service (WMS/WMTS Client)
• WFS — Web Feature Service (WFS and WFS-T Client)
• WFS-T — Web Feature Service - Transactional (WFS and WFS-T Client)
• WCS — Web Coverage Service (WCS Client)
• WPS — Web Processing Service
• CSW — Catalog Service for the Web
• SFS — Simple Features for SQL (PostGIS Layers)
• GML — Geography Markup Language
OGC services are increasingly being used to exchange geospatial data between different GIS implementations and
data stores. QGIS can deal with the above specifications as a client, being SFS (through support of the PostgreSQL
/ PostGIS data provider, see section PostGIS Layers).
15.1.1 WMS/WMTS Client
Overview of WMS Support
QGIS currently can act as a WMS client that understands WMS 1.1, 1.1.1 and 1.3 servers. In particular, it has
been tested against publicly accessible servers such as DEMIS.
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A WMS server acts upon requests by the client (e.g., QGIS) for a raster map with a given extent, set of layers,
symbolization style, and transparency. The WMS server then consults its local data sources, rasterizes the map,
and sends it back to the client in a raster format. For QGIS, this format would typically be JPEG or PNG.
WMS is generically a REST (Representational State Transfer) service rather than a full-blown Web service. As
such, you can actually take the URLs generated by QGIS and use them in a web browser to retrieve the same
images that QGIS uses internally. This can be useful for troubleshooting, as there are several brands of WMS
server on the market and they all have their own interpretation of the WMS standard.
WMS layers can be added quite simply, as long as you know the URL to access the WMS server, you have a
serviceable connection to that server, and the server understands HTTP as the data transport mechanism.
Additionally, QGIS will cache your WMS responses (i.e. images) for 24h as long as the GetCapabilities request is
not triggered. The GetCapabilities request is triggered everytime the [Connect] button in the [Add layer(s) from
WMS(T)S Server] dialog is used to retrieve the WMS server capabilities. This is an automatic feature meant to
optimize project loading time. If a project is saved with a WMS layer, the corresponding WMS tiles will be loaded
from the cache the next time the project is opened as long as they are no older than 24H.
Overview of WMTS Support
QGIS can also act as a WMTS client. WMTS is an OGC standard for distributing tile sets of geospatial data.
This is a faster and more efficient way of distributing data than WMS because with WMTS, the tile sets are pregenerated, and the client only requests the transmission of the tiles, not their production. A WMS request typically
involves both the generation and transmission of the data. A well-known example of a non-OGC standard for
viewing tiled geospatial data is Google Maps.
In order to display the data at a variety of scales close to what the user might want, the WMTS tile sets are
produced at several different scale levels and are made available for the GIS client to request them.
This diagram illustrates the concept of tile sets:
Fig. 15.1: Concept of WMTS tile sets
The two types of WMTS interfaces that QGIS supports are via Key-Value-Pairs (KVP) and RESTful. These two
interfaces are different, and you need to specify them to QGIS differently.
1. In order to access a WMTS KVP service, a QGIS user must open the WMS/WMTS interface and add the
following string to the URL of the WMTS tile service:
"?SERVICE=WMTS&REQUEST=GetCapabilities"
An example of this type of address is
http://opencache.statkart.no/gatekeeper/gk/gk.open_wmts?\
service=WMTS&request=GetCapabilities
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For testing the topo2 layer in this WMTS works nicely. Adding this string indicates that a WMTS web
service is to be used instead of a WMS service.
2. The RESTful WMTS service takes a different form, a straightforward URL. The format recommended by
the OGC is:
{WMTSBaseURL}/1.0.0/WMTSCapabilities.xml
This format helps you to recognize that it is a RESTful address. A RESTful WMTS is accessed in QGIS by
simply adding its address in the WMS setup in the URL field of the form. An example of this type of address
for the case of an Austrian basemap is http://maps.wien.gv.at/basemap/1.0.0/WMTSCapabilities.xml.
Note: You can still find some old services called WMS-C. These services are quite similar to WMTS (i.e., same
purpose but working a little bit differently). You can manage them the same as you do WMTS services. Just add
?tiled=true at the end of the url. See http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification for more
information about this specification.
When you read WMTS, you can often think WMS-C also.
Selecting WMS/WMTS Servers
The first time you use the WMS feature in QGIS, there are no servers defined.
Begin by clicking the
Add WMS layer
button on the toolbar, or selecting Layer → Add WMS Layer. . . .
The dialog Add Layer(s) from a Server for adding layers from the WMS server appears. You can add some servers
to play with by clicking the [Add default servers] button. This will add two WMS demo servers for you to use:
the WMS servers of the DM Solutions Group and Lizardtech. To define a new WMS server in the Layers tab, select
the [New] button. Then enter the parameters to connect to your desired WMS server, as listed in table_OGC_wms:
Name
URL
Username
Password
A name for this connection. This name will be used in the Server Connections drop-down
box so that you can distinguish it from other WMS servers.
URL of the server providing the data. This must be a resolvable host name – the same
format as you would use to open a telnet connection or ping a host.
Username to access a secured WMS server. This parameter is optional.
Password for a basic authenticated WMS server. This parameter is optional.
Ignore GetMap
URI
Ignore GetMap URI reported in capabilities. Use given URI from URL field above.
Ignore GetFeatureInfo URI
Ignore GetFeatureInfo URI reported in capabilities. Use given URI from URL field
above.
Table OGC 1: WMS Connection Parameters
If you need to set up a proxy server to be able to receive WMS services from the internet, you can add your proxy
server in the options. Choose Settings → Options and click on the Network & Proxy tab. There, you can add your
proxy settings and enable them by setting
proxy type from the Proxy type
Use proxy for web access. Make sure that you select the correct
drop-down menu.
Once the new WMS server connection has been created, it will be preserved for future QGIS sessions.
Tip: On WMS Server URLs
Be sure, when entering the WMS server URL, that you have the base URL only. For example, you shouldn’t have
fragments such as request=GetCapabilities or version=1.0.0 in your URL.
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Warning: Entering username and password in the Authentication tab will keep unprotected credentials in
the connection configuration. Those credentials will be visible if, for instance, you shared the project file
with someone. Therefore, it’s advisable to save your credentials in a Authentication configuration instead
(configurations tab). See Authentication System for more details.
Loading WMS/WMTS Layers
Once you have successfully filled in your parameters, you can use the [Connect] button to retrieve the capabilities
of the selected server. This includes the image encoding, layers, layer styles and projections. Since this is a
network operation, the speed of the response depends on the quality of your network connection to the WMS
server. While downloading data from the WMS server, the download progress is visualized in the lower left of the
WMS dialog.
Your screen should now look a bit like figure_OGC_add_wms, which shows the response provided by the European Soil Portal WMS server.
Fig. 15.2: Dialog for adding a WMS server, showing its available layers
Image Encoding
The Image encoding section lists the formats that are supported by both the client and server. Choose one depending on your image accuracy requirements.
Tip: Image Encoding
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You will typically find that a WMS server offers you the choice of JPEG or PNG image encoding. JPEG is a lossy
compression format, whereas PNG faithfully reproduces the raw raster data.
Use JPEG if you expect the WMS data to be photographic in nature and/or you don’t mind some loss in picture
quality. This trade-off typically reduces by five times the data transfer requirement compared with PNG.
Use PNG if you want precise representations of the original data and you don’t mind the increased data transfer
requirements.
Options
The Options area of the dialog provides a text field where you can add a Layer name for the WMS layer. This
name will appear in the legend after loading the layer.
Below the layer name, you can define Tile size if you want to set tile sizes (e.g., 256x256) to split up the WMS
request into multiple requests.
The Feature limit for GetFeatureInfo defines what features from the server to query.
If you select a WMS from the list, a field with the default projection provided by the mapserver appears. If the
[Change. . . ] button is active, you can click on it and change the default projection of the WMS to another CRS
provided by the WMS server.
Use contextual WMS-Legend if the WMS Server supports this feature. Then only the
Finally you can activate
relevant legend for your current map view extent will be shown and thus will not include legend items for things
you can’t see in the current map.
Layer Order
The Layer Order tab lists the selected layers available from the current connected WMS server. You may notice
that some layers are expandable; this means that the layer can be displayed in a choice of image styles.
You can select several layers at once, but only one image style per layer. When several layers are selected, they
will be combined at the WMS server and transmitted to QGIS in one go.
Tip: WMS Layer Ordering
WMS layers rendered by a server are overlaid in the order listed in the Layers section, from top to bottom of the
list. If you want to change the overlay order, you can use the Layer Order tab.
Transparency
In this version of QGIS, the Global transparency setting from the Layer Properties is hard coded to be always on,
where available.
Tip: WMS Layer Transparency
The availability of WMS image transparency depends on the image encoding used: PNG and GIF support transparency, whilst JPEG leaves it unsupported.
Coordinate Reference System
A coordinate reference system (CRS) is the OGC terminology for a QGIS projection.
Each WMS layer can be presented in multiple CRSs, depending on the capability of the WMS server.
To choose a CRS, select [Change. . . ] and a dialog similar to Custom CRS Dialog will appear. The main difference
with the WMS version of the dialog is that only those CRSs supported by the WMS server will be shown.
Server search
Within QGIS, you can search for WMS servers. Figure_OGC_search shows the Server Search tab with the Add
Layer(s) from a Server dialog.
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Fig. 15.3: Dialog for searching WMS servers after some keywords
As you can see, it is possible to enter a search string in the text field and hit the [Search] button. After a short
while, the search result will be populated into the list below the text field. Browse the result list and inspect your
search results within the table. To visualize the results, select a table entry, press the [Add selected row to WMS
list] button and change back to the Layers tab. QGIS has automatically updated your server list, and the selected
search result is already enabled in the list of saved WMS servers in the Layers tab. You only need to request the list
of layers by clicking the [Connect] button. This option is quite handy when you want to search maps by specific
keywords.
Basically, this option is a front end to the API of http://geopole.org.
Tilesets
When using WMTS (Cached WMS) services like
http://opencache.statkart.no/gatekeeper/gk/gk.open_wmts?\
service=WMTS&request=GetCapabilities
you are able to browse through the Tilesets tab given by the server. Additional information like tile size, formats
and supported CRS are listed in this table. In combination with this feature, you can use the tile scale slider by
selecting View → Panels ( or
Settings → Panels), then choosing Tile Scale Panel. This gives you the available
scales from the tile server with a nice slider docked in.
Using the Identify Tool
Once you have added a WMS server, and if any layer from a WMS server is queryable, you can then use the
Identify
tool to select a pixel on the map canvas. A query is made to the WMS server for each selection made. The
results of the query are returned in plain text. The formatting of this text is dependent on the particular WMS
server used. Format selection
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If multiple output formats are supported by the server, a combo box with supported formats is automatically added
to the identify results dialog and the selected format may be stored in the project for the layer. GML format
support
Identify
tool supports WMS server response (GetFeatureInfo) in GML format (it is called Feature in the
The
QGIS GUI in this context). If “Feature” format is supported by the server and selected, results of the Identify tool
are vector features, as from a regular vector layer. When a single feature is selected in the tree, it is highlighted
in the map and it can be copied to the clipboard and pasted to another vector layer. See the example setup of the
UMN Mapserver below to support GetFeatureInfo in GML format.
# in layer METADATA add which fields should be included and define geometry
˓→(example):
"gml_include_items"
"ows_geometries"
"ows_mygeom_type"
"all"
"mygeom"
"polygon"
# Then there are two possibilities/formats available, see a) and b):
# a) basic (output is generated by Mapserver and does not contain XSD)
# in WEB METADATA define formats (example):
"wms_getfeatureinfo_formatlist" "application/vnd.ogc.gml,text/html"
# b) using OGR (output is generated by OGR, it is send as multipart and contains
˓→XSD)
# in MAP define OUTPUTFORMAT (example):
OUTPUTFORMAT
NAME "OGRGML"
MIMETYPE "ogr/gml"
DRIVER "OGR/GML"
FORMATOPTION "FORM=multipart"
END
# in WEB METADATA define formats (example):
"wms_getfeatureinfo_formatlist" "OGRGML,text/html"
Viewing Properties
Once you have added a WMS server, you can view its properties by right-clicking on it in the legend and selecting
Properties. Metadata Tab
The tab Metadata displays a wealth of information about the WMS server, generally collected from the capabilities
statement returned from that server. Many definitions can be gleaned by reading the WMS standards (see OPENGEOSPATIAL-CONSORTIUM in Literature and Web References), but here are a few handy definitions:
• Server Properties
– WMS Version — The WMS version supported by the server.
– Image Formats — The list of MIME-types the server can respond with when drawing the map.
QGIS supports whatever formats the underlying Qt libraries were built with, which is typically at least
image/png and image/jpeg.
– Identity Formats — The list of MIME-types the server can respond with when you use the Identify
tool. Currently, QGIS supports the text-plain type.
• Layer Properties
– Selected — Whether or not this layer was selected when its server was added to this project.
– Visible — Whether or not this layer is selected as visible in the legend (not yet used in this version of
QGIS).
– Can Identify — Whether or not this layer will return any results when the Identify tool is used on it.
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– Can be Transparent — Whether or not this layer can be rendered with transparency. This version of
QGIS will always use transparency if this is Yes and the image encoding supports transparency.
– Can Zoom In — Whether or not this layer can be zoomed in by the server. This version of QGIS
assumes all WMS layers have this set to Yes. Deficient layers may be rendered strangely.
– Cascade Count — WMS servers can act as a proxy to other WMS servers to get the raster data for a
layer. This entry shows how many times the request for this layer is forwarded to peer WMS servers
for a result.
– Fixed Width, Fixed Height — Whether or not this layer has fixed source pixel dimensions. This
version of QGIS assumes all WMS layers have this set to nothing. Deficient layers may be rendered
strangely.
– WGS 84 Bounding Box — The bounding box of the layer, in WGS 84 coordinates. Some WMS
servers do not set this correctly (e.g., UTM coordinates are used instead). If this is the case, then
the initial view of this layer may be rendered with a very ‘zoomed-out’ appearance by QGIS. The
WMS webmaster should be informed of this error, which they may know as the WMS XML elements
LatLonBoundingBox, EX_GeographicBoundingBox or the CRS:84 BoundingBox.
– Available in CRS — The projections that this layer can be rendered in by the WMS server. These are
listed in the WMS-native format.
– Available in style — The image styles that this layer can be rendered in by the WMS server.
Show WMS legend graphic in table of contents and layout
The QGIS WMS data provider is able to display a legend graphic in the table of contents’ layer list and in the print
layout. The WMS legend will be shown only if the WMS server has GetLegendGraphic capability and the layer
has getCapability url specified, so you additionally have to select a styling for the layer.
If a legendGraphic is available, it is shown below the layer. It is little and you have to click on it to open it in real
dimension (due to QgsLegendInterface architectural limitation). Clicking on the layer’s legend will open a frame
with the legend at full resolution.
In the print layout, the legend will be integrated at it’s original (downloaded) dimension. Resolution of the legend
graphic can be set in the item properties under Legend –> WMS LegendGraphic to match your printing requirements
The legend will display contextual information based on your current scale. The WMS legend will be shown only
if the WMS server has GetLegendGraphic capability and the layer has getCapability url specified, so you have to
select a styling.
WMS Client Limitations
Not all possible WMS client functionality had been included in this version of QGIS. Some of the more noteworthy
exceptions follow.
Editing WMS Layer Settings
Add WMS layer
procedure, there is no way to change the settings. A work-around is
Once you’ve completed the
to delete the layer completely and start again.
WMS Servers Requiring Authentication
Currently, publicly accessible and secured WMS services are supported. The secured WMS servers can be accessed by public authentication. You can add the (optional) credentials when you add a WMS server. See section
Selecting WMS/WMTS Servers for details.
Tip: Accessing secured OGC-layers
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If you need to access secured layers with secured methods other than basic authentication, you can use InteProxy
as a transparent proxy, which does support several authentication methods. More information can be found in the
InteProxy manual at http://inteproxy.wald.intevation.org.
Tip: QGIS WMS Mapserver
Since Version 1.7.0, QGIS has its own implementation of a WMS 1.3.0 Mapserver. Read more about this in
chapter QGIS as OGC Data Server.
15.1.2 WCS Client
A Web Coverage Service (WCS) provides access to raster data in forms that are useful for client-side rendering, as input into scientific models, and for other clients. The WCS may be compared to the WFS and the WMS.
As WMS and WFS service instances, a WCS allows clients to choose portions of a server’s information holdings
based on spatial constraints and other query criteria.
QGIS has a native WCS provider and supports both version 1.0 and 1.1 (which are significantly different), but
currently it prefers 1.0, because 1.1 has many issues (i.e., each server implements it in a different way with various
particularities).
The native WCS provider handles all network requests and uses all standard QGIS network settings (especially
proxy). It is also possible to select cache mode (‘always cache’, ‘prefer cache’, ‘prefer network’, ‘always network’), and the provider also supports selection of time position, if temporal domain is offered by the server.
Warning: Entering username and password in the Authentication tab will keep unprotected credentials in
the connection configuration. Those credentials will be visible if, for instance, you shared the project file
with someone. Therefore, it’s advisable to save your credentials in a Authentication configuration instead
(configurations tab). See Authentication System for more details.
15.1.3 WFS and WFS-T Client
In QGIS, a WFS layer behaves pretty much like any other vector layer. You can identify and select features, and
view the attribute table. Since QGIS 1.6, editing WFS-T is also supported.
In general, adding a WFS layer is very similar to the procedure used with WMS. The difference is that there are
no default servers defined, so we have to add our own.
Loading a WFS Layer
As an example, we use the DM Solutions WFS server and display a layer. The URL is: http://www2.dmsolutions.
ca/cgi-bin/mswfs_gmap
1. Click on the
Add WFS Layer
tool on the Layers toolbar. The Add WFS Layer from a Server dialog appears.
2. Click on [New].
3. Enter ‘DM Solutions’ as name.
4. Enter the URL (see above).
5. Click [OK].
6. Choose ‘DM Solutions’ from the Server Connections
drop-down list.
7. Click [Connect].
8. Wait for the list of layers to be populated.
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9. Select the Parks layer in the list.
10. Click [Apply] to add the layer to the map.
Note that any proxy settings you may have set in your preferences are also recognized.
In the WFS settings dialog, you can define the maximal number of features downloaded, set up the version, force
to Ignore axis orientation (WFS 1.1/WFS 2.0) and force Inverse axis orientation.
Warning: Entering username and password in the Authentication tab will keep unprotected credentials in
the connection configuration. Those credentials will be visible if, for instance, you shared the project file
with someone. Therefore, it’s advisable to save your credentials in a Authentication configuration instead
(configurations tab). See Authentication System for more details.
Fig. 15.4: Adding a WFS layer
You’ll notice the download progress is visualized in the lower left of the QGIS main window. Once the layer is
loaded, you can identify and select a province or two and view the attribute table.
Note: About differences between WFS versions
WFS 1.0.0, 1.1.0 and 2.0 are supported. Background download and progressive rendering, on-disk caching of
downloaded features and version autodetection are now supported.
Only WFS 2.0 service supports GetFeature paging.
Tip: Finding WFS Servers
You can find additional WFS servers by using Google or your favourite search engine. There are a number of lists
with public URLs, some of them maintained and some not.
15.2 QGIS as OGC Data Server
QGIS Server is an open source WMS 1.3, WFS 1.0.0, WFS 1.1.0 and WCS 1.1.1 implementation that, in addition,
implements advanced cartographic features for thematic mapping. QGIS Server is a FastCGI/CGI (Common
Gateway Interface) application written in C++ that works together with a web server (e.g., Apache, Lighttpd).
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It has Python plugin support allowing for fast and efficient development and deployment of new features. The
original development of QGIS Server was funded by the EU projects Orchestra, Sany and the city of Uster in
Switzerland.
QGIS Server uses QGIS as back end for the GIS logic and for map rendering. Furthermore, the Qt library is
used for graphics and for platform-independent C++ programming. In contrast to other WMS software, the QGIS
Server uses cartographic rules as a configuration language, both for the server configuration and for the userdefined cartographic rules.
As QGIS desktop and QGIS Server use the same visualization libraries, the maps that are published on the web
look the same as in desktop GIS.
In the following sections, we will provide a sample configuration to set up a QGIS Server on Debian/Ubuntu
Linux. For more detailed installation instructions on other platforms or distributions and more information on
working with QGIS Server, we recommend reading the QGIS Server Training Manual or server_plugins.
15.2.1 Getting Started
Installation
At this point, we will give a short and simple installation how-to for a minimal working configuration on Debian
based systems. However, many other distributions and OSs provide packages for QGIS Server.
Requirements and steps to install QGIS Server on a Debian based system are provided in QGIS installers page.
Please refer to that section.
HTTP Server configuration
Apache
Configuration
Once QGIS Server is installed, let’s configure the environment and enable it.
Install the Apache server in a separate virtual host listening on port 80. Enable the rewrite module to pass HTTP
BASIC auth headers:
sudo a2enmod rewrite
cat /etc/apache2/conf-available/qgis-server-port.conf
Listen 80
sudo a2enconf qgis-server-port
This is the virtual host configuration, stored in /etc/apache2/sites-available/001-qgis-server.
conf:
<VirtualHost *:80>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/html
ErrorLog ${APACHE_LOG_DIR}/qgis-server-error.log
CustomLog ${APACHE_LOG_DIR}/qgis-server-access.log combined
# Longer timeout for WPS... default = 40
FcgidIOTimeout 120
FcgidInitialEnv LC_ALL "en_US.UTF-8"
FcgidInitialEnv PYTHONIOENCODING UTF-8
FcgidInitialEnv LANG "en_US.UTF-8"
ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/
<Directory "/usr/lib/cgi-bin">
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AllowOverride All
Options +ExecCGI -MultiViews +FollowSymLinks
# for apache2 > 2.4
Require all granted
#Allow from all
</Directory>
</VirtualHost>
Moreover, you can use some Environment variables to configure QGIS Server. With Apache as HTTP Server, you
have to use FcgidInitialEnv to define these variables as shown below:
FcgidInitialEnv QGIS_DEBUG 1
FcgidInitialEnv QGIS_SERVER_LOG_FILE /tmp/qgis-000.log
FcgidInitialEnv QGIS_SERVER_LOG_LEVEL 0
Start
Now enable the virtual host and restart Apache:
sudo a2ensite 001-qgis-server
sudo service apache2 restart
QGIS Server is now available at http://localhost/cgi-bin/qgis-server.cgi.
NGINX
You can also use QGIS Server with NGINX. Unlike Apache, NGINX does not automatically spawn a FastCGI
process. Actually, you have to use another component to start these processes.
To do that on Debian based systems, you may use fcgiwrap or spawn-fcgi based on your preferences to run QGIS
Server. In both case, you have to install NGINX:
sudo apt-get install nginx
fcgiwrap
If you want to use fcgiwrap to run QGIS Server, you firstly have to install the corresponding package:
sudo apt-get install fcgiwrap
Then, introduce the following block in your NGINX server configuration:
1
2
3
4
5
6
location /qgisserver {
gzip
off;
include
fastcgi_params;
fastcgi_pass
unix:/var/run/fcgiwrap.socket;
fastcgi_param SCRIPT_FILENAME /usr/lib/cgi-bin/qgis_mapserv.fcgi;
}
Finally, restart NGINX and fcgiwrap to take into account the new configuration:
sudo service nginx restart
sudo service fcgiwrap restart
QGIS Server is now available at http://localhost/qgisserver.
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spawn-fcgi
If you prefer to use spawn-fcgi instead of fcgiwrap, the first step is to install the package:
sudo apt-get install spawn-fcgi
Then, introduce the following block in your NGINX server configuration:
location /qgisserver {
gzip
off;
include
fastcgi_params;
fastcgi_pass
unix:/var/run/qgisserver.socket;
}
And restart NGINX to take into account the new configuration:
sudo service nginx restart
Finally, considering that there is no default service file for spawn-fcgi, you have to manually start QGIS Server in
your terminal:
sudo spawn-fcgi -f /usr/lib/bin/cgi-bin/qgis_mapserv.fcgi \
-s /var/run/qgisserver.socket \
-U www-data -G www-data -n
Of course, you may write an init script (like a qgisserver.service file with Systemd) to start QGIS Server
at boot time or whenever you want.
QGIS Server is now available at http://localhost/qgisserver.
Configuration
The include fastcgi_params; used in previous configuration is important as it adds the parameters from /etc/
nginx/fastcgi_params:
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
QUERY_STRING
REQUEST_METHOD
CONTENT_TYPE
CONTENT_LENGTH
$query_string;
$request_method;
$content_type;
$content_length;
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
SCRIPT_NAME
REQUEST_URI
DOCUMENT_URI
DOCUMENT_ROOT
SERVER_PROTOCOL
REQUEST_SCHEME
HTTPS
$fastcgi_script_name;
$request_uri;
$document_uri;
$document_root;
$server_protocol;
$scheme;
$https if_not_empty;
fastcgi_param
fastcgi_param
GATEWAY_INTERFACE
SERVER_SOFTWARE
CGI/1.1;
nginx/$nginx_version;
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
fastcgi_param
REMOTE_ADDR
REMOTE_PORT
SERVER_ADDR
SERVER_PORT
SERVER_NAME
$remote_addr;
$remote_port;
$server_addr;
$server_port;
$server_name;
# PHP only, required if PHP was built with --enable-force-cgi-redirect
fastcgi_param REDIRECT_STATUS
200;
Of course, you may override these variables in your own configuration. For example:
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include fastcgi_params;
fastcgi_param SERVER_NAME domain.name.eu;
Moreover, you can use some Environment variables to configure QGIS Server. With NGINX as HTTP Server,
you have to use fastcgi_param to define these variables as shown below:
fastcgi_param
fastcgi_param
fastcgi_param
QGIS_DEBUG
QGIS_SERVER_LOG_FILE
QGIS_SERVER_LOG_LEVEL
1;
/tmp/qgis-000.log;
0;
Note: When using spawn-fcgi, you may directly define environment variables before running the server. For
example: export QGIS_SERVER_LOG_FILE=/home/user/qgis.log
Xvfb
QGIS Server needs a running X Server to be fully usable. But if you don’t have one, you may use xvfb to have a
virtual X environment.
To install the package:
sudo apt-get install xvfb
Then, according to your HTTP server, you should configure the DISPLAY parameter or directly use xvfb-run.
For example with NGINX and spawn-fcgi using xvfb-run:
xvfb-run /usr/bin/spawn-fcgi -f /usr/lib/bin/cgi-bin/qgis_mapserv.fcgi \
-s /tmp/qgisserver.socket \
-G www-data -U www-data -n
The other option is to start a virtual X server environment with a specific display number thanks to Xvfb:
/usr/bin/Xvfb :99 -screen 0 1024x768x24 -ac +extension GLX +render -noreset
Then we just have to set the DISPLAY environment variable in the HTTP server configuration. For example with
NGINX:
fastcgi_param
DISPLAY
":99";
Or with Apache:
FcgidInitialEnv DISPLAY
":99"
Serve a project
Now that QGIS Server is installed and running, we just have to use it.
Obviously, we need a QGIS project to work on. Of course, you can fully customize your project by defining
contact information, precise some restrictions on CRS or even exclude some layers. Everything you need to know
about that is described later in Configure your project.
But for now, we are going to use a simple project already configured. To retrieve the project:
cd /home/user/
wget https://github.com/qgis/QGIS-Training-Data/archive/master.zip -O qgis-server˓→tutorial.zip
unzip qgis-server-tutorial.zip
mv QGIS-Training-Data-master/training_manual_data/qgis-server-tutorial-data ~
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The project file is qgis-server-tutorial-data-master/world.qgs. Of course, you can use your
favorite GIS software to open this file and take a look on the configuration and available layers.
By opening the project and taking a quick look on layers, we know that 4 layers are currently available:
• airports
• places
• countries
• countries_shapeburst
You don’t have to understand the full request for now but you may retrieve a map with some of the previous layers
thanks to QGIS Server by doing something like this in your web browser to retrieve the countries layer:
http://localhost/qgisserver?
MAP=/home/user/qgis-server-tutorial-data-master/world.qgs&
LAYERS=countries&
SERVICE=WMS&
REQUEST=GetMap&
CRS=EPSG:4326&
WIDTH=400&
HEIGHT=200
If you obtain the next image, then QGIS Server is running correctly:
Fig. 15.5: Server response to a basic GetMap request
Note that you may define PROJECT_FILE environment variable to use a project by default instead of giving a
MAP parameter (see Environment variables).
For example with spawn-fcgi:
export PROJECT_FILE=/home/user/qgis-server-tutorial-data-master/world.qgs
spawn-fcgi -f /usr/lib/bin/cgi-bin/qgis_mapserv.fcgi \
-s /var/run/qgisserver.socket \
-U www-data -G www-data -n
Configure your project
To provide a new QGIS Server WMS, WFS or WCS, you have to create a QGIS project file with some data or use
one of your current project. Define the colors and styles of the layers in QGIS and the project CRS, if not already
defined.
Then, go to the QGIS Server menu of the Project → Project Properties dialog and provide some information
about the OWS in the fields under Service Capabilities. This will appear in the GetCapabilities response of the
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Fig. 15.6: Definitions for a QGIS Server WMS/WFS/WCS project
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WMS, WFS or WCS. If you don’t check
Service capabilities, QGIS Server will use the information given in
the wms_metadata.xml file located in the cgi-bin folder.
Warning: If you’re using the QGIS project with styling based on SVG files using relative paths then you
should know that the server considers the path relative to its qgis_mapserv.fcgi file (not to the qgs
file). So, if you deploy a project on the server and the SVG files are not placed accordingly, the output images
may not respect the Desktop styling. To ensure this doesn’t happen, you can simply copy the SVG files relative
to the qgis_mapserv.fcgi. You can also create a symbolic link in the directory where the fcgi file resides
that points to the directory containing the SVG files (on Linux/Unix).
WMS capabilities
In the WMS capabilities section, you can define the extent advertised in the WMS GetCapabilities response by
entering the minimum and maximum X and Y values in the fields under Advertised extent. Clicking Use Current
CRS
Canvas Extent sets these values to the extent currently displayed in the QGIS map canvas. By checking
restrictions, you can restrict in which coordinate reference systems (CRS) QGIS Server will offer to render maps.
Use the
button below to select those CRSs from the Coordinate Reference System Selector, or click Used to
add the CRSs used in the QGIS project to the list.
If you have print layouts defined in your project, they will be listed in the GetProjectSettings response, and they
can be used by the GetPrint request to create prints, using one of the print layout layouts as a template. This is
a QGIS-specific extension to the WMS 1.3.0 specification. If you want to exclude any print layout from being
Exclude layouts and click the
button below. Then, select a print layout
published by the WMS, check
from the Select print layout dialog in order to add it to the excluded layouts list.
If you want to exclude any layer or layer group from being published by the WMS, check
Exclude Layers and
click the
button below. This opens the Select restricted layers and groups dialog, which allows you to choose
the layers and groups that you don’t want to be published. Use the Shift or Ctrl key if you want to select
multiple entries.
You can receive requested GetFeatureInfo as plain text, XML and GML. Default is XML, text or GML format
depends the output format chosen for the GetFeatureInfo request.
If you wish, you can check
Add geometry to feature response. This will include in the GetFeatureInfo response
the geometries of the features in a text format. If you want QGIS Server to advertise specific request URLs in the
WMS GetCapabilities response, enter the corresponding URL in the Advertised URL field. Furthermore, you can
restrict the maximum size of the maps returned by the GetMap request by entering the maximum width and height
into the respective fields under Maximums for GetMap request.
If one of your layers uses the Map Tip display (i.e. to show text using expressions) this will be listed inside the
GetFeatureInfo output. If the layer uses a Value Map for one of its attributes, this information will also be shown
in the GetFeatureInfo output.
WFS capabilities
In the WFS capabilities area you can select the layers you want to publish as WFS, and specify if they will allow
update, insert and delete operations. If you enter a URL in the Advertised URL field of the WFS capabilities
section, QGIS Server will advertise this specific URL in the WFS GetCapabilities response.
WCS capabilities
In the WCS capabilities area, you can select the layers that you want to publish as WCS. If you enter a URL in the
Advertised URL field of the WCS capabilities section, QGIS Server will advertise this specific URL in the WCS
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GetCapabilities response.
Fine tuning your OWS
For vector layers, the Fields menu of the Layer → Properties dialog allows you to define for each attribute if it
will be published or not. By default, all the attributes are published by your WMS and WFS. If you don’t want a
specific attribute to be published, uncheck the corresponding checkbox in the WMS or WFS column.
You can overlay watermarks over the maps produced by your WMS by adding text annotations or SVG annotations
to the project file. See the Annotation Tools section for instructions on creating annotations. For annotations to
be displayed as watermarks on the WMS output, the Fixed map position checkbox in the Annotation text dialog
must be unchecked. This can be accessed by double clicking the annotation while one of the annotation tools is
active. For SVG annotations, you will need either to set the project to save absolute paths (in the General menu
of the Project → Project Properties dialog) or to manually modify the path to the SVG image so that it represents
a valid relative path.
15.2.2 Services
QGIS Server is able to serve data according to standard protocols as described by the Open Geospatial Consortium (OGC):
• WMS 1.1.0 and 1.3.0
• WFS 1.0.0 and 1.1.0
• WCS 1.1.1
Extra vendor parameters and requests are supported in addition to the original standard that greatly enhance the
possibilities of customizing its behavior thanks to the QGIS rendering engine.
Web Map Service (WMS)
The 1.1.0 and 1.3.0 WMS standards implemented in QGIS Server provide a HTTP interface to request map or
legend images generated from a QGIS project. A typical WMS request defines the QGIS project to use, the layers
to render as well as the image format to generate. Basic support is also available for the Styled Layer Descriptor
(SLD) standard.
Specifications document according to the version number of the service:
• WMS 1.1.0
• WMS 1.3.0
Standard requests provided by QGIS Server:
Request
GetCapabilities
GetMap
GetFeatureInfo
GetLegendGraphics
Description
Return XML metadata with information about the server
Return a map
Retrieves data (geometry and values) for a pixel location
Returns legend symbols
Vendor requests provided by QGIS Server:
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Request
GetPrint
GetProjectSettings
GetDxf
Description
Returns a QGIS composition
Returns specific information about QGIS Server
Export layers in DXF format
GetMap
Standard parameters for the GetMap request according to the OGC WMS 1.1.0 and 1.3.0 specifications:
Parameter
SERVICE
VERSION
REQUEST
LAYERS
STYLES
SRS / CRS
BBOX
WIDTH
HEIGHT
FORMAT
TRANSPARENT
Required
Yes
No
Yes
No
No
Yes
No
Yes
Yes
No
No
Description
Name of the service
Version of the service
Name of the request
Layers to display
Layers’ style
Coordinate reference system
Map extent
Width of the image in pixels
Height of the image in pixels
Image format
Transparent background
In addition to the standard ones, QGIS Server supports the following extra parameters:
Parameter
MAP
BGCOLOR
DPI
IMAGE_QUALITY
OPACITIES
FILTER
SELECTION
Required
Yes
No
No
No
No
No
No
Description
Specify the QGIS project file
Specify the background color
Specify the output resolution
JPEG compression
Opacity for layer or group
Subset of features
Highlight features
SERVICE
This parameter has to be WMS in case of the GetMap request.
For example:
http://localhost/qgis_server?
SERVICE=WMS
&...
VERSION
This parameter allows to specify the version of the service to use. Available values for the VERSION parameter
are:
• 1.1.0
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• 1.3.0
If no version is indicated in the request, then 1.3.0 is used by default.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&VERSION=1.3.0
&...
According to the version number, slight differences have to be expected as explained later for the next parameters:
• CRS / SRS
• BBOX
REQUEST
This parameter is GetMap in case of the GetMap request.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&VERSION=1.3.0
&REQUEST=GetMap
&...
LAYERS
This parameter allows to specify the layers to display on the map. Names have to be separated by a comma.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&LAYERS=mylayer1,mylayer2
&...
In addition, QGIS Server introduced some options to select layers by:
• a short name
• the layer id
The short name of a layer may be configured through Properties → Metadata in layer menu. If the short name is
defined, then it’s used by default instead of the layer’s name:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&LAYERS=mynickname1,mynickname2
&...
Moreover, there’s a project option allowing to select layers by their id in OWS Server → WMS capabilities menu
of the Project → Project Properties dialog. To activate this option, the checkbox Use layer ids as names has to be
selected.
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
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&LAYERS=mylayerid1,mylayerid2
&...
STYLES
This parameter can be used to specify a layer’s style for the rendering step. Styles have to be separated by a
comma. The name of the default style is default.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&LAYERS=mylayer1,mylayer2,mylayer3
&STYLES=style1,default,style3
&...
SRS / CRS
This parameter allows to indicate the map output Spatial Reference System in WMS 1.1.0 and has to be formed
like EPSG:XXXX. Note that CRS is also supported if current version is 1.1.0.
For WMS 1.3.0, CRS parameter is preferable but SRS is also supported.
Note that if both CRS and SRS parameters are indicated in the request, then it’s the current version indicated in
VERSION parameter which is decisive.
In the next case, the SRS parameter is kept whatever the VERSION parameter because CRS is not indicated:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&SRS=EPSG:2854
&...
In the next case, the SRS parameter is kept instead of CRS because of the VERSION parameter:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.1.0
&CRS=EPSG:4326
&SRS=EPSG:2854
&...
In the next case, the CRS parameter is kept instead of SRS because of the VERSION parameter:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&CRS=EPSG:4326
&SRS=EPSG:2854
&...
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BBOX
This parameter allows to specify the map extent with units according to the current CRS. Coordinates have to be
separated by a comma.
However, a slight difference has to be noticed according to the current VERSION parameter. Actually, in WMS
1.1.0, coordinates are formed like minx,miny,maxx,maxy or minlong,minlat,maxlong,maxlat.
For example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.1.0
&SRS=epsg:4326
&BBOX=-180,-90,180,90
&...
But the axis is reversed in WMS 1.3.0, so coordinates are formed like: miny,minx,maxy,maxx or minlat,
minlong,maxlat,maxlong. For example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&CRS=epsg:4326
&BBOX=-90,-180,90,180
&...
WIDTH
This parameter allows to specify the width in pixels of the output image.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&WIDTH=400
&...
HEIGHT
This parameter allows to specify the height in pixels of the output image.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&HEIGHT=400
&...
FORMAT
This parameter may be used to specify the format of map image. Available values are:
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• jpg
• jpeg
• image/jpeg
• image/png
• image/png; mode=1bit
• image/png; mode=8bit
• image/png; mode=16bit
If the FORMAT parameter is different from one of these values, then the default format PNG is used instead.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&FORMAT=image/png; mode=8bit
&...
TRANSPARENT
This boolean parameter can be used to specify the background transparency. Available values are (not case sensitive):
• TRUE
• FALSE
However, this parameter is ignored if the format of the map image indicated with FORMAT is different from PNG.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&TRANSPARENT=TRUE
&...
MAP
This parameter allows to define the QGIS project file to use.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&MAP=/home/user/project.qgs
&...
As mentioned in GetMap parameters table, MAP is mandatory because a request needs a QGIS project to actually
work. However, the QGIS_PROJECT_FILE environment variable may be used to define a default QGIS project.
In this specific case, MAP is not longer a required parameter. For further information you may refer to Advanced
configuration.
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BGCOLOR
This parameter allows to indicate a background color for the map image. However it cannot be combined with
TRANSPARENT parameter in case of PNG images (transparency takes priority). The colour may be literal or in
hexadecimal notation.
URL example with the literal notation:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&BGCOLOR=green
&...
URL example with the hexadecimal notation:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&VERSION=1.3.0
&BGCOLOR=0x00FF00
&...
DPI
This parameter can be used to specify the requested output resolution.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&DPI=300
&...
IMAGE_QUALITY
This parameter is only used for JPEG images. By default, the JPEG compression is -1.
You can change the default per QGIS project in the OWS Server → WMS capabilities menu of the Project
→ Project Properties dialog. If you want to override it in a GetMap request you can do it using the
IMAGE_QUALITY parameter.
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&FORMAT=image/jpeg
&IMAGE_QUALITY=65
&...
OPACITIES
Opacity can be set on layer or group level. Allowed values range from 0 (fully transparent) to 255 (fully opaque).
URL example:
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http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&LAYERS=mylayer1,mylayer2
&OPACITIES=125,200
&...
FILTER
A subset of layers can be selected with the FILTER parameter. Syntax is basically the same as for the QGIS
subset string. However, there are some restrictions to avoid SQL injections into databases via QGIS Server. If a
dangerous string is found in the parameter, QGIS Server will return the next error:
Indeed, text strings need to be enclosed with quotes (single quotes for strings,
˓→double quotes for attributes). A space between each word / special character is
˓→mandatory. Allowed Keywords and special characters are 'AND','OR','IN','=','<','>
˓→=','>','>=','!=*,'(',')'. Semicolons in string expressions are not allowed
URL example:
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&LAYERS=mylayer1,mylayer2
&FILTER=mylayer1:"OBJECTID" = 3;mylayer2:'text' = 'blabla'
&...
Note: It is possible to make attribute searches via GetFeatureInfo and omit the X/Y parameter if a FILTER is
there. QGIS Server then returns info about the matching features and generates a combined bounding box in the
xml output.
SELECTION
The SELECTION parameter can highlight features from one or more layers. Vector features can be selected by
passing comma separated lists with feature ids.
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetMap
&LAYERS=mylayer1,mylayer2
&SELECTION=mylayer1:3,6,9;mylayer2:1,5,6
&...
The following image presents the response from a GetMap request using the SELECTION option e.g. http://
myserver.com/...&SELECTION=countries:171,65.
As those features id’s correspond in the source dataset to France and Romania they’re highlighted in yellow.
GetDxf
It is possible to export layers in the DXF format using the GetDxf Request. Only layers that have read access in
the WFS service are exported in the DXF format. Here is a valid REQUEST and a documentation of the available
parameters:
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Fig. 15.7: Server response to a GetMap request with SELECTION parameter
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http://your.server.address/wms/liegenschaftsentwaesserung/abwasser_werkplan?
˓→SERVICE=WMS&VERSION=1.3.0&REQUEST=GetDxf&LAYERS=Haltungen,Normschacht,
˓→Spezialbauwerke&STYLES=&CRS=EPSG%3A21781&BBOX=696136.28844801,245797.12108743,
˓→696318.91114315,245939.25832905&WIDTH=1042&HEIGHT=811&FORMAT_
˓→OPTIONS=MODE:SYMBOLLAYERSYMBOLOGY;SCALE:250&FILE_NAME=werkplan_abwasser.dxf
Parameters:
• FILE_NAME=yoursuggested_file_name_for_download.dxf
• FORMAT_OPTIONS=see options below, key:value pairs separated by Semicolon
FORMAT_OPTIONS Parameters:
• SCALE:scale to be used for symbology rules, filters and styles (not an actual scaling of the data - data
remains in the original scale).
• MODE:NOSYMBOLOGY|FEATURESYMBOLOGY|SYMBOLLAYERSYMBOLOGY corresponds
to the three export options offered in the QGIS Desktop DXF export dialog.
• LAYERSATTRIBUTES:yourcolumn_with_values_to_be_used_for_dxf_layernames - if not specified,
the original QGIS layer names are used.
• USE_TITLE_AS_LAYERNAME if enabled, the title of the layer will be used as layer name.
GetFeatureInfo
QGIS Server WMS GetFeatureInfo requests supports the following extra optional parameters to define the tolerance for point, line and polygon layers:
• FI_POINT_TOLERANCE parameter: Tolerance for point layers GetFeatureInfo request, in pixels.
• FI_LINE_TOLERANCE parameter: Tolerance for linestring layers GetFeatureInfo request, in pixels.
• FI_POLYGON_TOLERANCE parameter: Tolerance for polygon layers GetFeatureInfo request, in pixels.
QGIS Server also permits to make advanced GetFeatureInfo requests and select all the features that intersect any
given WKT geometry. It does that with the help of the FILTER_GEOM parameter.
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetFeatureInfo
&LAYERS=countries
&QUERY_LAYERS=countries
&INFO_FORMAT:text/xml
&FILTER_GEOM=POLYGON((16.04 53.51, 10.98 47.81, 21.33 47.53, 16.04 53.51))
&...
The content of map tips can be added to the GetFeatureInfo response by passing the WITH_MAPTIP vendor
parameter.
http://localhost/qgis_server?
SERVICE=WMS
&REQUEST=GetFeatureInfo
&LAYERS=countries
&QUERY_LAYERS=countries
&INFO_FORMAT:text/xml
&FILTER_GEOM=POLYGON((16.04 53.51, 10.98 47.81, 21.33 47.53, 16.04 53.51))
&WITH_MAPTIP=true
&...
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GetPrint
QGIS Server has the capability to create print layout output in pdf or pixel format. Print layout windows in the
published project are used as templates. In the GetPrint request, the client has the possibility to specify parameters
of the contained layout maps and labels.
Example:
The published project has two print layouts. In the GetProjectSettings response, they are listed as possible print
templates:
<WMS_Capabilities>
...
<ComposerTemplates xsi:type="wms:_ExtendedCapabilities">
<ComposerTemplate width="297" height="210" name="Druckzusammenstellung 1">
<ComposerMap width="171" height="133" name="map0"/>
<ComposerMap width="49" height="46" name="map1"/></ComposerTemplate>
</ComposerTemplates>
...
</WMS_Capabilities>
The client has now the information to request a print output:
http://myserver.com/cgi/qgis_mapserv.fcgi?...&REQUEST=GetPrint&
˓→TEMPLATE=Druckzusammenstellung 1&map0:EXTENT=xmin,ymin,xmax,ymax&
˓→map0:ROTATION=45&FORMAT=pdf&DPI=300
Parameters in the GetPrint request are:
• <map_id>:EXTENT gives the extent for a layout map item as xmin,ymin,xmax,ymax.
• <map_id>:ROTATION map rotation in degrees
• <map_id>:GRID_INTERVAL_X, <map_id>:GRID_INTERVAL_Y Grid line density for a map in xand y-direction
• <map_id>:SCALE Sets a map scale to a layout map item. This is useful to ensure scale based visibility of
layers and labels even if client and server may have different algorithms to calculate the scale denominator
• <map_id>:LAYERS, <map_id>:STYLES possibility to give layer and styles list for layout map item
(useful in case of overview maps which should have only a subset of layers)
GetLegendGraphics
Several additional parameters are available to change the size of the legend elements:
• BOXSPACE space between legend frame and content (mm)
• LAYERSPACE versical space between layers (mm)
• LAYERTITLESPACE vertical space between layer title and items following (mm)
• SYMBOLSPACE vertical space between symbol and item following (mm)
• ICONLABELSPACE horizontal space between symbol and label text (mm)
• SYMBOLWIDTH width of the symbol preview (mm)
• SYMBOLHEIGHT height of the symbol preview (mm)
These parameters change the font properties for layer titles and item labels:
• LAYERFONTFAMILY / ITEMFONTFAMILY font family for layer title / item text
• LAYERFONTBOLD / ITEMFONTBOLD TRUE to use a bold font
• LAYERFONTSIZE / ITEMFONTSIZE Font size in point
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• LAYERFONTITALIC / ITEMFONTITALIC TRUE to use italic font
• LAYERFONTCOLOR / ITEMFONTCOLOR Hex color code (e.g. #FF0000 for red)
• LAYERTITLE / RULELABEL set them to FALSE to get only the legend graphics without labels
Contest based legend. These parameters let the client request a legend showing only the symbols for the features
falling into the requested area:
• BBOX the geographical area for which the legend should be built
• CRS / SRS the coordinate reference system adopted to define the BBOX coordinates
• WIDTH / HEIGHT if set these should match those defined for the GetMap request, to let QGIS Server
scale symbols according to the map view image size.
Contest based legend features are based on the UMN MapServer implementation:
• SHOWFEATURECOUNT if set to TRUE adds in the legend the feature count of the features like in the
following image:
GetProjectSettings
This request type works similar to GetCapabilities, but it is more specific to QGIS Server and allows a client to
read additional information which is not available in the GetCapabilities output:
• initial visibility of layers
• information about vector attributes and their edit types
• information about layer order and drawing order
• list of layers published in WFS
Web Feature Service (WFS)
GetFeature
In the WFS GetFeature request, QGIS Server accepts two extra parameters in addition to the standard parameters
according to the OGC WFS 1.0.0 and 1.1.0 specification:
• GeometryName parameter: this parameter can be used to get the extent or the centroid as the geometry or
no geometry if none if used (ie attribute only). Allowed values are extent, centroid or none.
• StartIndex parameter: STARTINDEX is standard in WFS 2.0, but it’s an extension for WFS 1.0.0 which
is the only version implemented in QGIS Server. STARTINDEX can be used to skip some features in the
result set and in combination with MAXFEATURES will provide for the ability to use WFS GetFeature to
page through results. Note that STARTINDEX=0 means start with
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Extra parameters supported by all request types
• FILE_NAME parameter: if set, the server response will be sent to the client as a file attachment with the
specified file name.
• MAP parameter: Similar to MapServer, the MAP parameter can be used to specify the path to the QGIS
project file. You can specify an absolute path or a path relative to the location of the server executable
(qgis_mapserv.fcgi). If not specified, QGIS Server searches for .qgs files in the directory where the
server executable is located.
Example:
http://localhost/cgi-bin/qgis_mapserv.fcgi?\
REQUEST=GetMap&MAP=/home/qgis/mymap.qgs&...
Note:
You can define a QGIS_PROJECT_FILE as an environment variable to tell the server executable
where to find the QGIS project file. This variable will be the location where QGIS will look for the
project file. If not defined it will use the MAP parameter in the request and finally look at the server
executable directory.
the first feature, skipping none.
REDLINING
This feature is available and can be used with GetMap and GetPrint requests.
The redlining feature can be used to pass geometries and labels in the request which are overlapped by the server
over the standard returned image (map). This permits the user to put emphasis or maybe add some comments
(labels) to some areas, locations etc. that are not in the standard map.
The request is in the format:
http://qgisplatform.demo/cgi-bin/qgis_mapserv.fcgi?map=/world.qgs&SERVICE=WMS&
˓→VERSION=1.3.0&
REQUEST=GetMap
...
&HIGHLIGHT_GEOM=POLYGON((590000 5647000, 590000 6110620, 2500000 6110620, 2500000
˓→5647000, 590000 5647000))
&HIGHLIGHT_SYMBOL=<StyledLayerDescriptor><UserStyle><Name>Highlight</Name>
˓→<FeatureTypeStyle><Rule><Name>Symbol</Name><LineSymbolizer><Stroke><SvgParameter
˓→name="stroke">%23ea1173</SvgParameter><SvgParameter name="stroke-opacity">1</
˓→SvgParameter><SvgParameter name="stroke-width">1.6</SvgParameter></Stroke></
˓→LineSymbolizer></Rule></FeatureTypeStyle></UserStyle></StyledLayerDescriptor>
&HIGHLIGHT_LABELSTRING=Write label here
&HIGHLIGHT_LABELSIZE=16
&HIGHLIGHT_LABELCOLOR=%23000000
&HIGHLIGHT_LABELBUFFERCOLOR=%23FFFFFF
&HIGHLIGHT_LABELBUFFERSIZE=1.5
Here is the image outputed by the above request in which a polygon and a label are drawn on top of the normal
map:
You can see there are several parameters in this request:
• HIGHLIGHT_GEOM: You can add POINT, MULTILINESTRING, POLYGON etc. It supports multipart
geometries. Here is an example: HIGHLIGHT_GEOM=MULTILINESTRING((0 0, 0 1, 1 1)).
The coordinates should be in the CRS of the GetMap/GetPrint request.
• HIGHLIGHT_SYMBOL: This controls how the geometry is outlined and you can change the stroke width,
color and opacity.
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Fig. 15.8: Server response to a GetMap request with redlining parameters
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• HIGHLIGHT_LABELSTRING: You can pass your labeling text to this parameter.
• HIGHLIGHT_LABELSIZE: This parameter controls the size of the label.
• HIGHLIGHT_LABELCOLOR: This parameter controls the label color.
• HIGHLIGHT_LABELBUFFERCOLOR: This parameter controls the label buffer color.
• HIGHLIGHT_LABELBUFFERSIZE: This parameter controls the label buffer size.
15.2.3 Plugins
Installation
To install the HelloWorld example plugin for testing the servers, you firstly have to create a directory to hold server
plugins. This will be specified in the virtual host configuration and passed on to the server through an environment
variable:
sudo mkdir -p /opt/qgis-server/plugins
cd /opt/qgis-server/plugins
sudo wget https://github.com/elpaso/qgis-helloserver/archive/master.zip
In case unzip was not installed before:
sudo apt-get install unzip
sudo unzip master.zip
sudo mv qgis-helloserver-master HelloServer
HTTP Server configuration
Apache
To be able to use server plugins, FastCGI needs to know where to look. So, we have to modify the Apache
configuration file to indicate the QGIS_PLUGINPATH environment variable to FastCGI:
FcgidInitialEnv QGIS_SERVER_LOG_FILE /tmp/qgis-000.log
FcgidInitialEnv QGIS_SERVER_LOG_LEVEL 0
FcgidInitialEnv QGIS_PLUGINPATH "/opt/qgis-server/plugins"
Moreover, a basic HTTP authorization is necessary to play with the HelloWorld plugin previously introduced. So
we have to update the Apache configuration file a last time:
# Needed for QGIS HelloServer plugin HTTP BASIC auth
<IfModule mod_fcgid.c>
RewriteEngine on
RewriteCond %{HTTP:Authorization} .
RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
</IfModule>
Then, restart Apache:
sudo a2ensite 001-qgis-server
sudo service apache2 restart
Tip: If you work with a feature that has many nodes then modifying and adding a new feature will fail. In this
case it is possible to insert the following code into the 001-qgis-server.conf file:
<IfModule mod_fcgid.c>
FcgidMaxRequestLen 26214400
FcgidConnectTimeout 60
</IfModule>
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How to use a plugin
Test the server with the HelloWorld plugin:
wget -q -O - "http://localhost/cgi-bin/qgis_mapserv.fcgi?SERVICE=HELLO"
HelloServer!
You can have a look at the default GetCapabilities of the QGIS server at: http://localhost/cgi-bin/
qgis_mapserv.fcgi?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetCapabilities
15.2.4 Advanced configuration
Logging
To log requests sent to the server, you have to set the following environment variables:
• QGIS_SERVER_LOG_LEVEL
• QGIS_SERVER_LOG_FILE
Take a look on Environment variables to understand their meanings.
Environment variables
You can configure some aspects of QGIS Server by setting environment variables.
According to the HTTP server and how you run QGIS Server, there are several ways to define these variables.
This is fully described in HTTP Server configuration.
QGIS_OPTIONS_PATH
Specifies the path to the directory with settings. It works the same way as QGIS application --optionspath
option. It is looking for settings file in <QGIS_OPTIONS_PATH>/QGIS/QGIS3.ini.
QUERY_STRING
The query string, normally passed by the web server. This variable can be useful while testing QGIS server binary
from the command line.
QGIS_PROJECT_FILE
The .qgs project file, normally passed as a parameter in the query string (with MAP), you can also set it as an
environment variable (for example by using mod_rewrite Apache module).
QGIS_SERVER_LOG_FILE
Specify path and filename. Make sure that server has proper permissions for writing to file. File should be created
automatically, just send some requests to server. If it’s not there, check permissions.
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MAX_CACHE_LAYERS
Specify the maximum number of cached layers (default: 100).
DISPLAY
This is used to pass (fake) X server display number (needed on Unix-like systems).
QGIS_PLUGINPATH
Useful if you are using Python plugins for the server, this sets the folder that is searched for Python plugins.
QGIS_SERVER_LOG_LEVEL
Specify desired log level. Available values are:
• 0 or INFO (log all requests)
• 1 or WARNING
• 2 or CRITICAL (log just critical errors, suitable for production purposes)
QGIS_SERVER_PARALLEL_RENDERING
Activates parallel rendering for WMS GetMap requests. It’s disabled (false) by default. Available values are:
• 0 or false (case insensitive)
• 1 or true (case insensitive)
QGIS_SERVER_MAX_THREADS
Number of threads to use when parallel rendering is activated. Default value is -1 to use the number of processor
cores.
QGIS_SERVER_CACHE_DIRECTORY
Specifies the network cache directory on the filesystem. The default directory is named cache and located in the
profile directory.
QGIS_SERVER_CACHE_SIZE
Sets the network cache size in MB. The default value is 50 MB.
Settings summary
When QGIS Server is starting, you have a summary of all configurable parameters thanks to environment variables.
Moreover, the value currently used and the origin is also displayed.
For example with spawn-fcgi:
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export QGIS_OPTIONS_PATH=/home/user/.local/share/QGIS/QGIS3/profiles/default/
export QGIS_SERVER_LOG_FILE=/home/user/qserv.log
export QGIS_SERVER_LOG_LEVEL=2
spawn-fcgi -f /usr/lib/cgi-bin/qgis_mapserv.fcgi -s /tmp/qgisserver.sock -U www˓→data -G www-data -n
QGIS Server Settings:
- QGIS_OPTIONS_PATH / '' (Override the default path for user configuration): '/
home/user/.local/share/QGIS/QGIS3/profiles/default/' (read from ENVIRONMENT_
˓→VARIABLE)
˓→
- QGIS_SERVER_PARALLEL_RENDERING / '/qgis/parallel_rendering' (Activate/
Deactivate parallel rendering for WMS getMap request): 'true' (read from INI_
˓→FILE)
˓→
˓→
- QGIS_SERVER_MAX_THREADS / '/qgis/max_threads' (Number of threads to use when
parallel rendering is activated): '4' (read from INI_FILE)
- QGIS_SERVER_LOG_LEVEL / '' (Log level): '2' (read from ENVIRONMENT_VARIABLE)
˓→
- QGIS_SERVER_LOG_FILE / '' (Log file): '/tmp/qserv.log' (read from ENVIRONMENT_
VARIABLE)
- QGIS_PROJECT_FILE / '' (QGIS project file): '' (read from DEFAULT_VALUE)
˓→
- MAX_CACHE_LAYERS / '' (Specify the maximum number of cached layers): '100'
(read from DEFAULT_VALUE)
- QGIS_SERVER_CACHE_DIRECTORY / '/cache/directory' (Specify the cache
directory): '/root/.local/share/QGIS/QGIS3/profiles/default/cache' (read from
˓→DEFAULT_VALUE)
˓→
˓→
- QGIS_SERVER_CACHE_SIZE / '/cache/size' (Specify the cache size): '52428800'
(read from INI_FILE)
Ini file used to initialize settings: /home/user/.local/share/QGIS/QGIS3/profiles/
default/QGIS/QGIS3.ini
˓→
In
this
particular
case,
we
know
that
QGIS_SERVER_MAX_THREADS
and
QGIS_SERVER_PARALLEL_RENDERING values are read from the ini file found in
QGIS_OPTIONS_PATH directory (which is defined through an environment variable). The corresponding entries in the ini file are /qgis/max_threads and /qgis/parallel_rendering and their values are true and 4
threads.
Short name for layers, groups and project
A number of elements have both a <Name> and a <Title>. The Name is a text string used for machine-tomachine communication while the Title is for the benefit of humans.
For example, a dataset might have the descriptive Title “Maximum Atmospheric Temperature” and be requested
using the abbreviated Name “ATMAX”. User can already set title for layers, groups and project.
OWS name is based on the name used in layer tree. This name is more a label for humans than a name for
machine-to-machine communication.
QGIS Server supports:
• short name line edits to layers properties You can change this by right clicking on a layer, choose Properties
→ Metadata tab → Description → Short name.
• WMS data dialog to layer tree group (short name, title, abstract)
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By right clicking on a layer group and selecting the Set Group WMS data option you will get:
Fig. 15.9: Set group WMS data dialog
• short name line edits to project properties - add a regexp validator "^[A-Za-z][A-Za-z0-9\._-]*"
to short name line edit accessible through a static method
• add a regexp validator "^[A-Za-z][A-Za-z0-9\._-]*" to short name line edit accessible through a
static method
You can choose a short name for the project root by going to Project properties → OWS Server → Service
capabilities → Short name.
• add a TreeName element in the fullProjectSettings
If a short name has been set for layers, groups or project it is used by QGIS Sever as the layer name.
Connection to service file
In order to make apache aware of the PostgreSQL service file (see the PostgreSQL Service connection file section)
you need to make your *.conf file look like:
SetEnv PGSERVICEFILE /home/web/.pg_service.conf
<Directory "/home/web/apps2/bin/">
AllowOverride None
.....
Add fonts to your linux server
Keep in mind that you may use QGIS projects that point to fonts that may not exist by default on other machines.
This means that if you share the project, it may look different on other machines (if the fonts don’t exist on the
target machine).
In order to ensure this does not happen you just need to install the missing fonts on the target machine. Doing this
on desktop systems is usually trivial (double clicking the fonts).
For linux, if you don’t have a desktop environment installed (or you prefer the command line) you need to:
• On Debian based systems:
sudo su
mkdir -p /usr/local/share/fonts/truetype/myfonts && cd /usr/local/share/fonts/
˓→truetype/myfonts
# copy the fonts from their location
cp /fonts_location/* .
chown root *
cd .. && fc-cache -f -v
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• On Fedora based systems:
sudo su
mkdir /usr/share/fonts/myfonts && cd /usr/share/fonts/myfonts
# copy the fonts from their location
cp /fonts_location/* .
chown root *
cd .. && fc-cache -f -v
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16
Working with GPS Data
16.1 GPS Plugin
16.1.1 What is GPS?
GPS, the Global Positioning System, is a satellite-based system that allows anyone with a GPS receiver to find their
exact position anywhere in the world. GPS is used as an aid in navigation, for example in airplanes, in boats and
by hikers. The GPS receiver uses the signals from the satellites to calculate its latitude, longitude and (sometimes)
elevation. Most receivers also have the capability to store locations (known as waypoints), sequences of locations
that make up a planned route and a tracklog or track of the receiver’s movement over time. Waypoints, routes
and tracks are the three basic feature types in GPS data. QGIS displays waypoints in point layers, while routes
and tracks are displayed in linestring layers.
Note: QGIS supports also GNSS receivers. But we keep using the term GPS in this documentation.
16.1.2 Loading GPS data from a file
There are dozens of different file formats for storing GPS data. The format that QGIS uses is called GPX (GPS
eXchange format), which is a standard interchange format that can contain any number of waypoints, routes and
tracks in the same file.
To load a GPX file, you first need to load the plugin. Plugins →
Plugin Manager. . . opens the Plugin Manager
Dialog. Activate the
GPS Tools checkbox. When this plugin is loaded, a button with a small handheld GPS
device will show up in the toolbar and in Layer → Create Layer → :
•
GPS Tools
•
Create new GPX Layer
For working with GPS data, we provide an example GPX file available in the QGIS sample dataset:
qgis_sample_data/gps/national_monuments.gpx. See section Sample Data for more information
about the sample data.
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1. Select Vector → GPS → GPS Tools or click the
tab (see figure_GPS).
GPS Tools
icon in the toolbar and open the Load GPX file
2. Browse to the folder qgis_sample_data/gps/, select the GPX file national_monuments.gpx
and click [Open].
Fig. 16.1: The GPS Tools dialog window
Use the [Browse. . . ] button to select the GPX file, then use the checkboxes to select the feature types you want
to load from that GPX file. Each feature type will be loaded in a separate layer when you click [OK]. The file
national_monuments.gpx only includes waypoints.
Note: GPS units allow you to store data in different coordinate systems. When downloading a GPX file (from
your GPS unit or a web site) and then loading it in QGIS, be sure that the data stored in the GPX file uses WGS 84
(latitude/longitude). QGIS expects this, and it is the official GPX specification. See http://www.topografix.com/
GPX/1/1/.
16.1.3 GPSBabel
Since QGIS uses GPX files, you need a way to convert other GPS file formats to GPX. This can be done for many
formats using the free program GPSBabel, which is available at http://www.gpsbabel.org. This program can also
transfer GPS data between your computer and a GPS device. QGIS uses GPSBabel to do these things, so it is
recommended that you install it. However, if you just want to load GPS data from GPX files you will not need it.
Version 1.2.3 of GPSBabel is known to work with QGIS, but you should be able to use later versions without any
problems.
16.1.4 Importing GPS data
To import GPS data from a file that is not a GPX file, you use the tool Import other file in the GPS Tools dialog.
Here, you select the file that you want to import (and the file type), which feature type you want to import from it,
where you want to store the converted GPX file and what the name of the new layer should be. Note that not all
GPS data formats will support all three feature types, so for many formats you will only be able to choose between
one or two types.
16.1.5 Downloading GPS data from a device
QGIS can use GPSBabel to download data from a GPS device directly as new vector layers. For this we use
the Download from GPS tab of the GPS Tools dialog (see Figure_GPS_download). Here, we select the type of
GPS device, the port that it is connected to (or USB if your GPS supports this), the feature type that you want to
download, the GPX file where the data should be stored, and the name of the new layer.
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Fig. 16.2: The download tool
The device type you select in the GPS device menu determines how GPSBabel tries to communicate with your
GPS device. If none of the available types work with your GPS device, you can create a new type (see section
Defining new device types).
The port may be a file name or some other name that your operating system uses as a reference to the physical port
in your computer that the GPS device is connected to. It may also be simply USB, for USB-enabled GPS units.
•
On Linux, this is something like /dev/ttyS0 or /dev/ttyS1.
•
On Windows, it is COM1 or COM2.
When you click [OK], the data will be downloaded from the device and appear as a layer in QGIS.
16.1.6 Uploading GPS data to a device
You can also upload data directly from a vector layer in QGIS to a GPS device using the Upload to GPS tab of
the GPS Tools dialog. To do this, you simply select the layer that you want to upload (which must be a GPX
layer), your GPS device type, and the port (or USB) that it is connected to. Just as with the download tool, you
can specify new device types if your device isn’t in the list.
This tool is very useful in combination with the vector-editing capabilities of QGIS. It allows you to load a map,
create waypoints and routes, and then upload them and use them on your GPS device.
16.1.7 Defining new device types
There are lots of different types of GPS devices. The QGIS developers can’t test all of them, so if you have one
that does not work with any of the device types listed in the Download from GPS and Upload to GPS tools, you
can define your own device type for it. You do this by using the GPS device editor, which you start by clicking
the [Edit devices] button in the download or the upload tab.
To define a new device, you simply click the [New device] button, enter a name, enter download and upload
commands for your device, and click the [Update device] button. The name will be listed in the device menus in
the upload and download windows – it can be any string. The download command is the command that is used to
download data from the device to a GPX file. This will probably be a GPSBabel command, but you can use any
other command line program that can create a GPX file. QGIS will replace the keywords %type, %in, and %out
when it runs the command.
%type will be replaced by -w if you are downloading waypoints, -r if you are downloading routes and -t if
you are downloading tracks. These are command-line options that tell GPSBabel which feature type to download.
%in will be replaced by the port name that you choose in the download window and %out will be replaced by
the name you choose for the GPX file that the downloaded data should be stored in. So, if you create a device
type with the download command gpsbabel %type -i garmin -o gpx %in %out (this is actually the
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download command for the predefined device type ‘Garmin serial’) and then use it to download waypoints from
port /dev/ttyS0 to the file output.gpx, QGIS will replace the keywords and run the command gpsbabel
-w -i garmin -o gpx /dev/ttyS0 output.gpx.
The upload command is the command that is used to upload data to the device. The same keywords are used, but
%in is now replaced by the name of the GPX file for the layer that is being uploaded, and %out is replaced by
the port name.
You can learn more about GPSBabel and its available command line options at http://www.gpsbabel.org.
Once you have created a new device type, it will appear in the device lists for the download and upload tools.
16.1.8 Download of points/tracks from GPS units
As described in previous sections QGIS uses GPSBabel to download points/tracks directly in the project. QGIS
comes out of the box with a pre-defined profile to download from Garmin devices. Unfortunately there is a bug
#6318 that does not allow create other profiles, so downloading directly in QGIS using the GPS Tools is at the
moment limited to Garmin USB units.
Garmin GPSMAP 60cs
MS Windows
Install the Garmin USB drivers from http://www8.garmin.com/support/download_details.jsp?id=591
Connect the unit. Open GPS Tools and use type=garmin serial and port=usb: Fill the fields Layer
name and Output file. Sometimes it seems to have problems saving in a certain folder, using something like
c:\temp usually works.
Ubuntu/Mint GNU/Linux
It is first needed an issue about the permissions of the device, as described at https://wiki.openstreetmap.org/
wiki/USB_Garmin_on_GNU/Linux. You can try to create a file /etc/udev/rules.d/51-garmin.rules
containing this rule
ATTRS{idVendor}=="091e", ATTRS{idProduct}=="0003", MODE="666"
After that is necessary to be sure that the garmin_gps kernel module is not loaded
rmmod garmin_gps
and then you can use the GPS Tools. Unfortunately there seems to be a bug #7182 and usually QGIS freezes
several times before the operation work fine.
BTGP-38KM datalogger (only Bluetooth)
MS Windows
The already referred bug does not allow to download the data from within QGIS, so it is needed to use GPSBabel
from the command line or using its interface. The working command is
gpsbabel -t -i skytraq,baud=9600,initbaud=9600 -f COM9 -o gpx -F C:/GPX/aaa.gpx
Ubuntu/Mint GNU/Linux
Use same command (or settings if you use GPSBabel GUI) as in Windows. On Linux it maybe somehow common
to get a message like
skytraq: Too many read errors on serial port
it is just a matter to turn off and on the datalogger and try again.
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BlueMax GPS-4044 datalogger (both BT and USB)
MS Windows
Note: It needs to install its drivers before using it on Windows 7. See in the manufacturer site for the proper
download.
Downloading with GPSBabel, both with USB and BT returns always an error like
gpsbabel -t -i mtk -f COM12 -o gpx -F C:/temp/test.gpx
mtk_logger: Can't create temporary file data.bin
Error running gpsbabel: Process exited unsuccessfully with code 1
Ubuntu/Mint GNU/Linux
With USB
After having connected the cable use the dmesg command to understand what port is being used, for example
/dev/ttyACM3. Then as usual use GPSBabel from the CLI or GUI
gpsbabel -t -i mtk -f /dev/ttyACM3 -o gpx -F /home/user/bluemax.gpx
With Bluetooth
Use Blueman Device Manager to pair the device and make it available through a system port, then run GPSBabel
gpsbabel -t -i mtk -f /dev/rfcomm0 -o gpx -F /home/user/bluemax_bt.gpx
16.2 Live GPS tracking
To activate live GPS tracking in QGIS, you need to select Settings → Panels
new docked window on the left side of the canvas.
GPS information. You will get a
There are four possible screens in this GPS tracking window:
•
GPS position coordinates and an interface for manually entering vertices and features
•
GPS signal strength of satellite connections
•
GPS polar screen showing number and polar position of satellites
•
GPS options screen (see figure_gps_options)
With a plugged-in GPS receiver (has to be supported by your operating system), a simple click on [Connect] connects the GPS to QGIS. A second click (now on [Disconnect]) disconnects the GPS receiver from your computer.
For GNU/Linux, gpsd support is integrated to support connection to most GPS receivers. Therefore, you first have
to configure gpsd properly to connect QGIS to it.
Warning: If you want to record your position to the canvas, you have to create a new vector layer first and
switch it to editable status to be able to record your track.
16.2.1 Position and additional attributes
If the GPS is receiving signals from satellites, you will see your position in latitude, longitude and altitude
together with additional attributes.
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Fig. 16.3: GPS tracking position and additional attributes
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16.2.2 GPS signal strength
Here, you can see the signal strength of the satellites you are receiving signals from.
Fig. 16.4: GPS tracking signal strength
16.2.3 GPS polar window
If you want to know where in the sky all the connected satellites are, you have to switch to the polar screen.
You can also see the ID numbers of the satellites you are receiving signals from.
Fig. 16.5: GPS tracking polar window
16.2.4 GPS options
In case of connection problems, you can switch between:
•
Autodetect
•
Internal
•
Serial device
•
gpsd (selecting the Host, Port and Device your GPS is connected to)
A click on [Connect] again initiates the connection to the GPS receiver.
You can activate
Automatically save added features when you are in editing mode. Or you can activate
Automatically add points to the map canvas with a certain width and color.
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Fig. 16.6: GPS tracking options window
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Activating
canvas.
Cursor, you can use a slider
to shrink and grow the position cursor on the
Activating
Map centering allows you to decide in which way the canvas will be updated. This includes
‘always’, ‘when leaving’, if your recorded coordinates start to move out of the canvas, or ‘never’, to keep map
extent.
Finally, you can activate
logged.
Log file and define a path and a file where log messages about the GPS tracking are
If you want to set a feature manually, you have to go back to
point].
Position
and click on [Add Point] or [Add track
16.2.5 Connect to a Bluetooth GPS for live tracking
With QGIS you can connect a Bluetooth GPS for field data collection. To perform this task you need a GPS
Bluetooth device and a Bluetooth receiver on your computer.
At first you must let your GPS device be recognized and paired to the computer. Turn on the GPS, go to the
Bluetooth icon on your notification area and search for a New Device.
On the right side of the Device selection mask make sure that all devices are selected so your GPS unit will
probably appear among those available. In the next step a serial connection service should be available, select it
and click on [Configure] button.
Remember the number of the COM port assigned to the GPS connection as resulting by the Bluetooth properties.
After the GPS has been recognized, make the pairing for the connection. Usually the authorization code is 0000.
Now open GPS information panel and switch to GPS options screen. Select the COM port assigned to the GPS
connection and click the [Connect]. After a while a cursor indicating your position should appear.
If QGIS can’t receive GPS data, then you should restart your GPS device, wait 5-10 seconds then try to connect
again. Usually this solution work. If you receive again a connection error make sure you don’t have another
Bluetooth receiver near you, paired with the same GPS unit.
16.2.6 Using GPSMAP 60cs
MS Windows
Easiest way to make it work is to use a middleware (freeware, not open) called GPSGate.
Launch the program, make it scan for GPS devices (works for both USB and BT ones) and then in QGIS just click
[Connect] in the Live tracking panel using the
Autodetect mode.
Ubuntu/Mint GNU/Linux
As for Windows the easiest way is to use a server in the middle, in this case GPSD, so
sudo apt-get install gpsd
Then load the garmin_gps kernel module
sudo modprobe garmin_gps
And then connect the unit. Then check with dmesg the actual device being used bu the unit, for example /dev/
ttyUSB0. Now you can launch gpsd
gpsd /dev/ttyUSB0
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And finally connect with the QGIS live tracking tool.
16.2.7 Using BTGP-38KM datalogger (only Bluetooth)
Using GPSD (under Linux) or GPSGate (under Windows) is effortless.
16.2.8 Using BlueMax GPS-4044 datalogger (both BT and USB)
MS Windows
The live tracking works for both USB and BT modes, by using GPSGate or even without it, just use the
Autodetect mode, or point the tool the right port.
Ubuntu/Mint GNU/Linux
For USB
The live tracking works both with GPSD
gpsd /dev/ttyACM3
or without it, by connecting the QGIS live tracking tool directly to the device (for example /dev/ttyACM3).
For Bluetooth
The live tracking works both with GPSD
gpsd /dev/rfcomm0
or without it, by connecting the QGIS live tracking tool directly to the device (for example /dev/rfcomm0).
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17
Authentication System
17.1 Authentication System Overview
Fig. 17.1: Anatomy of authentication system
17.1.1 Authentication database
The new authentication system stores authentication configurations in an SQLite database file located, by default,
at <user home>/.qgis3/qgis-auth.db.
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This authentication database can be moved between QGIS installations without affecting other current QGIS user
preferences, as it is completely separate from normal QGIS settings. A configuration ID (a random 7-character
alphanumeric string) is generated when initially storing a configuration to the database. This represents the configuration, thereby allowing the ID to be stored in plain text application components, (such as project, plugin, or
settings files) without disclosure of its associated credentials.
Note:
The parent directory of the qgis-auth.db can be set using the following environment variable,
QGIS_AUTH_DB_DIR_PATH, or set on the command line during launch with the --authdbdirectory
option.
17.1.2 Master password
To store or access sensitive information within the database, a user must define a master password. A new master
password is requested and verified when initially storing any encrypted data to the database. Only when sensitive
information is accessed is the user prompted for the master password, which is then cached for the remainder of
the session (until application is quit), unless the user manually chooses an action to clear its cached value. Some
instances of using the authentication system do not require input of the master password, such as when selecting an
existing authentication configuration, or applying a configuration to a server configuration (such as when adding
a WMS layer).
Fig. 17.2: Input new master password
Note: A path to a file containing the master password can be set using the following environment variable,
QGIS_AUTH_PASSWORD_FILE.
Managing the master password
Once set, the master password can be reset; the current master password will be needed prior to resetting. During
this process, there is an option to generate a complete backup of the current database.
Fig. 17.3: Resetting master password
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If the user forgets the master password, there is no way to retrieve or override it. There is also no means of
retrieving encrypted information without knowing the master password.
If a user inputs their existing password incorrectly three times, the dialog will offer to erase the database.
Fig. 17.4: Password prompt after three invalid attempts
17.1.3 Authentication Configurations
You can manage authentication configurations from Configurations in the Authentication tab of the QGIS Options
dialog (Settings → Options).
Fig. 17.5: Configurations editor
Use the
button to add a new configuration, the
modify existing ones.
button to remove configurations, and the
button to
The same type of operations for authentication configuration management (Add, Edit and Remove) can be done
when configuring a given service connection, such as configuring an OWS service connection. For that, there are
action buttons within the configuration selector for fully managing configurations found within the authentication
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Fig. 17.6: Adding config from within Configuration editor
database. In this case, there is no need to go to the configurations in Authentication tab of QGIS options unless
you need to do more comprehensive configuration management.
When creating or editing an authentication configuration, the info required is a name, an authentication method
and any other info that the authentication method requires (see more about the available authentication types in
Authentication Methods).
17.1.4 Authentication Methods
Available authentications are provided by C++ plugins much in the same way data provider plugins are supported by QGIS. The method of authentication that can be selected is relative to the access needed for the resource/provider, e.g. HTTP(S) or database, and whether there is support in both QGIS code and a plugin. As such,
some authentication method plugins may not be applicable everywhere an authentication configuration selector is
shown. A list of available authentication method plugins and their compatible resource/providers can be accessed
going to Settings → Options and, in the Authentication tab, click the [Installed plugins] button.
Plugins can be created for new authentication methods that do not require QGIS to be recompiled. Since the
support for plugins is currently (since QGIS 2.12) C++-only, QGIS will need to be restarted for the new droppedin plugin to become available to the user. Ensure your plugin is compiled against the same target version of QGIS
if you intend to add it to an existing target install.
Note: The Resource URL is currently an unimplemented feature that will eventually allow a particular configuration to be auto-chosen when connecting to resources at a given URL.
17.1.5 Master Password and Auth Config Utilities
Under the Options menu (Settings → Options) in the Authentication tab, there are several utility actions to manage
the authentication database and configurations:
• Input master password:
– Opens the master password input dialog, independent of performing any authentication database command. Clear cached master password
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Fig. 17.7: WMS connection dialog showing [Add], [Edit], and [Remove] authentication configuration buttons
Fig. 17.8: Available method plugins list
Fig. 17.9: Basic HTTP authentication configs
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Fig. 17.10: PKI paths authentication configs
Fig. 17.11: PKI PKCS#12 file paths authentication configs
Fig. 17.12: Stored Identity authentication configs
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Fig. 17.13: Utilities menu
– Unsets the master password if it has been set via input dialog. Reset master password
– Opens a dialog to change the master password (the current password must be known) and optionally
back up the current database.
• Clear cached authentication configurations: Clears the internal lookup cache for configurations used to
speed up network connections. This does not clear QGIS’s core network access manager’s cache, which
requires a relaunch of QGIS.
• Reset master password: Replaces the current master password for a new one. The current master password
will be needed prior to resetting and a backup of database can be done.
• Remove all authentication configurations: Clears the database of all configuration records, without removing other stored records.
• Erase authentication database: Schedules a backup of the current database and complete rebuild of the
database table structure. These actions are scheduled for a later time, so as to ensure other operations like
project loading do not interrupt the operation or cause errors due to a temporarily missing database.
Fig. 17.14: DB erase verification menu
17.1.6 Using authentication configurations
Typically, an authentication configuration is selected in a configuration dialog for a network services (such as
WMS). However, the selector widget can be embedded anywhere authentication is needed or in non-core functionality, like in third-party PyQGIS or C++ plugins.
When using the selector, No authentication is displayed in the pop-up menu control when nothing is selected,
when there are no configurations to choose from, or when a previously assigned configuration can no longer be
found in the database. The Type and Id fields are read-only and provide a description of the authentication method
and the config’s ID respectively.
17.1.7 Python bindings
All classes and public functions have sip bindings, except QgsAuthCrypto, since management of the master
password hashing and auth database encryption should be handled by the main app, and not via Python. See
Security Considerations concerning Python access.
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Fig. 17.15: Authentication configuration selector with no authentication
Fig. 17.16: Authentication configuration selector with selected config
17.2 User Authentication Workflows
Fig. 17.17: Generic user workflow
17.2.1 HTTP(S) authentication
One of the most common resource connections is via HTTP(S), e.g. web mapping servers, and authentication
method plugins often work for these types of connections. Method plugins have access to the HTTP request
object and can manipulate both the request as well as its headers. This allows for many forms of internet-based
authentication. When connecting via HTTP(S) using the standard username/password authentication method will
attempt HTTP BASIC authentication upon connection.
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Fig. 17.18: Configuring a WMS connection for HTTP BASIC
17.2.2 Database authentication
Connections to database resources are generally stored as key=value pairs, which will expose usernames and
(optionally) passwords, if not using an authentication configuration. When configuring with the new auth system,
the key=value will be an abstracted representation of the credentials, e.g. authfg=81t21b9.
17.2.3 PKI authentication
When configuring PKI components within the authentication system, you have the option of importing components into the database or referencing component files stored on your filesystem. The latter may be useful if such
components change frequently, or where the components will be replaced by a system administrator. In either
instance you will need to store any passphrase needed to access private keys within the database.
All PKI components can be managed in separate editors within the Certificate Manager, which can be accessed
in the Authentication tab in QGIS Options dialog (Settings → Options) by clicking the [Manage certificates]
button.
In the Certificate Manager, there are editors for Identities, Servers and Authorities. Each of these are contained
in their own tabs, and are described below in the order they are encountered in the workflow chart above. The tab
order is relative to frequently accessed editors once you are accustomed to the workflow.
Note: Because all authentication system edits write immediately to the authentication database, there is no need
to click the Options dialog [OK] button for any changes to be saved. This is unlike other settings in the Options
dialog.
Authorities
You can manage available Certificate Authorities (CAs) from the Authorities tab in the Certificate manager from
the Authentication tab of the QGIS Options dialog.
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Fig. 17.19: Configuring a Postgres SSL-with-PKI connection
As referenced in the workflow chart above, the first step is to import or reference a file of CAs. This step is
optional, and may be unnecessary if your PKI trust chain originates from root CAs already installed in your
operating system (OS), such as a certificate from a commercial certificate vendor. If your authenticating root CA
is not in the OS’s trusted root CAs, it will need to be imported or have its file system path referenced. (Contact
your system administrator if unsure.)
By default, the root CAs from your OS are available; however, their trust settings are not inherited. You should
review the certificate trust policy settings, especially if your OS root CAs have had their policies adjusted. Any
certificate that is expired will be set to untrusted and will not be used in secure server connections, unless you
specifically override its trust policy. To see the QGIS-discoverable trust chain for any certificate, select it and click
the
Show information for certificate
You can edit the trust policy
.
for any selected certificate within the chain. Any change in trust policy to
Save certificate trust policy change to database
a selected certificate will not be saved to the database unless the
button is
clicked per selected certification. Closing the dialog will not apply the policy changes.
You can review the filtered CAs, both intermediate and root certificates, that will be trusted for secure connections
or change the default trust policy by clicking the
Options button.
Warning: Changing the default trust policy may result in problems with secure connections.
You can import CAs or save a file system path from a file that contains multiple CAs, or import individual CAs.
The standard PEM format for files that contain multiple CA chain certifications has the root cert at the bottom of
the file and all subsequently signed child certificates above, towards the beginning of the file.
The CA certificate import dialog will find all CA certificates within the file, regardless of order, and also offers the
option to import certificates that are considered invalid (in case you want to override their trust policy). You can
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Fig. 17.20: PKI configuration workflow
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Fig. 17.21: Opening the Certificate Manager
Fig. 17.22: Authorities editor
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Fig. 17.23: Certificate info dialog
Fig. 17.24: Saving the trust policy changes
Fig. 17.25: Authorities options menu
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override the trust policy upon import, or do so later within the Authorities editor.
Fig. 17.26: Import certificates dialog
Note: If you are pasting certificate information into the PEM text field, note that encrypted certificates are not
supported.
Identities
You can manage available client identity bundles from the Identities tab in the Certificate manager from the
Authentication tab of the QGIS Options dialog. An identity is what authenticates you against a PKI-enabled
service and usually consists of a client certificate and private key, either as separate files or combined into a single
“bundled” file. The bundle or private key is often passphrase-protected.
Once you have any Certificate Authorities (CAs) imported you can optionally import any identity bundles into the
authentication database. If you do not wish to store the identities, you can reference their component file system
paths within an individual authentication configuration.
When importing an identity bundle, it can be passphrase-protected or unprotected, and can contain CA certificates
forming a trust chain. Trust chain certifications will not be imported here; they can be added separately under the
Authorities tab.
Upon import the bundle’s certificate and private key will be stored in the database, with the key’s storage encrypted
using the QGIS master password. Subsequent usage of the stored bundle from the database will only require input
of the master password.
Personal identity bundles consisting of PEM/DER (.pem/.der) and PKCS#12 (.p12/.pfx) components are supported. If a key or bundle is passphrase-protected, the password will be required to validate the component prior
to import. Likewise, if the client certificate in the bundle is invalid (for example, its effective date has not yet
started or has elapsed) the bundle can not be imported.
17.2.4 Handling bad layers
Occasionally, the authentication configuration ID that is saved with a project file is no longer valid, possibly
because the current authentication database is different than when the project was last saved, or due to a credentials
mismatch. In such cases the Handle bad layers dialog will be presented upon QGIS launch.
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Fig. 17.27: Identities editor
Fig. 17.28: PEM/DER identity import
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Fig. 17.29: PKCS#12 identity import
Fig. 17.30: Handle bad layers with authentication
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If a data source is found to have an authentication configuration ID associated with it, you will be able to edit it.
Doing so will automatically edit the data source string, much in the same way as opening the project file in a text
editor and editing the string.
Fig. 17.31: Edit bad layer’s authentication config ID
17.2.5 Changing authentication config ID
Occasionally, you will need to change the authenticationn configuration ID that is associated with accessing a
resource. There are instances where this is useful:
• Resource auth config ID is no longer valid: This can occur when you have switched auth databases add
need to align a new configuration to the ID already associated with a resource.
• Shared project files: If you intended to share projects between users, e.g. via a shared file server, you can
predefine a 7-character (containing a-z and/or 0-9) that is associated with the resource. Then, individual
users change the ID of an authentication configuration that is specific to their credentials of the resource.
When the project is opened, the ID is found in the authentication database, but the credentials are different
per user.
Fig. 17.32: Changing a layer’s authentication config ID (unlocked yellow text field)
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Warning: Changing the auth config ID is considered an advanced operation and should only be done with
full knowledge as to why it is necessary. This is why there is a lock button that needs clicked, to unlock the
ID’s text field prior to editing the ID.
17.2.6 QGIS Server support
When using a project file, with layers that have authentication configurations, as a basis for a map in QGIS Server,
there are a couple of additional setup steps necessary for QGIS to load the resources:
• Authentication database needs to be available
• Authentication database’s master password needs to be available
When instantiating the authentication system, Server will create or use qgis-auth.db in ~/.qgis2/ or the
directory defined by the QGIS_AUTH_DB_DIR_PATH environment variable. It may be that the Server’s user has
no HOME directory, in which case, use the environment variable to define a directory that the Server’s user has
read/write permissions and is not located within the web-accessible directories.
To pass the master password to Server, write it to the first line of file at a path on the file system readable by the
Server processes user and defined using the QGIS_AUTH_PASSWORD_FILE environment variable. Ensure to
limit the file as only readable by the Server’s process user and to not store the file within web-accessible directories.
Note: QGIS_AUTH_PASSWORD_FILE variable will be removed from the Server environment immediately
after accessing.
17.2.7 SSL server exceptions
Fig. 17.33: SSL server exception
You can manage SSL server configurations and exceptions from the Servers tab in the Authentication section of
the QGIS Options dialog.
Sometimes, when connecting to an SSL server, there are errors with the SSL “handshake” or the server’s certificate. You can ignore those errors or create an SSL server configuration as an exception. This is similar to how
web browsers allow you to override SSL errors, but with more granular control.
Warning: You should not create an SSL server configuration unless you have complete knowledge of the
entire SSL setup between the server and client. Instead, report the issue to the server administrator.
Note: Some PKI setups use a completely different CA trust chain to validate client identities than the chain used
to validate the SSL server certificate. In such circumstances, any configuration created for the connecting server
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will not necessarily fix an issue with the validation of your client identity, and only your client identity’s issuer or
server administrator can fix the issue.
You can pre-configure an SSL server configuration by clicking the
button. Alternatively, you can add a
configuration when an SSL error occurs during a connection and you are presented with an SSL Error dialog
(where the error can be ignored temporarily or saved to the database and ignored):
Fig. 17.34: Manually adding configuration
Once an SSL configuration is saved to the database, it can be edited or deleted.
If you want to pre-configure an SSL configuration and the import dialog is not working for your server’s connection, you can manually trigger a connection via the Python Console by running the following code (replace
https://bugreports.qt-project.org with the URL of your server):
from PyQt4.QtNetwork import *
req = QNetworkRequest(QUrl('https://bugreports.qt-project.org'))
reply = QgsNetworkAccessManager.instance().get(req)
This will open an SSL error dialog if any errors occur, where you can choose to save the configuration to the
database.
17.3 Security Considerations
Once the master password is entered, the API is open to access authentication configs in the authentication
database, similar to how Firefox works. However, in the initial implementation, no wall against PyQGIS access has been defined. This may lead to issues where a user downloads/installs a malicious PyQGIS plugin or
standalone app that gains access to authentication credentials.
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Fig. 17.35: Adding configuration during SSL error
Fig. 17.36: Existing SSL configuration
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Fig. 17.37: Editing an existing SSL configuration
The quick solution for initial release of feature is to just not include most PyQGIS bindings for the authentication
system.
Another simple, though not robust, fix is to add a combobox in Settings → Options → Authentication (defaults to
“never”):
"Allow Python access to authentication system"
Choices: [ confirm once per session | always confirm | always allow | never]
Such an option’s setting would need to be saved in a location non-accessible to Python, e.g. the authentication
database, and encrypted with the master password.
• Another option may be to track which plugins the user has specifically
• allowed to access the authentication system, though it may be tricky to deduce which plugin is actually
making the call.
• Sandboxing plugins, possibly in their own virtual environments, would reduce ‘cross-plugin’ hacking of
authentication configs from another plugin that is authorized. This might mean limiting cross-plugin communication as well, but maybe only between third-party plugins.
• Another good solution is to issue code-signing certificates to vetted plugin authors. Then validate the plugin’s certificate upon loading. If need be the user can also directly set an untrusted policy for the certificate
associated with the plugin using existing certificate management dialogs.
• Alternatively, access to sensitive authentication system data from Python
• could never be allowed, and only the use of QGIS core widgets, or duplicating authentication system integrations, would allow the plugin to work with resources that have an authentication configuration, while
keeping master password and authentication config loading in the realm of the main app.
The same security concerns apply to C++ plugins, though it will be harder to restrict access, since there is no
function binding to simply be removed as with Python.
17.3.1 Restrictions
The confusing licensing and exporting issues associated with OpenSSL apply. In order for Qt to work with SSL
certificates, it needs access to the OpenSSL libraries. Depending upon how Qt was compiled, the default is to
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dynamically link to the OpenSSL libs at run-time (to avoid the export limitations).
QCA follows a similar tactic, whereby linking to QCA incurs no restrictions, because the qca-ossl (OpenSSL)
plugin is loaded at run-time. The qca-ossl plugin is directly linked to the OpenSSL libs. Packagers would be the
ones needing to ensure any OpenSSL-linking restrictions are met, if they ship the plugin. Maybe. I don’t really
know. I’m not a lawyer.
The authentication system safely disables itself when qca-ossl is not found at run-time.
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18
GRASS GIS Integration
GRASS integration provides access to GRASS GIS databases and functionalities (see GRASS-PROJECT in Literature and Web References). The integration consists of two parts: provider and plugin. The provider allows to
browse, manage and visualize GRASS raster and vector layers. The plugin can be used to create new GRASS
locations and mapsets, change GRASS region, create and edit vector layers and analyze GRASS 2-D and 3-D data
with more than 400 GRASS modules. In this section, we’ll introduce the provider and plugin functionalities and
give some examples of managing and working with GRASS data.
The provider supports GRASS version 6 and 7, the plugin supports GRASS 6 and 7 (starting from QGIS 2.12).
QGIS distribution may contain provider/plugin for either GRASS 6 or GRASS 7 or for both versions at the same
time (binaries have different file names). Only one version of the provider/plugin may be loaded on runtime
however.
18.1 Demo dataset
As an example, we will use the QGIS Alaska dataset (see section Sample Data). It includes a small sample GRASS
LOCATION with three vector layers and one raster elevation map. Create a new folder called grassdata, download the QGIS ‘Alaska’ dataset qgis_sample_data.zip from http://qgis.org/downloads/data/ and unzip the
file into grassdata.
More sample GRASS LOCATIONs are available at the GRASS website at http://grass.osgeo.org/download/
sample-data/.
18.2 Loading GRASS raster and vector layers
If the provider is loaded in QGIS, the location item with GRASS
icon is added in the browser tree under each
folder item which contains GRASS location. Go to the folder grassdata and expand location alaska and
mapset demo.
You can load GRASS raster and vector layers like any other layer from the browser either by double click on layer
item or by dragging and dropping to map canvas or legend.
Tip: GRASS Data Loading
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If you don’t see GRASS location item, verify in Help → About → Providers if GRASS vector provider is loaded.
18.3 Importing data into a GRASS LOCATION via drag and drop
This section gives an example of how to import raster and vector data into a GRASS mapset.
1. In QGIS browser navigate to the mapset you want to import data into.
2. In QGIS browser find a layer you want to import to GRASS, note that you can open another instance of the
browser (Browser Panel (2)) if source data are too far from the mapset in the tree.
3. Drag a layer and drop it on the target mapset. The import may take some time for larger layers, you will see
animated icon in front of new layer item until the import finishes.
When raster data are in different CRS, they can be reprojected using an Approximate (fast) or Exact (precise)
transformation. If a link to the source raster is created (using r.external), the source data are in the same CRS
and the format is known to GDAL, the source data CRS will be used. You can set these options in the Browser tab
in GRASS Options.
If a source raster has more bands, a new GRASS map is created for each layer with .<band number> suffix and
group of all maps with
icon is created. External rasters have a different icon
.
18.4 Managing GRASS data in QGIS Browser
• Copying maps: GRASS maps may be copied between mapsets within the same location using drag and
drop.
• Deleting maps: Right click on a GRASS map and select Delete from context menu.
• Renaming maps: Right click on a GRASS map and select Rename from context menu.
18.5 GRASS Options
GRASS options may be set in GRASS Options dialog, which can be opened by right clicking on the location or
mapset item in the browser and then choosing GRASS Options.
18.6 Starting the GRASS plugin
To use GRASS functionalities in QGIS, you must select and load the GRASS plugin using the Plugin Manager.
To do this, go to the menu Plugins →
Manage and Install Plugins. . . , select
GRASS and click [OK].
The following main features are provided with the GRASS menu (Plugins → GRASS) when you start the GRASS
plugin:
•
Open Mapset
•
New Mapset
•
Close Mapset
•
Open GRASS Tools
•
Display Current GRASS Region
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•
GRASS Options
18.7 Opening GRASS mapset
A GRASS mapset must be opened to get access to GRASS Tools in the plugin (the tools are disabled if no mapset
is open). You can open a mapset from the browser: right click on mapset item and then choose Open mapset from
context menu.
18.8 GRASS LOCATION and MAPSET
GRASS data are stored in a directory referred to as GISDBASE. This directory, often called grassdata, must
be created before you start working with the GRASS plugin in QGIS. Within this directory, the GRASS GIS
data are organized by projects stored in subdirectories called LOCATIONs. Each LOCATION is defined by its
coordinate system, map projection and geographical boundaries. Each LOCATION can have several MAPSETs
(subdirectories of the LOCATION) that are used to subdivide the project into different topics or subregions, or as
workspaces for individual team members (see Neteler & Mitasova 2008 in Literature and Web References). In
order to analyse vector and raster layers with GRASS modules, you generally have to import them into a GRASS
LOCATION. (This is not strictly true – with the GRASS modules r.external and v.external you can
create read-only links to external GDAL/OGR-supported datasets without importing them. This is not the usual
way for beginners to work with GRASS, therefore this functionality will not be described here.)
Fig. 18.1: GRASS data in the alaska LOCATION
18.9 Importing data into a GRASS LOCATION
See section Importing data into a GRASS LOCATION via drag and drop to find how data can be easily imported
by dragging and dropping in the browser.
This section gives an example of how to import raster and vector data into the ‘alaska’ GRASS LOCATION
provided by the QGIS ‘Alaska’ dataset in traditional way, using standard GRASS modules. Therefore, we use the
landcover raster map landcover.img and the vector GML file lakes.gml from the QGIS ‘Alaska’ dataset
(see Sample Data).
1. Start QGIS and make sure the GRASS plugin is loaded.
2. In the GRASS toolbar, click the
18.7. Opening GRASS mapset
Open MAPSET
icon to bring up the MAPSET wizard.
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3. Select as GRASS database the folder grassdata in the QGIS Alaska dataset, as LOCATION ‘alaska’, as
MAPSET ‘demo’ and click [OK].
4. Now click the
appears.
Open GRASS tools
icon. The GRASS Toolbox (see section The GRASS Toolbox) dialog
5. To import the raster map landcover.img, click the module r.in.gdal in the Modules Tree tab. This
GRASS module allows you to import GDAL-supported raster files into a GRASS LOCATION. The module
dialog for r.in.gdal appears.
6. Browse to the folder raster in the QGIS ‘Alaska’ dataset and select the file landcover.img.
7. As raster output name, define landcover_grass and click [Run]. In the Output tab, you
see the currently running GRASS command r.in.gdal -o input=/path/to/landcover.img
output=landcover_grass.
8. When it says Successfully finished, click [View output]. The landcover_grass raster layer is now
imported into GRASS and will be visualized in the QGIS canvas.
9. To import the vector GML file lakes.gml, click the module v.in.ogr in the Modules Tree tab. This
GRASS module allows you to import OGR-supported vector files into a GRASS LOCATION. The module
dialog for v.in.ogr appears.
10. Browse to the folder gml in the QGIS ‘Alaska’ dataset and select the file lakes.gml as OGR file.
11. As vector output name, define lakes_grass and click [Run]. You don’t have to care about the other
options in this example. In the Output tab you see the currently running GRASS command v.in.ogr -o
dsn=/path/to/lakes.gml output=lakes\_grass.
12. When it says Succesfully finished, click [View output]. The lakes_grass vector layer is now imported
into GRASS and will be visualized in the QGIS canvas.
18.9.1 Creating a new GRASS LOCATION
As an example, here is the sample GRASS LOCATION alaska, which is projected in the Albers Equal Area
projection using feet as units. This sample GRASS LOCATION alaska will be used for all examples and exercises in the following GRASS-related sections. It is useful to download and install the dataset on your computer
(see Sample Data).
1. Start QGIS and make sure the GRASS plugin is loaded.
2. Visualize the alaska.shp shapefile (see section Loading a layer from a file) from the QGIS Alaska
dataset (see Sample Data).
3. In the GRASS toolbar, click on the
New mapset
icon to bring up the MAPSET wizard.
4. Select an existing GRASS database (GISDBASE) folder grassdata, or create one for the new
LOCATION using a file manager on your computer. Then click [Next].
5. We can use this wizard to create a new MAPSET within an existing LOCATION (see section Adding
a new MAPSET) or to create a new LOCATION altogether. Select
Create new location (see figure_grass_new_location).
6. Enter a name for the LOCATION – we used ‘alaska’ – and click [Next].
7. Define the projection by clicking on the radio button
Projection to enable the projection list.
8. We are using Albers Equal Area Alaska (feet) projection. Since we happen to know that it is represented
by the EPSG ID 2964, we enter it in the search box. (Note: If you want to repeat this process for another
LOCATION and projection and haven’t memorized the EPSG ID, click on the
right-hand corner of the status bar (see section Working with Projections)).
CRS Status
icon in the lower
9. In Filter, insert 2964 to select the projection.
10. Click [Next].
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11. To define the default region, we have to enter the LOCATION bounds in the north, south, east, and west
directions. Here, we simply click on the button [Set current QGIS extent], to apply the extent of the
loaded layer alaska.shp as the GRASS default region extent.
12. Click [Next].
13. We also need to define a MAPSET within our new LOCATION (this is necessary when creating a new
LOCATION). You can name it whatever you like - we used ‘demo’. GRASS automatically creates a special
MAPSET called PERMANENT, designed to store the core data for the project, its default spatial extent and
coordinate system definitions (see Neteler & Mitasova 2008 in Literature and Web References).
14. Check out the summary to make sure it’s correct and click [Finish].
15. The new LOCATION, ‘alaska’, and two MAPSETs, ‘demo’ and ‘PERMANENT’, are created. The currently
opened working set is ‘demo’, as you defined.
16. Notice that some of the tools in the GRASS toolbar that were disabled are now enabled.
Fig. 18.2: Creating a new GRASS LOCATION or a new MAPSET in QGIS
If that seemed like a lot of steps, it’s really not all that bad and a very quick way to create a LOCATION. The
LOCATION ‘alaska’ is now ready for data import (see section Importing data into a GRASS LOCATION). You
can also use the already-existing vector and raster data in the sample GRASS LOCATION ‘alaska’, included in
the QGIS ‘Alaska’ dataset Sample Data, and move on to section The GRASS vector data model.
18.9.2 Adding a new MAPSET
A user has write access only to a GRASS MAPSET which he or she created. This means that besides access to
your own MAPSET, you can read maps in other users’ MAPSETs (and they can read yours), but you can modify
or remove only the maps in your own MAPSET.
All MAPSETs include a WIND file that stores the current boundary coordinate values and the currently selected
raster resolution (see Neteler & Mitasova 2008 in Literature and Web References, and section The GRASS region
tool).
1. Start QGIS and make sure the GRASS plugin is loaded.
2. In the GRASS toolbar, click on the
New mapset
icon to bring up the MAPSET wizard.
3. Select the GRASS database (GISDBASE) folder grassdata with the LOCATION ‘alaska’, where we
want to add a further MAPSET called ‘test’.
4. Click [Next].
5. We can use this wizard to create a new MAPSET within an existing LOCATION or to create a new
LOCATION altogether. Click on the radio button
Select location (see figure_grass_new_location) and
click [Next].
6. Enter the name text for the new MAPSET. Below in the wizard, you see a list of existing MAPSETs and
corresponding owners.
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7. Click [Next], check out the summary to make sure it’s all correct and click [Finish].
18.10 The GRASS vector data model
It is important to understand the GRASS vector data model prior to digitizing. In general, GRASS uses a topological vector model. This means that areas are not represented as closed polygons, but by one or more boundaries.
A boundary between two adjacent areas is digitized only once, and it is shared by both areas. Boundaries must be
connected and closed without gaps. An area is identified (and labelled) by the centroid of the area.
Besides boundaries and centroids, a vector map can also contain points and lines. All these geometry elements
can be mixed in one vector and will be represented in different so-called ‘layers’ inside one GRASS vector map.
So in GRASS, a layer is not a vector or raster map but a level inside a vector layer. This is important to distinguish
carefully. (Although it is possible to mix geometry elements, it is unusual and, even in GRASS, only used in
special cases such as vector network analysis. Normally, you should prefer to store different geometry elements
in different layers.)
It is possible to store several ‘layers’ in one vector dataset. For example, fields, forests and lakes can be stored in
one vector. An adjacent forest and lake can share the same boundary, but they have separate attribute tables. It is
also possible to attach attributes to boundaries. An example might be the case where the boundary between a lake
and a forest is a road, so it can have a different attribute table.
The ‘layer’ of the feature is defined by the ‘layer’ inside GRASS. ‘Layer’ is the number which defines if there is
more than one layer inside the dataset (e.g., if the geometry is forest or lake). For now, it can be only a number. In
the future, GRASS will also support names as fields in the user interface.
Attributes can be stored inside the GRASS LOCATION as dBase, SQLite3 or in external database tables, for
example, PostgreSQL, MySQL, Oracle, etc.
Attributes in database tables are linked to geometry elements using a ‘category’ value.
‘Category’ (key, ID) is an integer attached to geometry primitives, and it is used as the link to one key column in
the database table.
Tip: Learning the GRASS Vector Model
The best way to learn the GRASS vector model and its capabilities is to download one of the many GRASS
tutorials where the vector model is described more deeply. See http://grass.osgeo.org/documentation/manuals/ for
more information, books and tutorials in several languages.
18.11 Creating a new GRASS vector layer
To create a new GRASS vector layer, select one of following items from mapset context menu in the browser:
• New Point Layer
• New Line Layer
• New Polygon Layer
and enter a name in the dialog. A new vector map will be created and layer will be added to canvas and editing
started. Selecting type of the layer does not restrict geometry types which can be digitized in the vector map. In
GRASS, it is possible to organize all sorts of geometry types (point, line and polygon) in one vector map. The
type is only used to add the layer to the canvas, because QGIS requires a layer to have a specific type.
It is also possible to add layers to existing vector maps selecting one of the items described above from context
menu of existing vector map.
In GRASS, it is possible to organize all sorts of geometry types (point, line and area) in one layer, because GRASS
uses a topological vector model, so you don’t need to select the geometry type when creating a new GRASS vector.
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This is different from shapefile creation with QGIS, because shapefiles use the Simple Feature vector model (see
section Creating new vector layers).
18.12 Digitizing and editing a GRASS vector layer
GRASS vector layers can be digitized using the standard QGIS digitizing tools. There are however some particularities, which you should know about, due to
• GRASS topological model versus QGIS simple feature
• complexity of GRASS model
– multiple layers in single maps
– multiple geometry types in single map
– geometry sharing by multiple features from multiple layers
The particularities are discussed in the following sections.
Save, discard changes, undo, redo
Warning: All the changes done during editing are immediately written to vector map and related attribute
tables.
Changes are written after each operation, it is however, possible to do undo/redo or discard all changes when
closing editing. If undo or discard changes is used, original state is rewritten in vector map and attribute tables.
There are two main reasons for this behaviour:
• It is the nature of GRASS vectors coming from conviction that user wants to do what he is doing and it is
better to have data saved when the work is suddenly interrupted (for example, blackout)
• Necessity for effective editing of topological data is visualized information about topological correctness,
such information can only be acquired from GRASS vector map if changes are written to the map.
Toolbar
The ‘Digitizing Toolbar’ has some specific tools when a GRASS layer is edited:
Icon
Tool
Purpose
New Point
Digitize new point
New Line
Digitize new line
New Boundary
Digitize new boundary
New Centroid
Digitize new centroid (label existing area)
New Closed Boundary
Digitize new closed boundary
Table GRASS Digitizing: GRASS Digitizing Tools
Tip: Digitizing polygons in GRASS
If you want to create a polygon in GRASS, you first digitize the boundary of the polygon. Then you add a centroid
(label point) into the closed boundary. The reason for this is that a topological vector model links the attribute
information of a polygon always to the centroid and not to the boundary.
Category
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Category, often called cat, is sort of ID. The name comes from times when GRASS vectors had only singly
attribute “category”. Category is used as a link between geometry and attributes. A single geometry may have
multiple categories and thus represent multiple features in different layers. Currently it is possible to assign only
one category per layer using QGIS editing tools. New features have automatically assigned new unique category,
except boundaries. Boundaries usually only form areas and do not represent linear features, it is however possible
to define attributes for a boundary later, for example in different layer.
New categories are always created only in currently being edited layer.
It is not possible to assign more categories to geometry using QGIS editing, such data are properly represented as
multiple features, and individual features, even from different layers, may be deleted.
Attributes
Attributes of currently edited layer can only be modified. If the vector map contains more layers, features of other
layers will have all attributes set to ‘<not editable (layer #)>’ to warn you that such attribute is not editable. The
reason is, that other layers may have and usually have different set of fields while QGIS only supports one fixed
set of fields per layer.
If a geometry primitive does not have a category assigned, a new unique category is automatically assigned and
new record in attribute table is created when an attribute of that geometry is changed.
Tip: If you want to do bulk update of attributes in table, for example using ‘Field Calculator’ (Using the Field
Calculator), and there are features without category which you don’t want to update (typically boundaries), you
can filter them out by setting ‘Advanced Filter’ to cat is not null.
Editing style
The topological symbology is essential for effective editing of topological data. When editing starts, a specialized
‘GRASS Edit’ renderer is set on the layer automatically and original renderer is restored when editing is closed.
The style may be customized in layer properties ‘Style’ tab. The style can also be stored in project file or in
separate file as any other style. If you customize the style, do not change its name, because it is used to reset the
style when editing is started again.
Tip: Do not save project file when the layer is edited, the layer would be stored with ‘Edit Style’ which has no
meaning if layer is not edited.
The style is based on topological information which is temporarily added to attribute table as field ‘topo_symbol’.
The field is automatically removed when editing is closed.
Tip: Do not remove ‘topo_symbol’ field from attribute table, that would make features invisible because the
renderer is based on that column.
Snapping
To form an area, vertices of connected boundaries must have exactly the same coordinates. This can be achieved
using snapping tool only if canvas and vector map have the same CRS. Otherwise, due conversion from map
coordinates to canvas and back, the coordinate may become slightly different due to representation error and CRS
transformations.
Tip: Use layer’s CRS also for canvas when editing.
Limitations
Simultaneous editing of multiple layers within the same vector at the same time is not supported. This is mainly
due to the impossibility of handling multiple undo stacks for a single data source.
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On Linux and macOS only one GRASS layer can be edited at time. This is due to a bug in GRASS which
does not allow to close database drivers in random order. This is being solved with GRASS developers.
Tip: GRASS Edit Permissions
You must be the owner of the GRASS MAPSET you want to edit. It is impossible to edit data layers in a MAPSET
that is not yours, even if you have write permission.
18.13 The GRASS region tool
The region definition (setting a spatial working window) in GRASS is important for working with raster layers.
Vector analysis is by default not limited to any defined region definitions. But all newly created rasters will have
the spatial extension and resolution of the currently defined GRASS region, regardless of their original extension
and resolution. The current GRASS region is stored in the $LOCATION/$MAPSET/WIND file, and it defines
north, south, east and west bounds, number of columns and rows, horizontal and vertical spatial resolution.
It is possible to switch on and off the visualization of the GRASS region in the QGIS canvas using the
Display current GRASS region
button.
The region can be modified in ‘Region’ tab in ‘GRASS Tolls’ dock widget. Type in the new region bounds and
resolution, and click [Apply]. If you click on [Select the extent by dragging on canvas] you can select a new
region interactively with your mouse on the QGIS canvas dragging a rectangle.
The GRASS module g.region provides a lot more parameters to define an appropriate region extent and resolution for your raster analysis. You can use these parameters with the GRASS Toolbox, described in section The
GRASS Toolbox.
18.14 The GRASS Toolbox
Open GRASS Tools
box provides GRASS module functionalities to work with data inside a selected GRASS
The
LOCATION and MAPSET. To use the GRASS Toolbox you need to open a LOCATION and MAPSET that you
have write permission for (usually granted, if you created the MAPSET). This is necessary, because new raster or
vector layers created during analysis need to be written to the currently selected LOCATION and MAPSET.
18.14.1 Working with GRASS modules
The GRASS shell inside the GRASS Toolbox provides access to almost all (more than 300) GRASS modules in
a command line interface. To offer a more user-friendly working environment, about 200 of the available GRASS
modules and functionalities are also provided by graphical dialogs within the GRASS plugin Toolbox.
A complete list of GRASS modules available in the graphical Toolbox in QGIS version 2.18 is available in the
GRASS wiki at http://grass.osgeo.org/wiki/GRASS-QGIS_relevant_module_list.
It is also possible to customize the GRASS Toolbox content. This procedure is described in section Customizing
the GRASS Toolbox.
As shown in figure_grass_toolbox, you can look for the appropriate GRASS module using the thematically
grouped Modules Tree or the searchable Modules List tab.
By clicking on a graphical module icon, a new tab will be added to the Toolbox dialog, providing three new
sub-tabs: Options, Output and Manual.
Options
The Options tab provides a simplified module dialog where you can usually select a raster or vector layer visualized
in the QGIS canvas and enter further module-specific parameters to run the module.
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Fig. 18.3: GRASS Toolbox and Module Tree
Fig. 18.4: GRASS Toolbox Module Options
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The provided module parameters are often not complete to keep the dialog simple. If you want to use further
module parameters and flags, you need to start the GRASS shell and run the module in the command line.
A new feature since QGIS 1.8 is the support for a Show Advanced Options button below the simplified module
dialog in the Options tab. At the moment, it is only added to the module v.in.ascii as an example of use, but
it will probably be part of more or all modules in the GRASS Toolbox in future versions of QGIS. This allows you
to use the complete GRASS module options without the need to switch to the GRASS shell.
Output
Fig. 18.5: GRASS Toolbox Module Output
The Output tab provides information about the output status of the module. When you click the [Run] button, the
module switches to the Output tab and you see information about the analysis process. If all works well, you will
finally see a Successfully finished message.
Manual
The Manual tab shows the HTML help page of the GRASS module. You can use it to check further module
parameters and flags or to get a deeper knowledge about the purpose of the module. At the end of each module
manual page, you see further links to the Main Help index, the Thematic index and the Full index.
These links provide the same information as the module g.manual.
Tip: Display results immediately
If you want to display your calculation results immediately in your map canvas, you can use the ‘View Output’
button at the bottom of the module tab.
18.14.2 GRASS module examples
The following examples will demonstrate the power of some of the GRASS modules.
Creating contour lines
The first example creates a vector contour map from an elevation raster (DEM). Here, it is assumed that you have
the Alaska LOCATION set up as explained in section Importing data into a GRASS LOCATION.
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Fig. 18.6: GRASS Toolbox Module Manual
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• First, open the location by clicking the
• Now open the Toolbox with the
Open mapset
Open GRASS tools
button and choosing the Alaska location.
button.
• In the list of tool categories, double-click Raster → Surface Management → Generate vector contour lines.
• Now a single click on the tool r.contour will open the tool dialog as explained above (see Working with
GRASS modules).
• In the Name of input raster map enter gtopo30.
• Type into the Increment between Contour levels
intervals of 100 meters.)
the value 100. (This will create contour lines at
• Type into the Name for output vector map the name ctour_100.
• Click [Run] to start the process. Wait for several moments until the message Successfully finished
appears in the output window. Then click [View Output] and [Close].
Since this is a large region, it will take a while to display. After it finishes rendering, you can open the layer
properties window to change the line color so that the contours appear clearly over the elevation raster, as in The
Vector Properties Dialog.
Next, zoom in to a small, mountainous area in the center of Alaska. Zooming in close, you will notice that the
contours have sharp corners. GRASS offers the v.generalize tool to slightly alter vector maps while keeping
their overall shape. The tool uses several different algorithms with different purposes. Some of the algorithms
(i.e., Douglas Peuker and Vertex Reduction) simplify the line by removing some of the vertices. The resulting
vector will load faster. This process is useful when you have a highly detailed vector, but you are creating a very
small-scale map, so the detail is unnecessary.
Tip: The simplify tool
Note that QGIS has a Vector → Geometry Tools → Simplify geometries tool that works just like the GRASS
v.generalize Douglas-Peuker algorithm.
However, the purpose of this example is different. The contour lines created by r.contour have sharp angles
that should be smoothed. Among the v.generalize algorithms, there is Chaiken’s, which does just that (also
Hermite splines). Be aware that these algorithms can add additional vertices to the vector, causing it to load even
more slowly.
• Open the GRASS Toolbox and double-click the categories Vector → Develop map → Generalization, then
click on the v.generalize module to open its options window.
• Check that the ‘ctour_100’ vector appears as the Name of input vector.
• From the list of algorithms, choose Chaiken’s. Leave all other options at their default, and scroll down to
the last row to enter in the field Name for output vector map ‘ctour_100_smooth’, and click [Run].
• The process takes several moments. Once Successfully finished appears in the output windows,
click [View output] and then [Close].
• You may change the color of the vector to display it clearly on the raster background and to contrast with
the original contour lines. You will notice that the new contour lines have smoother corners than the original
while staying faithful to the original overall shape.
Tip: Other uses for r.contour
The procedure described above can be used in other equivalent situations. If you have a raster map of precipitation
data, for example, then the same method will be used to create a vector map of isohyetal (constant rainfall) lines.
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Fig. 18.7: GRASS module v.generalize to smooth a vector map
Creating a Hillshade 3-D effect
Several methods are used to display elevation layers and give a 3-D effect to maps. The use of contour lines, as
shown above, is one popular method often chosen to produce topographic maps. Another way to display a 3-D
effect is by hillshading. The hillshade effect is created from a DEM (elevation) raster by first calculating the slope
and aspect of each cell, then simulating the sun’s position in the sky and giving a reflectance value to each cell.
Thus, you get sun-facing slopes lighted; the slopes facing away from the sun (in shadow) are darkened.
• Begin this example by loading the gtopo30 elevation raster. Start the GRASS Toolbox, and under the
Raster category, double-click to open Spatial analysis → Terrain analysis.
• Then click r.shaded.relief to open the module.
• Change the azimuth angle
270 to 315.
• Enter gtopo30_shade for the new hillshade raster, and click [Run].
• When the process completes, add the hillshade raster to the map. You should see it displayed in grayscale.
• To view both the hillshading and the colors of the gtopo30 together, move the hillshade map below the
gtopo30 map in the table of contents, then open the Properties window of gtopo30, switch to the
Transparency tab and set its transparency level to about 25%.
You should now have the gtopo30 elevation with its colormap and transparency setting displayed above the
grayscale hillshade map. In order to see the visual effects of the hillshading, turn off the gtopo30_shade map,
then turn it back on.
Using the GRASS shell
The GRASS plugin in QGIS is designed for users who are new to GRASS and not familiar with all the modules
and options. As such, some modules in the Toolbox do not show all the options available, and some modules do
not appear at all. The GRASS shell (or console) gives the user access to those additional GRASS modules that
do not appear in the Toolbox tree, and also to some additional options to the modules that are in the Toolbox with
the simplest default parameters. This example demonstrates the use of an additional option in the r.shaded.relief
module that was shown above.
The module r.shaded.relief can take a parameter zmult, which multiplies the elevation values relative to the X-Y
coordinate units so that the hillshade effect is even more pronounced.
• Load the gtopo30 elevation raster as above, then start the GRASS Toolbox and click on the
GRASS shell.
In the shell window, type the command r.shaded.relief map=gtopo30
shade=gtopo30_shade2 azimuth=315 zmult=3 and press [Enter].
• After the process finishes, shift to the Browse tab and double-click on the new gtopo30_shade2 raster
to display it in QGIS.
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Fig. 18.8: The GRASS shell, r.shaded.relief module
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• As explained above, move the shaded relief raster below the gtopo30 raster in the table of contents, then
check the transparency of the colored gtopo30 layer. You should see that the 3-D effect stands out more
strongly compared with the first shaded relief map.
Fig. 18.9: Displaying shaded relief created with the GRASS module r.shaded.relief
Raster statistics in a vector map
The next example shows how a GRASS module can aggregate raster data and add columns of statistics for each
polygon in a vector map.
• Again using the Alaska data, refer to Importing data into a GRASS LOCATION to import the trees shapefile
from the shapefiles directory into GRASS.
• Now an intermediate step is required: centroids must be added to the imported trees map to make it a
complete GRASS area vector (including both boundaries and centroids).
• From the Toolbox, choose Vector → Manage features, and open the module v.centroids.
• Enter as the output vector map ‘forest_areas’ and run the module.
• Now load the forest_areas vector and display the types of forests - deciduous, evergreen, mixed - in
different colors: In the layer Properties window, Symbology tab, choose from Legend type
‘Unique
value’ and set the Classification field to ‘VEGDESC’. (Refer to the explanation of the symbology tab in
Symbology Properties of the vector section.)
• Next, reopen the GRASS Toolbox and open Vector → Vector update by other maps.
• Click on the v.rast.stats module. Enter gtopo30 and forest_areas.
• Only one additional parameter is needed: Enter column prefix elev, and click [Run]. This is a computationally heavy operation, which will run for a long time (probably up to two hours).
• Finally, open the forest_areas attribute table, and verify that several new columns have been added,
including elev_min, elev_max, elev_mean, etc., for each forest polygon.
18.14.3 Customizing the GRASS Toolbox
Nearly all GRASS modules can be added to the GRASS Toolbox. An XML interface is provided to parse the
pretty simple XML files that configure the modules’ appearance and parameters inside the Toolbox.
A sample XML file for generating the module v.buffer (v.buffer.qgm) looks like this:
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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE qgisgrassmodule SYSTEM "http://mrcc.com/qgisgrassmodule.dtd">
<qgisgrassmodule label="Vector buffer" module="v.buffer">
<option key="input" typeoption="type" layeroption="layer" />
<option key="buffer"/>
<option key="output" />
</qgisgrassmodule>
The parser reads this definition and creates a new tab inside the Toolbox when you select the module. A more
detailed description for adding new modules, changing a module’s group, etc., can be found at https://qgis.org/en/
site/getinvolved/development/addinggrasstools.html.
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19
QGIS processing framework
19.1 Introduction
This chapter introduces the QGIS processing framework, a geoprocessing environment that can be used to call
native and third-party algorithms from QGIS, making your spatial analysis tasks more productive and easy to
accomplish.
In the following sections, we will review how to use the graphical elements of this framework and make the most
out of each one of them.
There are four basic elements in the framework GUI, which are used to run algorithms for different purposes.
Choosing one tool or another will depend on the kind of analysis that is to be performed and the particular
characteristics of each user and project. All of them (except for the batch processing interface, which is called
from the toolbox or the algorithm execution dialog, as we will see) can be accessed from the Processing menu
item. (You will see more than four entries. The remaining ones are not used to execute algorithms and will be
explained later in this chapter).
• The Toolbox. The main element of the GUI, it is used to execute a single algorithm or run a batch process
based on that algorithm.
• The Graphical modeler. Several algorithms can be combined graphically using the modeler to define a
workflow, creating a single process that involves several subprocesses.
• The History manager. All actions performed using any of the aforementioned elements are stored in a
history file and can be later easily reproduced using the history manager.
• The Batch processing interface. This interface allows you to execute batch processes and automate the
execution of a single algorithm on multiple datasets.
In the following sections, we will review each one of these elements in detail.
Note: About Vector Menu
Some processing algorithms can be accessed via the Vector menu which lists some tools from the processing
framework but also from plugins.
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Fig. 19.1: Processing Toolbox
Fig. 19.2: Processing Modeler
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Fig. 19.3: Processing History
Fig. 19.4: Batch Processing interface
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19.2 Vector menu
Vector menu provides a one-stop resource for many common vector-based GIS tasks. It provides a growing suite
of spatial data management and analysis functions that are both fast and functional. These features are part of
the processing framework. Note that some plugins can add other features so, in this chapter, only default features
from processing framework will be listed.
Processing framework is automatically installed and, like any plugin, can be enabled and disabled using the Plugin
Manager (see The Plugins Dialog). When enabled, the Processing plugin adds a Vector menu to QGIS, providing
functions ranging from Analysis and Research Tools to Geometry and Geoprocessing Tools, as well as several
useful Data Management Tools.
See QGIS processing framework for more information about the processing framework.
19.2.1 Analysis tools
Icon Tool
Purpose
Distance matrix
Measure distances between two point layers, and output results as a) Square distance
matrix, b) Linear distance matrix, or c) Summary of distances. Can limit distances to
the k nearest features.
Sum
length
Calculate the total sum of line lengths for each polygon of a polygon vector layer.
Points
polygon
line
in
Count the number of points that occur in each polygon of an input polygon vector
layer.
List unique
values
List all unique values in an input vector layer field.
Basic statistics for text
field
Compute basic statistics (mean, std dev, N, sum, CV) on an input text field.
Basic statistics
for
numeric field
Compute basic statistics (mean, std dev, N, sum, CV) on an input numeric field
Nearest
neighbor
analysis
Compute nearest neighbor statistics to assess the level of clustering in a point vector
layer.
Mean coordinate(s)
Compute either the normal or weighted mean center of an entire vector layer, or multiple features based on a unique ID field.
Line intersections
Locate intersections between lines, and output results as a point shapefile. Useful for
locating road or stream intersections, ignores line intersections with length > 0.
Table tools 1: Default tools in Analysis group
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19.2.2 Research tools
Icon Tool
Purpose
Random selection
Randomly select n number of features, or n percentage of features.
Random selection
within subsets
Randomly select features within subsets based on a unique ID field.
Random points inside polygons
Generate pseudo-random points over a polygon layer (variable number of point
or fixed number of point).
Random points in
extent
Generate pseudo-random points over a given extent.
Random points in
layer bounds
Generate pseudo-random points over bounds of a given input layer.
Regular points
Generate a regular grid of points over a specified region and export them as a
point shapefile.
Vector grid
Generate a line or polygon grid based on user-specified grid spacing.
Select by location
Select features based on their location relative to another layer to form a new
selection, or add or subtract from the current selection.
Polygon from layer
extent
Create a single rectangular polygon layer from the extent of an input raster or
vector layer.
Table Tools 2: Default tools in Research group
19.2.3 Geoprocessing tools
Icon Tool
Purpose
Convex hull(s)
Create minimum convex hull(s) for an input layer, or based on an ID field.
Buffer with * fixed distance
* distance field
Create buffer(s) around features * based on fixed distance * based on
distance field
Intersect
Overlay layers such that output contains areas where both layers intersect.
Union
Overlay layers such that output contains intersecting and non-intersecting
areas.
Symmetrical difference
Overlay layers such that output contains those areas of the input and difference layers that do not intersect.
Clip
Overlay layers such that output contains areas that intersect the clip layer.
Difference
Overlay layers such that output contains areas not intersecting the clip
layer.
Dissolve
Merge features based on input field. All features with identical input
values are combined to form one single feature.
Eliminate sliver polygons
Merges selected features with the neighboring polygon with the largest
area or largest common boundary.
Table Tools 3: Default tools in Geoprocessing group
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19.2.4 Geometry tools
Icon Tool
Check
geometry
validity
Purpose
Check polygons for intersections, closed holes, and fix node ordering. You can choose the
engine used by the in the options dialog, digitizing tab Change the Validate geometries value.
There is two engines: QGIS and GEOS which have pretty different behavior. Another tools
exists which shows different result as well: Topology Checker plugin and ‘must not have
invalid geometries’ rule.
ExAdd vector layer geometry info to point (XCOORD, YCOORD), line (LENGTH), or polygon
port/Add (AREA, PERIMETER) layer.
geometry
columns
Polygon
centroids
Calculate the true centroids for each polygon in an input polygon layer.
DeCalculate and output (as polygons) the Delaunay triangulation of an input point vector layer.
launay
triangulation
Voronoi Calculate Voronoi polygons of an input point vector layer.
polygons
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Simplify
geometry
Densify
geometry
Generalize lines or polygons with a modified Douglas-Peucker algorithm.
Multipart
to singleparts
Convert multipart features to multiple singlepart features. Creates simple polygons and lines.
Singleparts
to multipart
Merge multiple features to a single multipart feature based on a unique ID field.
Polygons to
lines
Convert polygons to lines, multipart polygons to multiple singlepart lines.
Lines
to
polygons
Convert lines to polygons, multipart lines to multiple singlepart polygons.
Extract
nodes
Extract nodes from line and polygon layers and output them as points.
Densify lines or polygons by adding vertices.
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Table Tools 4: Default tools in Geometry group
Note: The Simplify geometry tool can be used to remove duplicate nodes in line and polygon geometries. Just set
the Simplify tolerance parameter to 0 and this will do the trick.
19.2.5 Data management tools
Icon Tool
Purpose
Define current projection
Specify the CRS for shapefiles whose CRS has not been defined.
Join
attributes by
location
Join additional attributes to vector layer based on spatial relationship. Attributes from
one vector layer are appended to the attribute table of another layer and exported as a
shapefile.
Split vector
layer
Split input layer into multiple separate layers based on input field.
Merge
shapefiles to
one
Create spatial index
Merge several shapefiles within a folder into a new shapefile based on the layer type
(point, line, area).
Create a spatial index for OGR- supported formats.
Table Tools 5: Default tools in Data management group
19.3 The toolbox
The Toolbox is the main element of the processing GUI, and the one that you are more likely to use in your daily
work. It shows the list of all available algorithms grouped in different blocks, and it is the access point to run them,
whether as a single process or as a batch process involving several executions of the same algorithm on different
sets of inputs.
The toolbox contains all the available algorithms, divided into so-called “Providers”.
Providers can be (de)activated in the settings dialog. A label in the bottom part of the toolbox will remind you
of that whenever there are inactive providers. Use the link in the label to open the settings window and set up
providers. We will discuss the settings dialog later in this manual.
By default, only providers that do not rely on third-party applications (that is, those that only require QGIS
elements to be run) are active. Algorithms requiring external applications might need additional configuration.
Configuring providers is explained in a later chapter in this manual.
In the upper part of the toolbox, you will find a text box. To reduce the number of algorithms shown in the toolbox
and make it easier to find the one you need, you can enter any word or phrase on the text box. Notice that, as you
type, the number of algorithms in the toolbox is reduced to just those that contain the text you have entered in their
names.
If there are algorithms that match your search but belong to a provider that is not active, an additional label will
be shown in the lower part of the toolbox.
If you click on the link in that label, the list of algorithms will also include those from inactive providers, which
will be shown in light gray. A link to active each inactive provider is also shown.
To execute an algorithm, just double-click on its name in the toolbox.
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Fig. 19.5: Processing Toolbox
Fig. 19.6: Processing Toolbox showing search results
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Fig. 19.7: Processing Toolbox showing search results
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19.3.1 The algorithm dialog
Once you double-click on the name of the algorithm that you want to execute, a dialog similar to that in the figure
below is shown (in this case, the dialog corresponds to the ‘Polygon centroids’ algorithm).
Fig. 19.8: Parameters Dialog
This dialog is used to set the input values that the algorithm needs to be executed. It shows a list of input values
and configuration parameters to be set. It of course has a different content, depending on the requirements of the
algorithm to be executed, and is created automatically based on those requirements.
Although the number and type of parameters depend on the characteristics of the algorithm, the structure is similar
for all of them. The parameters found in the table can be of one of the following types.
• A raster layer, to select from a list of all such layers available (currently opened) in QGIS. The selector
contains as well a button on its right-hand side, to let you select filenames that represent layers currently not
loaded in QGIS.
• A vector layer, to select from a list of all vector layers available in QGIS. Layers not loaded in QGIS can
be selected as well, as in the case of raster layers, but only if the algorithm does not require a table field
selected from the attributes table of the layer. In that case, only opened layers can be selected, since they
need to be open so as to retrieve the list of field names available.
You will see an iterator button by each vector layer selector, as shown in the figure below.
Fig. 19.9: Vector iterator button
If the algorithm contains several of them, you will be able to toggle just one of them. If the button corresponding to a vector input is toggled, the algorithm will be executed iteratively on each one of its features,
instead of just once for the whole layer, producing as many outputs as times the algorithm is executed. This
allows for automating the process when all features in a layer have to be processed separately.
• A table, to select from a list of all available in QGIS. Non-spatial tables are loaded into QGIS like vector
layers, and in fact they are treated as such by the program. Currently, the list of available tables that you will
see when executing an algorithm that needs one of them is restricted to tables coming from files in dBase
(.dbf) or Comma-Separated Values (.csv) formats.
• An option, to choose from a selection list of possible options.
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• A numerical value, to be introduced in a spin box. You will find a button by its side. Clicking on it, you will
open the expression builder that allows you to enter a mathematical expression, so you can use it as a handy
calculator. Some useful variables related to data loaded into QGIS can be added to your expression, so you
can select a value derived from any of these variables, such as the cell size of a layer or the northernmost
coordinate of another one.
Fig. 19.10: Expression based input
• A range, with min and max values to be introduced in two text boxes.
• A text string, to be introduced in a text box.
• A field, to choose from the attributes table of a vector layer or a single table selected in another parameter.
• A coordinate reference system. You can type the EPSG code directly in the text box, or select it from the
CRS selection dialog that appears when you click on the button on the right-hand side.
• An extent, to be entered by four numbers representing its xmin, xmax, ymin, ymax limits. Clicking on
the button on the right-hand side of the value selector, a pop-up menu will appear, giving you three options:
– to select the value from a layer or the current canvas extent,
– to define it by dragging directly onto the map canvas, or
– to use the minimum coverage from all input layers.
Fig. 19.11: Extent selector
If you select the first option, you will see a window like the next one.
If you select the second one, the parameters window will hide itself, so you can click and drag onto the
canvas. Once you have defined the selected rectangle, the dialog will reappear, containing the values in the
extent text box.
• A list of elements (whether raster layers, vector layers or tables), to select from the list of such layers
available in QGIS. To make the selection, click on the small button on the left side of the corresponding row
to see a dialog like the following one.
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Fig. 19.12: Extent List
Fig. 19.13: Extent Drag
Fig. 19.14: Multiple Selection
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• A small table to be edited by the user. These are used to define parameters like lookup tables or convolution
kernels, among others.
Click on the button on the right side to see the table and edit its values.
Fig. 19.15: Fixed Table
Depending on the algorithm, the number of rows can be modified or not by using the buttons on the right
side of the window.
Along with the Parameters tab, you will find another tab named Log. Information provided by the algorithm
during its execution is written in this tab, and allow you to track the execution and be aware and have more details
about the algorithm as it runs. Notice that not all algorithms write information to this tab, and many of them might
run silently without producing any output other than the final files.
On the right hand side of the dialog you wil find a short description of the algorithm, which will help you understand its purpose and its basic ideas. If such a description is not available, the description panel will not be
shown.
Some algorithms might have a more detailed help file, which might include description of every parameter it uses,
or examples. In that case, you will find a Help tab in the parameters dialog.
A note on projections
Algorithms that are run from the processing framework — this is also true for most of the external applications
whose algorithms are exposed through it — do not perform any reprojection on input layers and assume that all
of them are already in a common coordinate system and ready to be analyzed. Whenever you use more than one
layer as input to an algorithm, whether vector or raster, it is up to you to make sure that they are all in the same
coordinate system.
Note that, due to QGIS’s on-the-fly reprojecting capabilities, although two layers might seem to overlap and match,
that might not be true if their original coordinates are used without reprojecting them onto a common coordinate
system. That reprojection should be done manually, and then the resulting files should be used as input to the
algorithm. Also, note that the reprojection process can be performed with the algorithms that are available in the
processing framework itself.
By default, the parameters dialog will show a description of the CRS of each layer along with its name, making it
easy to select layers that share the same CRS to be used as input layers. If you do not want to see this additional
information, you can disable this functionality in the Processing settings dialog, unchecking the Show CRS option.
If you try to execute an algorithm using as input two or more layers with unmatching CRSs, a warning dialog will
be shown.
You still can execute the algorithm, but be aware that in most cases that will produce wrong results, such as empty
layers due to input layers not overlapping.
19.3.2 Data objects generated by algorithms
Data objects generated by an algorithm can be of any of the following types:
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• A raster layer
• A vector layer
• A table
• An HTML file (used for text and graphical outputs)
These are all saved to disk, and the parameters table will contain a text box corresponding to each one of these
outputs, where you can type the output channel to use for saving it. An output channel contains the information
needed to save the resulting object somewhere. In the most usual case, you will save it to a file, but in the case of
vector layers, and when they are generated by native algorithms (algorithms not using external applications) you
can also save to a PostGIS or Spatialite database, or a memory layer.
To select an output channel, just click on the button on the right side of the text box, and you will see a small
context menu with the available options.
In the most usual case, you will select saving to a file. If you select that option, you will be prompted with a
save file dialog, where you can select the desired file path. Supported file extensions are shown in the file format
selector of the dialog, depending on the kind of output and the algorithm.
The format of the output is defined by the filename extension. The supported formats depend on what is supported
by the algorithm itself. To select a format, just select the corresponding file extension (or add it, if you are directly
typing the file path instead). If the extension of the file path you entered does not match any of the supported
formats, a default extension will be appended to the file path, and the file format corresponding to that extension
will be used to save the layer or table. Default extensions are .dbf for tables, .tif for raster layers and .shp
for vector layers. These can be modified in the setting dialog, selecting any other of the formats supported by
QGIS.
If you do not enter any filename in the output text box (or select the corresponding option in the context menu),
the result will be saved as a temporary file in the corresponding default file format, and it will be deleted once you
exit QGIS (take care with that, in case you save your project and it contains temporary layers).
You can set a default folder for output data objects. Go to the settings dialog (you can open it from the Processing
menu), and in the General group, you will find a parameter named Output folder. This output folder is used as the
default path in case you type just a filename with no path (i.e., myfile.shp) when executing an algorithm.
When running an algorithm that uses a vector layer in iterative mode, the entered file path is used as the base path
for all generated files, which are named using the base name and appending a number representing the index of
the iteration. The file extension (and format) is used for all such generated files.
Apart from raster layers and tables, algorithms also generate graphics and text as HTML files. These results are
shown at the end of the algorithm execution in a new dialog. This dialog will keep the results produced by any
algorithm during the current session, and can be shown at any time by selecting Processing → Results viewer from
the QGIS main menu.
Some external applications might have files (with no particular extension restrictions) as output, but they do not
belong to any of the categories above. Those output files will not be processed by QGIS (opened or included into
the current QGIS project), since most of the time they correspond to file formats or elements not supported by
QGIS. This is, for instance, the case with LAS files used for LiDAR data. The files get created, but you won’t see
anything new in your QGIS working session.
For all the other types of output, you will find a checkbox that you can use to tell the algorithm whether to load
the file once it is generated by the algorithm or not. By default, all files are opened.
Optional outputs are not supported. That is, all outputs are created. However, you can uncheck the corresponding
checkbox if you are not interested in a given output, which essentially makes it behave like an optional output (in
other words, the layer is created anyway, but if you leave the text box empty, it will be saved to a temporary file
and deleted once you exit QGIS).
19.3.3 Configuring the processing framework
As has been mentioned, the configuration menu gives access to a new dialog where you can configure how algorithms work. Configuration parameters are structured in separate blocks that you can select on the left-hand side
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of the dialog.
Along with the aforementioned Output folder entry, the General block contains parameters for setting the default
rendering style for output layers (that is, layers generated by using algorithms from any of the framework GUI
components). Just create the style you want using QGIS, save it to a file, and then enter the path to that file in the
settings so the algorithms can use it. Whenever a layer is loaded by Processing and added to the QGIS canvas, it
will be rendered with that style.
Rendering styles can be configured individually for each algorithm and each one of its outputs. Just right-click on
the name of the algorithm in the toolbox and select Edit rendering styles for outputs. You will see a dialog like the
one shown next.
Fig. 19.16: Rendering Styles
Select the style file (.qml) that you want for each output and press [OK].
Other configuration parameters in the General group are listed below:
• Use filename as layer name. The name of each resulting layer created by an algorithm is defined by the
algorithm itself. In some cases, a fixed name might be used, meaning that the same output name will be
used, no matter which input layer is used. In other cases, the name might depend on the name of the input
layer or some of the parameters used to run the algorithm. If this checkbox is checked, the name will be
taken from the output filename instead. Notice that, if the output is saved to a temporary file, the filename
of this temporary file is usually a long and meaningless one intended to avoid collision with other already
existing filenames.
• Keep dialog open after running algorithm. Once an algorithm has finished execution and its output layers
are loaded into the QGIS project, the algorithm dialog is closed. If you want to keep it open (to run the
algorithm again with different parameters, or to better check the output that is written to the log tab), check
this option
• Use only selected features. If this option is selected, whenever a vector layer is used as input for an algorithm, only its selected features will be used. If the layer has no selected features, all features will be
used.
• Pre-execution script file and Post-execution script file. These parameters refer to scripts written using the
processing scripting functionality, and are explained in the section covering scripting and the console.
Apart from the General block in the settings dialog, you will also find a block for algorithm providers. Each
entry in this block contains an Activate item that you can use to make algorithms appear or not in the toolbox.
Also, some algorithm providers have their own configuration items, which we will explain later when covering
particular algorithm providers.
19.4 The history manager
19.4.1 The processing history
Every time you execute an algorithm, information about the process is stored in the history manager. Along with
the parameters used, the date and time of the execution are also saved.
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This way, it is easy to track and control all the work that has been developed using the Processing framework, and
to reproduce it.
The history manager is a set of registry entries grouped according to their date of execution, making it easier to
find information about an algorithm executed at any particular moment.
Fig. 19.17: History
Process information is kept as a command-line expression, even if the algorithm was launched from the toolbox.
This makes it also useful for those learning how to use the command-line interface, since they can call an algorithm
using the toolbox and then check the history manager to see how that same algorithm could be called from the
command line.
Apart from browsing the entries in the registry, you can also re-execute processes by simply double-clicking on
the corresponding entry.
19.4.2 The processing log
The history dialog only contains the execution calls, but not the information produced by the algorithm when
executed. That information is written to the QGIS log, in a Processing tab.
Third-party algorithms are usually executed by calling their command-line interfaces, which communicate with
the user via the console. Although that console is not shown, usually a full dump of it is written to the log each
time you run one of those algorithms. To avoid cluttering the log with that information, you can disable it for each
provider, looking for the corresponding option in the provider entry of the settings dialog.
Some algorithms, even if they can produce a result with the given input data, might add comments or additional
information to log if they detect potential problems with the data, in order to warn you. Make sure you check
those messages in the log if you are having unexpected results.
19.5 The graphical modeler
The graphical modeler allows you to create complex models using a simple and easy-to-use interface. When
working with a GIS, most analysis operations are not isolated, but rather part of a chain of operations instead. Using the graphical modeler, that chain of processes can be wrapped into a single process, so it is as more convenient
to execute as a single process later on a different set of inputs. No matter how many steps and different algorithms
it involves, a model is executed as a single algorithm, thus saving time and effort, especially for larger models.
The modeler can be opened from the processing menu.
The modeler has a working canvas where the structure of the model and the workflow it represents are shown. On
the left part of the window, a panel with two tabs can be used to add new elements to the model.
Creating a model involves two steps:
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Fig. 19.18: Modeler
1. Definition of necessary inputs. These inputs will be added to the parameters window, so the user can set their
values when executing the model. The model itself is an algorithm, so the parameters window is generated
automatically as it happens with all the algorithms available in the processing framework.
2. Definition of the workflow. Using the input data of the model, the workflow is defined by adding algorithms
and selecting how they use those inputs or the outputs generated by other algorithms already in the model.
19.5.1 Definition of inputs
The first step to create a model is to define the inputs it needs. The following elements are found in the Inputs tab
on the left side of the modeler window:
• Raster layer
• Vector layer
• String
• Table field
• Table
• Extent
• Number
• Boolean
• File
Double-clicking on any of these elements, a dialog is shown to define its characteristics. Depending on the
parameter itself, the dialog may contain just one basic element (the description, which is what the user will see
when executing the model) or more of them. For instance, when adding a numerical value, as can be seen in the
next figure, apart from the description of the parameter, you have to set a default value and a range of valid values.
For each added input, a new element is added to the modeler canvas.
You can also add inputs by dragging the input type from the list and dropping it in the modeler canvas, in the
position where you want to place it.
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Fig. 19.19: Model Parameters Definition
Fig. 19.20: Model Parameters in canvas
19.5.2 Definition of the workflow
Once the inputs have been defined, it is time to define the algorithms to apply on them. Algorithms can be found
in the Algorithms tab, grouped much in the same way as they are in the toolbox.
To add an algorithm to a model, double-click on its name or drag and drop it, just like it was done when adding
inputs. An execution dialog will appear, with a content similar to the one found in the execution panel that is shown
when executing the algorithm from the toolbox. The one shown next corresponds to the SAGA ‘Convergence
index’ algorithm.
As you can see, some differences exist. Instead of the file output box that was used to set the file path for output
layers and tables, a simple text box is used here. If the layer generated by the algorithm is just a temporary result
that will be used as the input of another algorithm and should not be kept as a final result, just do not edit that text
box. Typing anything in it means that the result is final and the text that you supply will be the description for the
output, which will be the output the user will see when executing the model.
Selecting the value of each parameter is also a bit different, since there are important differences between the
context of the modeler and that of the toolbox. Let’s see how to introduce the values for each type of parameter.
• Layers (raster and vector) and tables. These are selected from a list, but in this case, the possible values are
not the layers or tables currently loaded in QGIS, but the list of model inputs of the corresponding type, or
other layers or tables generated by algorithms already added to the model.
• Numerical values. Literal values can be introduced directly in the text box. Clicking on the button beside
the text box, expresions can be entered. Available variables for expressions include numerical inputs of the
model, and also statistical values from available raster layers within the model.
• String. Literal strings can be typed in the corresponding text box. Cliking on the button beside the text box,
expressions can be entered, as in the case of numerical values.
• Table field. The fields of the parent table or layer cannot be known at design time, since they depend on the
selection of the user each time the model is executed. To set the value for this parameter, type the name of
a field directly in the text box, or use the list to select a table field input already added to the model. The
validity of the selected field will be checked at run time.
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Fig. 19.21: Model Inputs
Fig. 19.22: Model Algorithm parameters
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In all cases, you will find an additional parameter named Parent algorithms that is not available when calling
the algorithm from the toolbox. This parameter allows you to define the order in which algorithms are executed
by explicitly defining one algorithm as a parent of the current one, which will force the parent algorithm to be
executed before the current one.
When you use the output of a previous algorithm as the input of your algorithm, that implicitly sets the previous
algorithm as parent of the current one (and places the corresponding arrow in the modeler canvas). However,
in some cases an algorithm might depend on another one even if it does not use any output object from it (for
instance, an algorithm that executes a SQL sentence on a PostGIS database and another one that imports a layer
into that same database). In that case, just select the previous algorithm in the Parent algorithms parameter and
the two steps will be executed in the correct order.
Once all the parameters have been assigned valid values, click on [OK] and the algorithm will be added to the
canvas. It will be linked to all the other elements in the canvas, whether algorithms or inputs, that provide objects
that are used as inputs for that algorithm.
Elements can be dragged to a different position within the canvas, to change the way the module structure is
displayed and make it more clear and intuitive. Links between elements are updated automatically. You can zoom
in and out by using the mouse wheel.
You can run your algorithm any time by clicking on the [Run] button. However, in order to use the algorithm
from the toolbox, it has to be saved and the modeler dialog closed, to allow the toolbox to refresh its contents.
19.5.3 Saving and loading models
Use the [Save] button to save the current model and the [Open] button to open any model previously saved.
Models are saved with the .model extension. If the model has been previously saved from the modeler window,
you will not be prompted for a filename. Since there is already a file associated with that model, the same file will
be used for any subsequent saves.
Before saving a model, you have to enter a name and a group for it, using the text boxes in the upper part of the
window.
Models saved on the models folder (the default folder when you are prompted for a filename to save the model)
will appear in the toolbox in the corresponding branch. When the toolbox is invoked, it searches the models
folder for files with the .model extension and loads the models they contain. Since a model is itself an algorithm,
it can be added to the toolbox just like any other algorithm.
The models folder can be set from the Processing configuration dialog, under the Modeler group.
Models loaded from the models folder appear not only in the toolbox, but also in the algorithms tree in the
Algorithms tab of the modeler window. That means that you can incorporate a model as a part of a bigger model,
just as you add any other algorithm.
19.5.4 Editing a model
You can edit the model you are currently creating, redefining the workflow and the relationships between the
algorithms and inputs that define the model itself.
If you right-click on an algorithm in the canvas representing the model, you will see a context menu like the one
shown next:
Fig. 19.23: Modeler Right Click
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Selecting the Remove option will cause the selected algorithm to be removed. An algorithm can be removed only
if there are no other algorithms depending on it. That is, if no output from the algorithm is used in a different one
as input. If you try to remove an algorithm that has others depending on it, a warning message like the one you
can see below will be shown:
Fig. 19.24: Cannot Delete Algorithm
Selecting the Edit option will show the parameters dialog of the algorithm, so you can change the inputs and
parameter values. Not all input elements available in the model will appear in this case as available inputs. Layers
or values generated at a more advanced step in the workflow defined by the model will not be available if they
cause circular dependencies.
Select the new values and then click on the [OK] button as usual. The connections between the model elements
will change accordingly in the modeler canvas.
A model can be run partially, by deactivating some of its algorithms. To do it, select the Deactivate option in the
context menu that appears when right-clicking on an algorithm element. The selected algorithm, and all the ones
in the model that depend on it will be displayed in grey and will not be executed as part of the model.
Fig. 19.25: Model With Deactivated Algorithms
When right-clicking on an algorithm that is not active, you will instead see a Activate menu option that you can
use to activate it back.
19.5.5 Editing model help files and meta-information
You can document your models from the modeler itself. Just click on the [Edit model help] button and a dialog
like the one shown next will appear.
On the right-hand side, you will see a simple HTML page, created using the description of the input parameters
and outputs of the algorithm, along with some additional items like a general description of the model or its author.
The first time you open the help editor, all these descriptions are empty, but you can edit them using the elements
on the left-hand side of the dialog. Select an element on the upper part and then write its description in the text
box below.
Model help is saved as part of the model itself.
19.5.6 Exporting a model as a Python script
As we will see in a later chapter, Processing algorithms can be called from the QGIS Python console, and new
Processing algorithms can be created as well using Python. A quick way of creating such a Python script is to
create a model and then to export is as a Python file.
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Fig. 19.26: Help Edition
To do so, click on the Export as Python script button. Select the output file in the file chooser dialog, and
Processing will write in it the Python commands that perform the same operations defined in the current model.
19.5.7 About available algorithms
You might notice that some algorithms that can be be executed from the toolbox do not appear in the list of
available algorithms when you are designing a model. To be included in a model, an algorithm must have a
correct semantic, so as to be properly linked to others in the workflow. If an algorithm does not have such a
well-defined semantic (for instance, if the number of output layers cannot be known in advance), then it is not
possible to use it within a model, and thus, it does not appear in the list of algorithms that you can find in the
modeler dialog.
19.6 The batch processing interface
19.6.1 Introduction
All algorithms (including models) can be executed as a batch process. That is, they can be executed using not just
a single set of inputs, but several of them, executing the algorithm as many times as needed. This is useful when
processing large amounts of data, since it is not necessary to launch the algorithm many times from the toolbox.
To execute an algorithm as a batch process, right-click on its name in the toolbox and select the Execute as batch
process option in the pop-up menu that will appear.
If you have the execution dialog of the algorithm open, you can also start the batch processing interface from
there, clicking on the Run as batch process. . . button.
19.6.2 The parameters table
Executing a batch process is similar to performing a single execution of an algorithm. Parameter values have to
be defined, but in this case we need not just a single value for each parameter, but a set of them instead, one for
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Fig. 19.27: Batch Processing from right-click
Fig. 19.28: Batch Processing From Algorithm Dialog
each time the algorithm has to be executed. Values are introduced using a table like the one shown next.
Each line of this table represents a single execution of the algorithm, and each cell contains the value of one of the
parameters. It is similar to the parameters dialog that you see when executing an algorithm from the toolbox, but
with a different arrangement.
By default, the table contains just two rows. You can add or remove rows using the buttons on the lower part of
the window.
Once the size of the table has been set, it has to be filled with the desired values.
19.6.3 Filling the parameters table
For most parameters, setting the value is trivial. Just type the value or select it from the list of available options,
depending on the parameter type.
Filenames for input data objects are introduced directly typing or, more conveniently, clicking on the
button
on the right hand of the cell, which will show a context menu with two option: one for selecting from the layers
currently opened and another to select from the filesystem. This second option, when selected, shows a typical
file chooser dialog. Multiple files can be selected at once. If the input parameter represents a single data object
and several files are selected, each one of them will be put in a separate row, adding new ones if needed. If the
parameter represents a multiple input, all the selected files will be added to a single cell, separated by semicolons
(;).
Layer identifiers can be directly typed in the parameter text box. You can enter the full path to a file or the name of
a layer that is currently loaded in the current QGIS project. The name of the layer will be automatically resolved
to its source path. Notice that, if several layers have the same name, this might cause unexpected results due to
ambiguity.
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Fig. 19.29: Batch Processing
Output data objects are always saved to a file and, unlike when executing an algorithm from the toolbox, saving
to a temporary file or database is not permitted. You can type the name directly or use the file chooser dialog that
appears when clicking on the accompanying button.
Once you select the file, a new dialog is shown to allow for autocompletion of other cells in the same column
(same parameter).
Fig. 19.30: Batch Processing Save
If the default value (‘Do not autocomplete’) is selected, it will just put the selected filename in the selected cell
from the parameters table. If any of the other options is selected, all the cells below the selected one will be
automatically filled based on a defined criteria. This way, it is much easier to fill the table, and the batch process
can be defined with less effort.
Automatic filling can be done by simply adding correlative numbers to the selected file path, or by appending the
value of another field at the same row. This is particularly useful for naming output data objects according to input
ones.
19.6.4 Executing the batch process
To execute the batch process once you have introduced all the necessary values, just click on [OK]. Progress of
the global batch task will be shown in the progress bar in the lower part of the dialog.
19.7 Using processing algorithms from the console
The console allows advanced users to increase their productivity and perform complex operations that cannot be
performed using any of the other GUI elements of the processing framework. Models involving several algorithms
can be defined using the command-line interface, and additional operations such as loops and conditional sentences
can be added to create more flexible and powerful workflows.
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Fig. 19.31: Batch Processing File Path
There is not a processing console in QGIS, but all processing commands are available instead from the QGIS
built-in Python console. That means that you can incorporate those commands into your console work and connect
processing algorithms to all the other features (including methods from the QGIS API) available from there.
The code that you can execute from the Python console, even if it does not call any specific processing method,
can be converted into a new algorithm that you can later call from the toolbox, the graphical modeler or any other
component, just like you do with any other algorithm. In fact, some algorithms that you can find in the toolbox
are simple scripts.
In this section, we will see how to use processing algorithms from the QGIS Python console, and also how to write
algorithms using Python.
19.7.1 Calling algorithms from the Python console
The first thing you have to do is to import the processing functions with the following line:
>>> import processing
Now, there is basically just one (interesting) thing you can do with that from the console: execute an algorithm.
That is done using the runalg() method, which takes the name of the algorithm to execute as its first parameter,
and then a variable number of additional parameters depending on the requirements of the algorithm. So the first
thing you need to know is the name of the algorithm to execute. That is not the name you see in the toolbox, but
rather a unique command–line name. To find the right name for your algorithm, you can use the algslist()
method. Type the following line in your console:
>>> processing.alglist()
You will see something like this.
Accumulated Cost (Anisotropic)-------->saga:accumulatedcost(anisotropic)
Accumulated Cost (Isotropic)---------->saga:accumulatedcost(isotropic)
Add Coordinates to points------------->saga:addcoordinatestopoints
Add Grid Values to Points------------->saga:addgridvaluestopoints
Add Grid Values to Shapes------------->saga:addgridvaluestoshapes
Add Polygon Attributes to Points------>saga:addpolygonattributestopoints
Aggregate----------------------------->saga:aggregate
Aggregate Point Observations---------->saga:aggregatepointobservations
Aggregation Index--------------------->saga:aggregationindex
Analytical Hierarchy Process---------->saga:analyticalhierarchyprocess
Analytical Hillshading---------------->saga:analyticalhillshading
Average With Mask 1------------------->saga:averagewithmask1
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Average With Mask 2------------------->saga:averagewithmask2
Average With Thereshold 1------------->saga:averagewiththereshold1
Average With Thereshold 2------------->saga:averagewiththereshold2
Average With Thereshold 3------------->saga:averagewiththereshold3
B-Spline Approximation---------------->saga:b-splineapproximation
...
That’s a list of all the available algorithms, alphabetically ordered, along with their corresponding command-line
names.
You can use a string as a parameter for this method. Instead of returning the full list of algorithms, it will only
display those that include that string. If, for instance, you are looking for an algorithm to calculate slope from a
DEM, type alglist("slope") to get the following result:
DTM Filter (slope-based)-------------->saga:dtmfilter(slope-based)
Downslope Distance Gradient----------->saga:downslopedistancegradient
Relative Heights and Slope Positions-->saga:relativeheightsandslopepositions
Slope Length-------------------------->saga:slopelength
Slope, Aspect, Curvature-------------->saga:slopeaspectcurvature
Upslope Area-------------------------->saga:upslopearea
Vegetation Index[slope based]--------->saga:vegetationindex[slopebased]
This result might change depending on the algorithms you have available.
It is easier now to find the algorithm you are looking for and its command-line name, in this case
saga:slopeaspectcurvature.
Once you know the command-line name of the algorithm, the next thing to do is to determine the right syntax to
execute it. That means knowing which parameters are needed and the order in which they have to be passed when
calling the runalg() method. There is a method to describe an algorithm in detail, which can be used to get a
list of the parameters that an algorithm requires and the outputs that it will generate. To get this information, you
can use the alghelp(name_of_the_algorithm) method. Use the command-line name of the algorithm,
not the full descriptive name.
Calling the method with saga:slopeaspectcurvature as parameter, you get the following description:
>>> processing.alghelp("saga:slopeaspectcurvature")
ALGORITHM: Slope, Aspect, Curvature
ELEVATION <ParameterRaster>
METHOD <ParameterSelection>
SLOPE <OutputRaster>
ASPECT <OutputRaster>
CURV <OutputRaster>
HCURV <OutputRaster>
VCURV <OutputRaster>
Now you have everything you need to run any algorithm. As we have already mentioned, there is only one single
command to execute algorithms: runalg(). Its syntax is as follows:
>>> processing.runalg(name_of_the_algorithm, param1, param2, ..., paramN,
Output1, Output2, ..., OutputN)
The list of parameters and outputs to add depends on the algorithm you want to run, and is exactly the list that the
alghelp() method gives you, in the same order as shown.
Depending on the type of parameter, values are introduced differently. The next list gives a quick review of how
to introduce values for each type of input parameter:
• Raster Layer, Vector Layer or Table. Simply use a string with the name that identifies the data object to use
(the name it has in the QGIS Table of Contents) or a filename (if the corresponding layer is not opened, it
will be opened but not added to the map canvas). If you have an instance of a QGIS object representing the
layer, you can also pass it as parameter. If the input is optional and you do not want to use any data object,
use None.
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• Selection. If an algorithm has a selection parameter, the value of that parameter should be entered using an
integer value. To know the available options, you can use the algoptions() command, as shown in the
following example:
>>> processing.algoptions("saga:slopeaspectcurvature")
METHOD(Method)
0 - [0] Maximum Slope (Travis et al. 1975)
1 - [1] Maximum Triangle Slope (Tarboton 1997)
2 - [2] Least Squares Fitted Plane (Horn 1981, Costa-Cabral & Burgess 1996)
3 - [3] Fit 2.Degree Polynom (Bauer, Rohdenburg, Bork 1985)
4 - [4] Fit 2.Degree Polynom (Heerdegen & Beran 1982)
5 - [5] Fit 2.Degree Polynom (Zevenbergen & Thorne 1987)
6 - [6] Fit 3.Degree Polynom (Haralick 1983)
In this case, the algorithm has one such parameter, with seven options. Notice that ordering is zero-based.
• Multiple input. The value is a string with input descriptors separated by semicolons (;). As in the case of
single layers or tables, each input descriptor can be the data object name, or its file path.
• Table Field from XXX. Use a string with the name of the field to use. This parameter is case-sensitive.
• Fixed Table. Type the list of all table values separated by commas (,) and enclosed between quotes (").
Values start on the upper row and go from left to right. You can also use a 2-D array of values representing
the table.
• CRS. Enter the EPSG code number of the desired CRS.
• Extent. You must use a string with xmin, xmax, ymin and ymax values separated by commas (,).
Boolean, file, string and numerical parameters do not need any additional explanations.
Input parameters such as strings, booleans, or numerical values have default values. To use them, specify None
in the corresponding parameter entry.
For output data objects, type the file path to be used to save it, just as it is done from the toolbox. If you want to
save the result to a temporary file, use None. The extension of the file determines the file format. If you enter
a file extension not supported by the algorithm, the default file format for that output type will be used, and its
corresponding extension appended to the given file path.
Unlike when an algorithm is executed from the toolbox, outputs are not added to the map canvas if you execute
that same algorithm from the Python console. If you want to add an output to the map canvas, you have to do it
yourself after running the algorithm. To do so, you can use QGIS API commands, or, even easier, use one of the
handy methods provided for such tasks.
The runalg method returns a dictionary with the output names (the ones shown in the algorithm description)
as keys and the file paths of those outputs as values. You can load those layers by passing the corresponding file
paths to the load() method.
19.7.2 Additional functions for handling data
Apart from the functions used to call algorithms, importing the processing package will also import some
additional functions that make it easier to work with data, particularly vector data. They are just convenience
functions that wrap some functionality from the QGIS API, usually with a less complex syntax. These functions
should be used when developing new algorithms, as they make it easier to operate with input data.
Below is a list of some of these commands. More information can be found in the classes under the
processing/tools package, and also in the example scripts provided with QGIS.
• getObject(obj): Returns a QGIS object (a layer or table) from the passed object, which can be a
filename or the name of the object in the QGIS Layers List
• values(layer, fields): Returns the values in the attributes table of a vector layer, for the passed
fields. Fields can be passed as field names or as zero-based field indices. Returns a dict of lists, with the
passed field identifiers as keys. It considers the existing selection.
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• features(layer): Returns an iterator over the features of a vector layer, considering the existing
selection.
• uniqueValues(layer, field): Returns a list of unique values for a given attribute. Attributes can
be passed as a field name or a zero-based field index. It considers the existing selection.
19.7.3 Creating scripts and running them from the toolbox
You can create your own algorithms by writing the corresponding Python code and adding a few extra lines to
supply additional information needed to define the semantics of the algorithm. You can find a Create new script
menu under the Tools group in the Script algorithms block of the toolbox. Double-click on it to open the script
editing dialog. That’s where you should type your code. Saving the script from there in the scripts folder (the
default folder when you open the save file dialog) with .py extension will automatically create the corresponding
algorithm.
The name of the algorithm (the one you will see in the toolbox) is created from the filename, removing its extension
and replacing low hyphens with blank spaces.
Let’s have a look at the following code, which calculates the Topographic Wetness Index (TWI) directly from a
DEM.
##dem=raster
##twi=output
ret_slope = processing.runalg("saga:slopeaspectcurvature", dem, 0, None,
None, None, None, None)
ret_area = processing.runalg("saga:catchmentarea(mass-fluxmethod)", dem,
0, False, False, False, False, None, None, None, None, None)
processing.runalg("saga:topographicwetnessindex(twi), ret_slope['SLOPE'],
ret_area['AREA'], None, 1, 0, twi)
As you can see, the calculation involves three algorithms, all of them coming from SAGA. The last one calculates
the TWI, but it needs a slope layer and a flow accumulation layer. We do not have these layers, but since we have
the DEM, we can calculate them by calling the corresponding SAGA algorithms.
The part of the code where this processing takes place is not difficult to understand if you have read the previous
sections in this chapter. The first lines, however, need some additional explanation. They provide the information
that is needed to turn your code into an algorithm that can be run from any of the GUI components, like the toolbox
or the graphical modeler.
These lines start with a double Python comment symbol (##) and have the following structure:
[parameter_name]=[parameter_type] [optional_values]
Here is a list of all the parameter types that are supported in processing scripts, their syntax and some examples.
• raster. A raster layer.
• vector. A vector layer.
• table. A table.
• number. A numerical value. A default value must be provided. For instance, depth=number 2.4.
• string. A text string. As in the case of numerical values, a default value must be added. For instance,
name=string Victor.
• boolean. A boolean value. Add True or False after it to set the default value. For example,
verbose=boolean True.
• multiple raster. A set of input raster layers.
• multiple vector. A set of input vector layers.
• field. A field in the attributes table of a vector layer. The name of the layer has to be added after the
field tag. For instance, if you have declared a vector input with mylayer=vector, you could use
myfield=field mylayer to add a field from that layer as parameter.
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• folder. A folder.
• file. A filename.
The parameter name is the name that will be shown to the user when executing the algorithm, and also the variable
name to use in the script code. The value entered by the user for that parameter will be assigned to a variable with
that name.
When showing the name of the parameter to the user, the name will be edited to improve its appearance, replacing
low hyphens with spaces. So, for instance, if you want the user to see a parameter named A numerical
value, you can use the variable name A_numerical_value.
Layers and table values are strings containing the file path of the corresponding object. To turn them into a QGIS
object, you can use the processing.getObjectFromUri() function. Multiple inputs also have a string
value, which contains the file paths to all selected object, separated by semicolons (;).
Outputs are defined in a similar manner, using the following tags:
• output raster
• output vector
• output table
• output html
• output file
• output number
• output string
The value assigned to the output variables is always a string with a file path. It will correspond to a temporary file
path in case the user has not entered any output filename.
When you declare an output, the algorithm will try to add it to QGIS once it is finished. That is why, although the
runalg() method does not load the layers it produces, the final TWI layer will be loaded (using the case of our
previous example), since it is saved to the file entered by the user, which is the value of the corresponding output.
Do not use the load() method in your script algorithms, just when working with the console line. If a layer is
created as output of an algorithm, it should be declared as such. Otherwise, you will not be able to properly use
the algorithm in the modeler, since its syntax (as defined by the tags explained above) will not match what the
algorithm really creates.
Hidden outputs (numbers and strings) do not have a value. Instead, you have to assign a value to them. To do so,
just set the value of a variable with the name you used to declare that output. For instance, if you have used this
declaration,
##average=output number
the following line will set the value of the output to 5:
average = 5
In addition to the tags for parameters and outputs, you can also define the group under which the algorithm will
be shown, using the group tag.
If your algorithm takes a long time to process, it is a good idea to inform the user. You have a global named
progress available, with two possible methods: setText(text) and setPercentage(percent) to
modify the progress text and the progress bar.
Several examples are provided. Please check them to see real examples of how to create algorithms using the
processing framework classes. You can right-click on any script algorithm and select Edit script to edit its code or
just to see it.
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19.7.4 Documenting your scripts
As in the case of models, you can create additional documentation for your scripts, to explain what they do and
how to use them. In the script editing dialog, you will find an [Edit script help] button. Click on it and it will
take you to the help editing dialog. Check the section about the graphical modeler to know more about this dialog
and how to use it.
Help files are saved in the same folder as the script itself, adding the .help extension to the filename. Notice that
you can edit your script’s help before saving the script for the first time. If you later close the script editing dialog
without saving the script (i.e., you discard it), the help content you wrote will be lost. If your script was already
saved and is associated to a filename, saving the help content is done automatically.
19.7.5 Pre- and post-execution script hooks
Scripts can also be used to set pre- and post-execution hooks that are run before and after an algorithm is run. This
can be used to automate tasks that should be performed whenever an algorithm is executed.
The syntax is identical to the syntax explained above, but an additional global variable named alg is available,
representing the algorithm that has just been (or is about to be) executed.
In the General group of the processing configuration dialog, you will find two entries named Pre-execution script
file and Post-execution script file where the filename of the scripts to be run in each case can be entered.
19.8 Writing new Processing algorithms as python scripts
You can create your own algorithms by writing the corresponding Python code and adding a few extra lines to
supply additional information needed to define the semantics of the algorithm. You can find a Create new script
menu under the Tools group in the Script algorithms block of the toolbox. Double-click on it to open the script
edition dialog. That’s where you should type your code. Saving the script from there in the scripts folder (the
default one when you open the save file dialog), with .py extension, will automatically create the corresponding
algorithm.
The name of the algorithm (the one you will see in the toolbox) is created from the filename, removing its extension
and replacing underscores with blank spaces.
Let’s have the following code, which calculates the Topographic Wetness Index (TWI) directly from a DEM
##dem=raster
##twi=output raster
ret_slope = processing.runalg("saga:slopeaspectcurvature", dem, 0, None,
None, None, None, None)
ret_area = processing.runalg("saga:catchmentarea", dem,
0, False, False, False, False, None, None, None, None, None)
processing.runalg("saga:topographicwetnessindextwi, ret_slope['SLOPE'],
ret_area['AREA'], None, 1, 0, twi)
As you can see, it involves 3 algorithms, all of them coming from SAGA. The last one of them calculates the TWI,
but it needs a slope layer and a flow accumulation layer. We do not have these, but since we have the DEM, we
can calculate them by calling the corresponding SAGA algorithms.
The part of the code where this processing takes place is not difficult to understand if you have read the previous
chapter. The first lines, however, need some additional explanation. They provide the information that is needed to
turn your code into an algorithm that can be run from any of the GUI components, like the toolbox or the graphical
modeler.
These lines start with a double Python comment symbol (##) and have the following structure
[parameter_name]=[parameter_type] [optional_values]
Here is a list of all the parameter types that are supported in processing scripts, their syntax and some examples.
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• raster. A raster layer
• vector. A vector layer
• table. A table
• number. A numerical value. A default value must be provided. For instance, depth=number 2.4
• string. A text string. As in the case of numerical values, a default value must be added. For instance,
name=string Victor
• longstring. Same as string, but a larger text box will be shown, so it is better suited for long strings,
such as for a script expecting a small code snippet.
• boolean. A boolean value. Add True or False after it to set the default value. For example,
verbose=boolean True.
• multiple raster. A set of input raster layers.
• multiple vector. A set of input vector layers.
• field. A field in the attributes table of a vector layer. The name of the layer has to be added after the
field tag. For instance, if you have declared a vector input with mylayer=vector, you could use
myfield=field mylayer to add a field from that layer as parameter.
• extent. A spatial extent defined by xmin, xmax, ymin, ymax
• folder. A folder
• file. A filename
• crs. A Coordinate Reference System
• selection. A dropdown menu that allows the user to select from a pre-populated list. For example
units=selection sq_km;sq_miles;sq_degrees
• name. Name of the script. This will be displayed as the algorithm name in the processing toolbox. For
example My Algorithm Name=name
• group. Folder name where the script will appear in the Processing Toolbox. For Example, adding
Utils=groups will put the script within a Utils folder within Scripts.
The parameter name is the name that will be shown to the user when executing the algorithm, and also the variable
name to use in the script code. The value entered by the user for that parameter will be assigned to a variable with
that name.
When showing the name of the parameter to the user, the name will be edited to improve its appearance, replacing
underscores with spaces. So, for instance, if you want the user to see a parameter named A numerical value,
you can use the variable name A_numerical_value.
Layers and tables values are strings containing the filepath of the corresponding object. To turn them into a QGIS
object, you can use the processing.getObjectFromUri() function. Multiple inputs also have a string
value, which contains the filepaths to all selected objects, separated by semicolons (;).
Outputs are defined in a similar manner, using the following tags:
• output raster
• output vector
• output table
• output html
• output file
• output number
• output string
• output extent
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The value assigned to the output variables is always a string with a filepath. It will correspond to a temporary
filepath in case the user has not entered any output filename.
In addition to the tags for parameters and outputs, you can also define the group under which the algorithm will
be shown, using the group tag.
The last tag that you can use in your script header is ##nomodeler. Use that when you do not want your
algorithm to be shown in the modeler window. This should be used for algorithms that do not have a clear syntax
(for instance, if the number of layers to be created is not known in advance, at design time), which make them
unsuitable for the graphical modeler
19.8.1 Handing data produced by the algorithm
When you declare an output representing a layer (raster, vector or table), the algorithm will try to add it to QGIS
once it is finished. That is the reason why, although the runalg() method does not load the layers it produces,
the final TWI layer will be loaded, since it is saved to the file entered by the user, which is the value of the
corresponding output.
Do not use the load() method in your script algorithms, but just when working with the console line. If a layer
is created as output of an algorithm, it should be declared as such. Otherwise, you will not be able to properly
use the algorithm in the modeler, since its syntax (as defined by the tags explained above) will not match what the
algorithm really creates.
Hidden outputs (numbers and strings) do not have a value. Instead, it is you who has to assign a value to them. To
do so, just set the value of a variable with the name you used to declare that output. For instance, if you have used
this declaration,
##average=output number
the following line will set the value of the output to 5:
average = 5
19.8.2 Communicating with the user
If your algorithm takes a long time to process, it is a good idea to inform the user. You have a global named
progress available, with two available methods: setText(text) and setPercentage(percent) to
modify the progress text and the progress bar.
If you have to provide some information to the user, not related to the progress of the algorithm, you can use the
setInfo(text) method, also from the progress object.
If your script has some problem, the correct way of propagating it is to raise an exception of type
GeoAlgorithmExecutionException(). You can pass a message as argument to the constructor of the
exception. Processing will take care of handling it and communicating with the user, depending on where the
algorithm is being executed from (toolbox, modeler, Python console. . . )
19.8.3 Documenting your scripts
As in the case of models, you can create additional documentation for your script, to explain what they do and
how to use them. In the script editing dialog you will find a [Edit script help] button. Click on it and it will take
you to the help editing dialog. Check the chapter about the graphical modeler to find out more about this dialog
and how to use it.
Help files are saved in the same folder as the script itself, adding the .help extension to the filename. Note that
you can edit your script’s help before saving it for the first time. If you later close the script editing dialog without
saving the script (i.e. you discard it), the help content you wrote will be lost. If your script was already saved and
is associated with a filename, saving is done automatically.
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19.8.4 Example scripts
Several examples are available in the on-line collection of scripts, which you can access by selecting the Get script
from on-line script collection tool under the Scripts/tools entry in the toolbox.
Fig. 19.32: Processing Get Script
Please, check them to see real examples of how to create algorithms using the processing framework classes. You
can right-click on any script algorithm and select Edit script to edit its code or just to see it.
19.8.5 Best practices for writing script algorithms
Here’s a quick summary of ideas to consider when creating your script algorithms and, especially, if you want to
share with other QGIS users. Following these simple rules will ensure consistency across the different Processing
elements such as the toolbox, the modeler or the batch processing interface.
• Do not load resulting layers. Let Processing handle your results and load your layers if needed.
• Always declare the outputs your algorithm creates. Avoid things such as declaring one output and then
using the destination filename set for that output to create a collection of them. That will break the correct
semantics of the algorithm and make it impossible to use it safely in the modeler. If you have to write an
algorithm like that, make sure you add the ##nomodeler tag.
• Do not show message boxes or use any GUI element from the script. If you want to communicate with the
user, use the setInfo() method or throw an GeoAlgorithmExecutionException
• As a rule of thumb, do not forget that your algorithm might be executed in a context other than the Processing
toolbox.
19.8.6 Pre- and post-execution script hooks
Scripts can also be used to set pre- and post-execution hooks that are run before and after an algorithm is run. This
can be used to automate tasks that should be performed whenever an algorithm is executed.
The syntax is identical to the syntax explained above, but an additional global variable named alg is available,
representing the algorithm that has just been (or is about to be) executed.
In the General group of the processing config dialog you will find two entries named Pre-execution script file and
Post-execution script file where the filename of the scripts to be run in each case can be entered.
19.9 Configuring external applications
The processing framework can be extended using additional applications. Currently, SAGA, GRASS and R are
supported. Algorithms relying on an external applications are managed by their own algorithm provider. Additional providers can be found as separate plugins, and installed using the QGIS Plugin Manager.
This section will show you how to configure the Processing framework to include these additional applications,
and it will explain some particular features of the algorithms based on them. Once you have correctly configured
the system, you will be able to execute external algorithms from any component like the toolbox or the graphical
modeler, just like you do with any other geoalgorithm.
By default, all algorithms that rely on an external application not shipped with QGIS are not enabled. You can
enable them in the settings dialog. Make sure that the corresponding application is already installed in your
system.
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19.9.1 A note for Windows users
If you are not an advanced user and you are running QGIS on Windows, you might not be interested in reading
the rest of this chapter. Make sure you install QGIS in your system using the standalone installer. That will
automatically install SAGA and GRASS in your system and configure them so they can be run from QGIS. All
the algorithms from these providers will be ready to be run without needing any further configuration. If installing
through OSGeo4W application, make sure you select for installation SAGA and GRASS as well.
19.9.2 A note on file formats
When using an external software, opening a file in QGIS does not mean that it can be opened and processed as
well in that other software. In most cases, other software can read what you have opened in QGIS, but in some
cases, that might not be true. When using databases or uncommon file formats, whether for raster or vector layers,
problems might arise. If that happens, try to use well-known file formats that you are sure are understood by both
programs, and check the console output (in the history and log dialog) to know more about what is going wrong.
Using GRASS raster layers is, for instance, one case in which you might have trouble and not be able to complete
your work if you call an external algorithm using such a layer as input. For this reason, these layers will not appear
as available to algorithms.
You should, however, find no problems at all with vector layers, since QGIS automatically converts from the
original file format to one accepted by the external application before passing the layer to it. This adds extra
processing time, which might be significant if the layer has a large size, so do not be surprised if it takes more
time to process a layer from a DB connection than it does to process one of a similar size stored in a shapefile.
Providers not using external applications can process any layer that you can open in QGIS, since they open it for
analysis through QGIS.
Regarding output formats, all formats supported by QGIS as output can be used, both for raster and vector layers.
Some providers do not support certain formats, but all can export to common formats that can later be transformed
by QGIS automatically. As in the case of input layers, if this conversion is needed, that might increase the
processing time.
19.9.3 A note on vector layer selections
External applications may also be made aware of the selections that exist in vector layers within QGIS. However,
that requires rewriting all input vector layers, just as if they were originally in a format not supported by the
external application. Only when no selection exists, or the Use only selected features option is not enabled in the
processing general configuration, can a layer be directly passed to an external application.
In other cases, exporting only selected features is needed, which causes execution times to be longer.
19.9.4 SAGA
SAGA algorithms can be run from QGIS if you have SAGA installed in your system and you configure the processing framework properly so it can find SAGA executables. In particular, the SAGA command-line executable
is needed to run SAGA algorithms.
If you are running Windows, both the stand-alone installer and the OSGeo4W installer include SAGA along with
QGIS, and the path is automatically configured, so there is no need to do anything else.
If you have installed SAGA yourself and your QGIS installer did not include it, the path to the SAGA executable
must be configured. To do this, open the configuration dialog. In the SAGA block, you will find a setting named
SAGA Folder. Enter the path to the folder where SAGA is installed. Close the configuration dialog, and now you
are ready to run SAGA algorithms from QGIS.
If you are running Linux, SAGA binaries are not included with Processing, so you have to download and install
the software yourself. Please check the SAGA website for more information.
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In this case, there is no need to configure the path to the SAGA executable, and you will not see those folder entries.
Instead, you must make sure that SAGA is properly installed and its folder is added to the PATH environment
variable. Just open a console and type saga_cmd to check that the system can find where the SAGA binaries are
located.
About SAGA grid system limitations
Most SAGA algorithms that require several input raster layers require them to have the same grid system. That is,
they must cover the same geographic area and have the same cell size, so their corresponding grids match. When
calling SAGA algorithms from QGIS, you can use any layer, regardless of its cell size and extent. When multiple
raster layers are used as input for a SAGA algorithm, QGIS resamples them to a common grid system and then
passes them to SAGA (unless the SAGA algorithm can operate with layers from different grid systems).
The definition of that common grid system is controlled by the user, and you will find several parameters in the
SAGA group of the settings window to do so. There are two ways of setting the target grid system:
• Setting it manually. You define the extent by setting the values of the following parameters:
– Resampling min X
– Resampling max X
– Resampling min Y
– Resampling max Y
– Resampling cellsize
Notice that QGIS will resample input layers to that extent, even if they do not overlap with it.
• Setting it automatically from input layers. To select this option, just check the Use min covering grid system
for resampling option. All the other settings will be ignored and the minimum extent that covers all the
input layers will be used. The cell size of the target layer is the maximum of all cell sizes of the input layers.
For algorithms that do not use multiple raster layers, or for those that do not need a unique input grid system, no
resampling is performed before calling SAGA, and those parameters are not used.
Limitations for multi-band layers
Unlike QGIS, SAGA has no support for multi-band layers. If you want to use a multiband layer (such as an RGB or
multispectral image), you first have to split it into single-banded images. To do so, you can use the ‘SAGA/Grid
- Tools/Split RGB image’ algorithm (which creates three images from an RGB image) or the ‘SAGA/Grid Tools/Extract band’ algorithm (to extract a single band).
Limitations in cell size
SAGA assumes that raster layers have the same cell size in the X and Y axis. If you are working with a layer with
different values for horizontal and vertical cell size, you might get unexpected results. In this case, a warning will
be added to the processing log, indicating that an input layer might not be suitable to be processed by SAGA.
Logging
When QGIS calls SAGA, it does so using its command-line interface, thus passing a set of commands to perform
all the required operations. SAGA shows its progress by writing information to the console, which includes the
percentage of processing already done, along with additional content. This output is filtered and used to update
the progress bar while the algorithm is running.
Both the commands sent by QGIS and the additional information printed by SAGA can be logged along with other
processing log messages, and you might find them useful to track in detail what is going on when QGIS runs a
SAGA algorithm. You will find two settings, namely Log console output and Log execution commands, to activate
that logging mechanism.
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Most other providers that use an external application and call it through the command-line have similar options,
so you will find them as well in other places in the processing settings list.
19.9.5 R. Creating R scripts
R integration in QGIS is different from that of SAGA in that there is not a predefined set of algorithms you can run
(except for a few examples). Instead, you should write your scripts and call R commands, much like you would do
from R, and in a very similar manner to what we saw in the section dedicated to processing scripts. This section
shows you the syntax to use to call those R commands from QGIS and how to use QGIS objects (layers, tables)
in them.
The first thing you have to do, as we saw in the case of SAGA, is to tell QGIS where your R binaries are located.
You can do this using the R folder entry in the processing configuration dialog. Once you have set that parameter,
you can start creating and executing your own R scripts.
Note: for Windows user, usually the R executable file is in the C:\Program Files\R\R-3.2 folder. Add
just the folder and NOT the binary!
Once again, this is different in Linux, and you just have to make sure that the R folder is included in the PATH
environment variable. If you can start R just typing R in a console, then you are ready to go.
To add a new algorithm that calls an R function (or a more complex R script that you have developed and you
would like to have available from QGIS), you have to create a script file that tells the processing framework how
to perform that operation and the corresponding R commands to do so.
R script files have the extension .rsx, and creating them is pretty easy if you just have a basic knowledge of R
syntax and R scripting. They should be stored in the R scripts folder. You can set this folder in the R settings group
(available from the processing settings dialog), just like you do with the folder for regular processing scripts.
Let’s have a look at a very simple script file, which calls the R method spsample to create a random grid within
the boundary of the polygons in a given polygon layer. This method belongs to the maptools package. Since
almost all the algorithms that you might like to incorporate into QGIS will use or generate spatial data, knowledge
of spatial packages like maptools and, especially, sp, is mandatory.
##polyg=vector
##numpoints=number 10
##output=output vector
##sp=group
pts=spsample(polyg,numpoints,type="random")
output=SpatialPointsDataFrame(pts, as.data.frame(pts))
The first lines, which start with a double Python comment sign (##), tell QGIS the inputs of the algorithm described in the file and the outputs that it will generate. They work with exactly the same syntax as the Processing
scripts that we have already seen, so they will not be described here again.
Please have a look at the R Intro and the R Syntax Training Manual Chapters to have more information on how to
write your own R scriptsWhen you declare an input parameter, QGIS uses that information for two things: creating the user interface to
ask the user for the value of that parameter and creating a corresponding R variable that can later be used as input
for R commands.
In the above example, we are declaring an input of type vector named polyg. When executing the algorithm,
QGIS will open in R the layer selected by the user and store it in a variable also named polyg. So, the name of
a parameter is also the name of the variable that we can use in R for accessing the value of that parameter (thus,
you should avoid using reserved R words as parameter names).
Spatial elements such as vector and raster layers are read using the readOGR() and brick() commands (you
do not have to worry about adding those commands to your description file – QGIS will do it), and they are stored
as Spatial*DataFrame objects. Table fields are stored as strings containing the name of the selected field.
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Tables are opened using the read.csv() command. If a table entered by the user is not in CSV format, it will
be converted prior to importing it into R.
Additionally, raster files can be read using the readGDAL() command instead of brick() by using the
##usereadgdal.
If you are an advanced user and do not want QGIS to create the object representing the layer, you can use the
##passfilenames tag to indicate that you prefer a string with the filename instead. In this case, it is up to you
to open the file before performing any operation on the data it contains.
With the above information, we can now understand the first line of our first example script (the first line not
starting with a Python comment).
pts=spsample(polyg,numpoints,type="random")
The variable polygon already contains a SpatialPolygonsDataFrame object, so it can be used to call the
spsample method, just like the numpoints one, which indicates the number of points to add to the created
sample grid.
Since we have declared an output of type vector named out, we have to create a variable named out and store a
Spatial*DataFrame object in it (in this case, a SpatialPointsDataFrame). You can use any name for
your intermediate variables. Just make sure that the variable storing your final result has the same name that you
used to declare it, and that it contains a suitable value.
In this case, the result obtained from the spsample method has to be converted explicitly into a
SpatialPointsDataFrame object, since it is itself an object of class ppp, which is not a suitable class
to be returned to QGIS.
If your algorithm generates raster layers, the way they are saved will depend on whether or not you have used the
##dontuserasterpackage option. If you have used it, layers are saved using the writeGDAL() method.
If not, the writeRaster() method from the raster package will be used.
If you have used the ##passfilenames option, outputs are generated using the raster package (with
writeRaster()), even though it is not used for the inputs.
If your algorithm does not generate any layer, but rather a text result in the console instead, you have to indicate
that you want the console to be shown once the execution is finished. To do so, just start the command lines that
produce the results you want to print with the > (‘greater’) sign. The output of all other lines will not be shown.
For instance, here is the description file of an algorithm that performs a normality test on a given field (column) of
the attributes of a vector layer:
##layer=vector
##field=field layer
##nortest=group
library(nortest)
>lillie.test(layer[[field]])
The output of the last line is printed, but the output of the first is not (and neither are the outputs from other
command lines added automatically by QGIS).
If your algorithm creates any kind of graphics (using the plot() method), add the following line:
##showplots
This will cause QGIS to redirect all R graphical outputs to a temporary file, which will be opened once R execution
has finished.
Both graphics and console results will be shown in the processing results manager.
For more information, please check the script files provided with Processing. Most of them are rather simple and
will greatly help you understand how to create your own scripts.
Note: rgdal and raster libraries are loaded by default, so you do not have to add the corresponding
library() commands (you just have to make sure that those two packages are installed in your R dis-
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tribution). However, other additional libraries that you might need have to be explicitly loaded by typing,
library(ggplot2). If the package is not already installed on your machine, Processing will download and
install it. In this way the package will be also available in R Standalone. Be aware that if the package has to be
downloaded, the first time you run the script it might take a long time.
19.9.6 GRASS
Configuring GRASS is not much different from configuring SAGA. First, the path to the GRASS folder has to be
defined, but only if you are running Windows.
By default, the Processing framework tries to configure its GRASS connector to use the GRASS distribution that
ships along with QGIS. This should work without problems in most systems, but if you experience problems, you
might have to configure the GRASS connector manually. Also, if you want to use a different GRASS installation,
you can change that setting and point to the folder where the other version is installed. GRASS 7 is needed for
algorithms to work correctly.
If you are running Linux, you just have to make sure that GRASS is correctly installed, and that it can be run
without problem from a console.
GRASS algorithms use a region for calculations. This region can be defined manually using values similar to
the ones found in the SAGA configuration, or automatically, taking the minimum extent that covers all the input
layers used to execute the algorithm each time. If the latter approach is the behavior you prefer, just check the Use
min covering region option in the GRASS configuration parameters.
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20
Plugins
20.1 QGIS Python console
As you will see later in this chapter, QGIS has been designed with a plugin architecture. Plugins can be written in
Python, a very famous language in the geospatial world.
QGIS brings a Python API (see PyQGIS Developer Cookbook for some code sample) to let the user interact with
its objects (layers, feature or interface). QGIS also has a Python console.
The QGIS Python Console is an interactive shell for the python command executions. It also has a python file
editor that allows you to edit and save your python scripts. Both console and editor are based on PyQScintilla2
package. To open the console go to Plugins → Python Console (Ctrl+Alt+P).
20.1.1 The Interactive Console
The interactive console is composed of a toolbar, an input area and an output one.
Toolbar
The toolbar proposes the following tools:
•
Clear console to wipe the output area;
•
Import class: Processing, PyQt4.QtCore or PyQt4.QtGui class;
•
Run command available in the input area: same as pressing Enter;
•
Show editor: toggles The Code Editor visibility;
•
Options. . . ;
•
Help. . . .
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Console
The console main features are:
• Code completion, highlighting syntax and calltips for the following APIs:
– Python
– PyQGIS
– PyQt4
– QScintilla2
– osgeo-gdal-ogr
• Ctrl+Alt+Space to view the auto-completion list if enabled in the Options;
• Execute code snippets from the input area by typing and pressing Enter or Run Command;
• Execute code snippets from the output area using the Enter selected from the contextual menu or pressing
Ctrl+E;
• Browse the command history from the input area using the Up and Down arrow keys and execute the
command you want;
• Ctrl+Shift+Space to view the command history: double-clicking a row will execute the command.
The Command History dialog can also be accessed from context menu of input area;
• Save and clear the command history.
console_history.txt;
The history will be saved into the file ~/.qgis2/
• Open QGIS API documentation by typing _api;
• Open PyQGIS Cookbook by typing _pyqgis.
Tip: Reuse executed commands from the output panel
You can execute code snippets from the output panel by selecting some text and pressing Ctrl+E. No matter if
selected text contains the interpreter prompt (>>>, ...).
Fig. 20.1: The Python Console
20.1.2 The Code Editor
Show editor
Use the
button to enable the editor widget. It allows editing and saving Python files and offers
advanced functionalities to manage your code (comment and uncomment code, check syntax, share the code via
codepad.org and much more). Main features are:
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• Code completion, highlighting syntax and calltips for the following APIs:
– Python
– PyQGIS
– PyQt4
– QScintilla2
– osgeo-gdal-ogr
• Ctrl+Space to view the auto-completion list.
• Sharing code snippets via codepad.org.
• Ctrl+4 Syntax check.
• Search bar (open it with the default Desktop Environment shorcut, usually Ctrl+F):
– Use the default Desktop Environment shortcut to find next/previous (Ctrl+G and Shift+Ctrl+G);
– Automatically find first match when typing in find box;
– Set initial find string to selection when opening find;
– Pressing Esc closes the find bar.
• Object inspector: a class and function browser;
• Go to an object definition with a mouse click (from Object inspector);
• Execute code snippets with the Enter selected command;
• Execute the whole script with the Run script command (this creates a byte-compiled file with the extension
.pyc).
Note: Running partially or totally a script from the Code Editor outputs the result in the Console output area.
Fig. 20.2: The Python Console editor
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20.1.3 Options
Accessible either from the Console toolbar or the contextual menu of Console output panel or Code Editor, this
adds further settings to manage and control the Python console behavior:
• Autocompletion: If checked the code completion is enabled. You can get autocompletion from current
document, from installed APIs and both from APIs and current document.
• Autocompletion threshold: Sets the threshold to display the autocompletion list (in chars typed).
• Automatic parentheses insertion: If checked enables the autoclosing for bracket.
• Auto-save script before running: Allows you to save automatically the script to be executed in order to
avoid to save it after any modification. This action will store a temporary file into the temporary system
directory that will be automatically deleted after running.
• Using preloaded APIs file: You can choose whether use the preload APIs file or load some APIs files saved
on your system.
• Using prepared APIs file: If checked the *.pap file will be used for code completion. To generate a
prepared APIs file you have to load at least an *.api file and then compile it by clicking on [Compile
Apis. . . ] button.
Tip: Save the options
To save the state of console’s widgets you have to close the Python Console from the close button. This allows
you to save the geometry to be restored to the next start.
20.2 QGIS Plugins
QGIS has been designed with a plugin architecture. This allows many new features and functions to be easily
added to the application. Some of the features in QGIS are actually implemented as plugins.
20.2.1 Core and External plugins
QGIS plugins are implemented either as Core Plugins or External Plugins.
Core Plugins are maintained by the QGIS Development Team and are automatically part of every QGIS distribution. They are written in one of two languages: C++ or Python.
Most of External Plugins are currently written in Python. They are stored either in the ‘Official’ QGIS Repository
at http://plugins.qgis.org/plugins/ or in external repositories and are maintained by the individual authors. Detailed
documentation about the usage, minimum QGIS version, home page, authors,and other important information
are provided for the plugins in the Official repository. For other external repositories, documentation might be
available with the external plugins themselves. External plugins documentation is not included in this manual.
To install or activate a plugin, go to Plugins menu and select
Manage and install plugins. . . .
Installed external python plugins are placed under the ~/(UserProfile)/python/plugins folder where:
• ~ represents the HOME directory which on
Windows is usually something like C:\Documents and
Settings\(user) (on Windows XP or earlier) or C:\Users\(user);
• and (UserProfile) represents the active profile folder, which for the QGIS default user profile is:
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–
.local/share/QGIS/QGIS3/profiles/default;
–
AppData\Roaming\QGIS\QGIS3\profiles\default;
–
Library/Application Support/QGIS/QGIS3/profiles/default.
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Paths to Custom C++ plugins libraries can also be added under Settings → Options → System.
Note: According to the plugin manager settings, QGIS main interface can display an icon on the right of the
status bar to inform you that there are updates for your installed plugins or new plugins available.
20.2.2 The Plugins Dialog
The tabs in the Plugins dialog allow the user to install, uninstall and upgrade plugins in different ways. Each
plugin has some metadata displayed in the right panel:
• information on whether the plugin is experimental
• description
• rating vote(s) (you can vote for your preferred plugin!)
• tags
• some useful links to the home page, tracker and code repository
• author(s)
• version available
At the top of the dialog, a Search function helps you find any plugin using metadata information (author, name,
description. . . ). It is available in nearly every tab (except
Settings).
The Settings tab
Settings tab is the main place you can configure which plugins can be displayed in your application. You
The
can use the following options:
•
Check for updates on startup. Whenever a new plugin or a plugin update is available, QGIS will inform
you ‘every time QGIS starts’, ‘once a day’, ‘every 3 days’, ‘every week’, ‘every 2 weeks’ or ‘every month’.
•
Show also experimental plugins. QGIS will show you plugins in early stages of development, which are
generally unsuitable for production use.
•
Show also deprecated plugins. Because they use functions that are no longer available in QGIS, these
plugins are set deprecated and generally unsuitable for production use. They appear among invalid plugins
list.
By default, QGIS provides you with its official plugin repository with the URL https://plugins.qgis.org/plugins/
plugins.xml?qgis=3.0 (in case of QGIS 3.0) in the Plugin repositories section. To add external author repositories,
click [Add. . . ] and fill in the Repository Details form with a name and the URL. The URL can be of http://
or file:// protocol type.
The default QGIS repository is an open repository and you don’t need any authentication to access it. You can
however deploy your own plugin repository and require an authentication (basic authentication, PKI). You can get
more information on QGIS authentication support in Authentication chapter.
If you do not want one or more of the added repositories, they can be disabled from the Settings tab via the
[Edit. . . ] button, or completely removed with the [Delete] button.
The All tab
All tab, all the available plugins are listed, including both core and external plugins. Use [Upgrade
In the
all] to look for new versions of the plugins. Furthermore, you can use [Install plugin] if a plugin is listed but
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Fig. 20.3: The
Settings tab
not installed, [Uninstall plugin] as well as [Reinstall plugin] if a plugin is installed. An installed plugin can be
temporarily de/activated using the checkbox.
The Installed tab
In the
Installed tab, you’ll find listed the Core plugins, that you can not uninstall. You can extend this list
with external plugins that can be uninstalled and reinstalled any time, using the [Uninstall plugin] and [Reinstall
plugin] buttons. You can [Upgrade all] the plugins here as well.
The Not installed tab
The
Not installed tab lists all plugins available that are not installed. You can use the [Install plugin] button
to implement a plugin into QGIS.
The Upgradeable and New tabs
The
Upgradeable and
New tabs are enabled when new plugins are added to the repository or a new version
of an installed plugin is released. If you activated
Show also experimental plugins in the
those also appear in the list giving you opportunity to early test upcoming tools.
Settings menu,
Installation can be done with the [Install plugin], [Upgrade plugin] or [Upgrade all] buttons.
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Fig. 20.4: The
Fig. 20.5: The
20.2. QGIS Plugins
All tab
Installed tab
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Fig. 20.6: The
Not installed tab
Fig. 20.7: The
Upgradeable tab
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The Invalid tab
Invalid tab lists all installed plugins that are currently broken for any reason (missing dependency, errors
The
while loading, incompatible functions with QGIS version. . . ). You can try the [Reinstall plugin] button to fix
an invalidated plugin but most of the times the fix will be elsewhere (install some libraries, look for another
compatible plugin or help to upgrade the broken one).
Fig. 20.8: The
Invalid tab
The Install from ZIP tab
Install from ZIP tab provides a file selector widget to import plugins in a zipped format, e.g. plugins
The
downloaded directly from their repository.
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20.3 Using QGIS Core Plugins
Icon
Plugin
Description
Manual Reference
Coordinate Capture
Capture mouse coordinate in different CRS
Coordinate Capture Plugin
DB Manager
Manage your databases within QGIS
DB Manager Plugin
eVis
Event Visualization Tool
eVis Plugin
GDAL Tools
GDAL raster functionality
GDAL Tools Plugin
Geometry Checker
Check and repair errors in vector geometries
Geometry Checker Plugin
Georeferencer GDAL
Georeference rasters with GDAL
Georeferencer Plugin
GPS Tools
Tools for loading and importing GPS data
GPS Plugin
GRASS
GRASS functionality
GRASS GIS Integration
Interact with metadata catalog services
(CSW)
MetaSearch
Client
Offline Editing
Offline editing and synchronizing with
database
Offline Editing Plugin
Plugin Manager
Manage core and external plugins
The Plugins Dialog
Processing
Spatial data processing framework
QGIS processing framework
Topology Checker
Find topological errors in vector layers
Topology Checker Plugin
Metasearch
Client
Catalog
Catalog
20.4 Coordinate Capture Plugin
The coordinate capture plugin is easy to use and provides the ability to display coordinates on the map canvas for
two selected coordinate reference systems (CRS).
Fig. 20.9: Coordinate Capture Plugin
1. Start QGIS, select
Project Properties from the Settings (KDE, Windows) or File (Gnome, macOS) menu
and click on the Projection tab. As an alternative, you can also click on the
right-hand corner of the status bar.
CRS status
icon in the lower
2. Click on the
Enable on the fly projection checkbox and select a projected coordinate system of your
choice (see also Working with Projections).
3. Activate the coordinate capture plugin in the Plugin Manager (see The Plugins Dialog) and ensure that the
dialog is visible by going to View → Panels and ensuring that
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coordinate capture dialog appears as shown in Figure figure_coordinate_capture. Alternatively, you can
also go to Vector → Coordinate Capture and see if
4. Click on the
selected above.
Coordinate Capture is enabled.
Click to the select the CRS to use for coordinate display
icon and select a different CRS from the one you
5. To start capturing coordinates, click on [Start capture]. You can now click anywhere on the map canvas
and the plugin will show the coordinates for both of your selected CRS.
6. To enable mouse coordinate tracking, click the
mouse tracking
icon.
7. You can also copy selected coordinates to the clipboard.
20.5 DB Manager Plugin
The DB Manager Plugin is officially part of the QGIS core and is intended to be the main tool to integrate and
manage spatial database formats supported by QGIS (PostGIS, SpatiaLite, GeoPackage, Oracle Spatial, Virtual
DB Manager
layers) in one user interface. The
Plugin provides several features. You can drag layers from the
QGIS Browser into the DB Manager, and it will import your layer into your spatial database. You can drag and
drop tables between spatial databases and they will get imported.
Fig. 20.10: DB Manager dialog
The Database menu allows you to connect to an existing database, to start the SQL window and to exit the DB
Manager Plugin. Once you are connected to an existing database, the menus Schema and Table additionally
appear.
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The Schema menu includes tools to create and delete (empty) schemas and, if topology is available (e.g., PostGIS
2), to start a TopoViewer.
The Table menu allows you to create and edit tables and to delete tables and views. It is also possible to empty
tables and to move tables from one schema to another. As further functionality, you can perform a VACUUM
and then an ANALYZE for each selected table. Plain VACUUM simply reclaims space and makes it available
for reuse. ANALYZE updates statistics to determine the most efficient way to execute a query. Finally, you can
import layers/files, if they are loaded in QGIS or exist in the file system. And you can export database tables to
shape with the Export File feature.
The Tree window lists all existing databases supported by QGIS. With a double-click, you can connect to the
database. With the right mouse button, you can rename and delete existing schemas and tables. Tables can also be
added to the QGIS canvas with the context menu.
If connected to a database, the main window of the DB Manager offers three tabs. The Info tab provides information about the table and its geometry, as well as about existing fields, constraints and indexes. It also allows you
to run Vacuum Analyze and to create a spatial index on a selected table, if not already done. The Table tab shows
all attributes, and the Preview tab renders the geometries as preview.
20.5.1 Working with the SQL Window
You can also use the DB Manager to execute SQL queries against your spatial database and then view the spatial
output for queries by adding the results to QGIS as a query layer. It is possible to highlight a portion of the SQL
and only that portion will be executed when you press F5 or click the Execute (F5) button.
Fig. 20.11: Executing SQL queries in the DB Manager SQL window
Note: The SQL Window can also be used to create Virtual Layers. In that case, instead of selecting a database,
select QGIS Layers under Virtual Layers before opening the SQL Window. See Creating virtual layers for
instructions on the SQL syntax to use.
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20.6 eVis Plugin
(This section is derived from Horning, N., K. Koy, P. Ersts. 2009. eVis (v1.1.0) User’s Guide. American Museum
of Natural History, Center for Biodiversity and Conservation. Available from http://biodiversityinformatics.amnh.
org/, and released under the GNU FDL.)
The Biodiversity Informatics Facility at the American Museum of Natural History’s (AMNH) Center for Biodiversity and Conservation (CBC) has developed the Event Visualization Tool (eVis), another software tool to add
to the suite of conservation monitoring and decision support tools for guiding protected area and landscape planning. This plugin enables users to easily link geocoded (i.e., referenced with latitude and longitude or X and Y
coordinates) photographs, and other supporting documents, to vector data in QGIS.
eVis is now automatically installed and enabled in new versions of QGIS, and as with all plugins, it can be disabled
and enabled using the Plugin Manager (see The Plugins Dialog).
The eVis plugin is made up of three modules: the ‘Database Connection tool’, ‘Event ID tool’, and the ‘Event
Browser’. These work together to allow viewing of geocoded photographs and other documents that are linked to
features stored in vector files, databases, or spreadsheets.
20.6.1 Event Browser
The Event Browser module provides the functionality to display geocoded photographs that are linked to vector
features displayed in the QGIS map window. Point data, for example, can be from a vector file that can be input
using QGIS or it can be from the result of a database query. The vector feature must have attribute information
associated with it to describe the location and name of the file containing the photograph and, optionally, the
compass direction the camera was pointed when the image was acquired. Your vector layer must be loaded into
QGIS before running the Event Browser.
Launch the Event Browser module
To launch the Event Browser module, click on Database → eVis → eVis Event Browser. This will open the
Generic Event Browser window.
The Event Browser window has three tabs displayed at the top of the window. The Display tab is used to view the
photograph and its associated attribute data. The Options tab provides a number of settings that can be adjusted to
control the behavior of the eVis plugin. Lastly, the Configure External Applications tab is used to maintain a table
of file extensions and their associated application to allow eVis to display documents other than images.
Understanding the Display window
To see the Display window, click on the Display tab in the Event Browser window. The Display window is used
to view geocoded photographs and their associated attribute data.
1. Display window: A window where the photograph will appear.
2. Zoom in button: Zoom in to see more detail. If the entire image cannot be displayed in the display window,
scroll bars will appear on the left and bottom sides of the window to allow you to pan around the image.
3. Zoom out button: Zoom out to see more area.
4. Zoom to full extent button: Displays the full extent of the photograph.
5. Attribute information window: All of the attribute information for the point associated with the photograph being viewed is displayed here. If the file type being referenced in the displayed record is not an
image but is of a file type defined in the Configure External Applications tab, then when you double-click
on the value of the field containing the path to the file, the application to open the file will be launched to
view or hear the contents of the file. If the file extension is recognized, the attribute data will be displayed
in green.
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Fig. 20.12: The eVis display window
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6. Navigation buttons: Use the Previous and Next buttons to load the previous or next feature when more
than one feature is selected.
Understanding the Options window
Fig. 20.13: The eVis Options window
1. File path: A drop-down list to specify the attribute field that contains the directory path or URL for the
photographs or other documents being displayed. If the location is a relative path, then the checkbox must
be clicked. The base path for a relative path can be entered in the Base Path text box below. Information
about the different options for specifying the file location are noted in the section Specifying the location
and name of a photograph below.
2. Compass bearing: A drop-down list to specify the attribute field that contains the compass bearing associated with the photograph being displayed. If compass bearing information is available, it is necessary to
click the checkbox below the drop-down menu title.
3. Compass offset: Compass offsets can be used to compensate for declination (to adjust bearings collected
using magnetic bearings to true north bearings). Click the
Manual radio button to enter the offset in
the text box or click the
From Attribute radio button to select the attribute field containing the offsets.
For both of these options, east declinations should be entered using positive values, and west declinations
should use negative values.
4. Directory base path: The base path onto which the relative path defined in Figure_eVis_options (A) will
be appended.
5. Replace path: If this checkbox is checked, only the file name from A will be appended to the base path.
6. Apply rule to all documents: If checked, the same path rules that are defined for photographs will be used
for non-image documents such as movies, text documents, and sound files. If not checked, the path rules
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will only apply to photographs, and other documents will ignore the base path parameter.
7. Remember settings: If the checkbox is checked, the values for the associated parameters will be saved for
the next session when the window is closed or when the [Save] button below is pressed.
8. Reset values: Resets the values on this line to the default setting.
9. Restore defaults: This will reset all of the fields to their default settings. It has the same effect as clicking
all of the [Reset] buttons.
10. Save: This will save the settings without closing the Options pane.
Understanding the Configure External Applications window
Fig. 20.14: The eVis External Applications window
1. File reference table: A table containing file types that can be opened using eVis. Each file type needs a
file extension and the path to an application that can open that type of file. This provides the capability of
opening a broad range of files such as movies, sound recordings, and text documents instead of only images.
2. Add new file type: Add a new file type with a unique extension and the path for the application that can
open the file.
3. Delete current row: Delete the file type highlighted in the table and defined by a file extension and a path
to an associated application.
20.6.2 Specifying the location and name of a photograph
The location and name of the photograph can be stored using an absolute or relative path, or a URL if the photograph is available on a web server. Examples of the different approaches are listed in Table evis_examples.
X
780596
780596
780819
Y
1784017
1784017
1784015
780596
1784017
FILE
C:\Workshop\eVis_Data\groundphotos\DSC_0168.JPG
/groundphotos/DSC_0169.JPG
http://biodiversityinformatics.amnh.org/\
evis_testdata/DSC_0170.JPG
pdf:http://www.testsite.com/attachments.php?\
attachment_id-12
BEARING
275
80
10
76
20.6.3 Specifying the location and name of other supporting documents
Supporting documents such as text documents, videos, and sound clips can also be displayed or played by eVis. To
do this, it is necessary to add an entry in the file reference table that can be accessed from the Configure External
Applications window in the Generic Event Browser that matches the file extension to an application that can be
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used to open the file. It is also necessary to have the path or URL to the file in the attribute table for the vector
layer. One additional rule that can be used for URLs that don’t contain a file extension for the document you want
to open is to specify the file extension before the URL. The format is — file extension:URL. The URL
is preceded by the file extension and a colon; this is particularly useful for accessing documents from wikis and
other web sites that use a database to manage the web pages (see Table evis_examples).
20.6.4 Using the Event Browser
When the Event Browser window opens, a photograph will appear in the display window if the document referenced in the vector file attribute table is an image and if the file location information in the Options window is
properly set. If a photograph is expected and it does not appear, it will be necessary to adjust the parameters in the
Options window.
If a supporting document (or an image that does not have a file extension recognized by eVis) is referenced in the
attribute table, the field containing the file path will be highlighted in green in the attribute information window if
that file extension is defined in the file reference table located in the Configure External Applications window. To
open the document, double-click on the green-highlighted line in the attribute information window. If a supporting
document is referenced in the attribute information window and the file path is not highlighted in green, then it
will be necessary to add an entry for the file’s filename extension in the Configure External Applications window.
If the file path is highlighted in green but does not open when double-clicked, it will be necessary to adjust the
parameters in the Options window so the file can be located by eVis.
If no compass bearing is provided in the Options window, a red asterisk will be displayed on top of the vector
feature that is associated with the photograph being displayed. If a compass bearing is provided, then an arrow will
appear pointing in the direction indicated by the value in the compass bearing display field in the Event Browser
window. The arrow will be centered over the point that is associated with the photograph or other document.
To close the Event Browser window, click on the [Close] button from the Display window.
20.6.5 Event ID Tool
The ‘Event ID’ module allows you to display a photograph by clicking on a feature displayed in the QGIS map
window. The vector feature must have attribute information associated with it to describe the location and name of
the file containing the photograph and, optionally, the compass direction the camera was pointed when the image
was acquired. This layer must be loaded into QGIS before running the ‘Event ID’ tool.
Launch the Event ID module
Event ID
icon or click on Database → eVis → Event ID
To launch the ‘Event ID’ module, either click on the
Tool. This will cause the cursor to change to an arrow with an ‘i’ on top of it signifying that the ID tool is active.
To view the photographs linked to vector features in the active vector layer displayed in the QGIS map window,
move the Event ID cursor over the feature and then click the mouse. After clicking on the feature, the Event
Browser window is opened and the photographs on or near the clicked locality are available for display in the
browser. If more than one photograph is available, you can cycle through the different features using the [Previous] and [Next] buttons. The other controls are described in the Event Browser section of this guide.
20.6.6 Database connection
The ‘Database Connection’ module provides tools to connect to and query a database or other ODBC resource,
such as a spreadsheet.
eVis can directly connect to the following types of databases: PostgreSQL, MySQL, and SQLite; it can also
read from ODBC connections (e.g., MS Access). When reading from an ODBC database (such as an Excel
spreadsheet), it is necessary to configure your ODBC driver for the operating system you are using.
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Launch the Database Connection module
eVis Database Connection
or click
To launch the ‘Database Connection’ module, either click on the appropriate icon
on Database → eVis → Database Connection. This will launch the Database Connection window. The window
has three tabs: Predefined Queries, Database Connection, and SQL Query. The Output Console window at the
bottom of the window displays the status of actions initiated by the different sections of this module.
Connect to a database
Click on the Database Connection tab to open the database connection interface. Next, use the Database Type
combo box to select the type of database that you want to connect to. If a password or username is required,
that information can be entered in the Username and Password textboxes.
Enter the database host in the Database Host textbox. This option is not available if you selected ‘MS Access’ as
the database type. If the database resides on your desktop, you should enter “localhost”.
Enter the name of the database in the Database Name textbox. If you selected ‘ODBC’ as the database type, you
need to enter the data source name.
When all of the parameters are filled in, click on the [Connect] button. If the connection is successful, a message
will be written in the Output Console window stating that the connection was established. If a connection was not
established, you will need to check that the correct parameters were entered above.
Fig. 20.15: The eVis Database connection window
1. Database Type: A drop-down list to specify the type of database that will be used.
2. Database Host: The name of the database host.
3. Port: The port number if a MySQL or PostgreSQL database type is selected.
4. Database Name: The name of the database.
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5. Connect: A button to connect to the database using the parameters defined above.
6. Output Console: The console window where messages related to processing are displayed.
7. Username: Username for use when a database is password protected.
8. Password: Password for use when a database is password protected.
9. Predefined Queries: Tab to open the “Predefined Queries” window.
10. Database Connection: Tab to open the “Database Connection” window.
11. SQL Query: Tab to open the “SQL Query” window.
12. Help: Displays the online help.
13. OK: Closes the main “Database Connection” window.
Running SQL queries
SQL queries are used to extract information from a database or ODBC resource. In eVis, the output from these
queries is a vector layer added to the QGIS map window. Click on the SQL Query tab to display the SQL query
interface. SQL commands can be entered in this text window. A helpful tutorial on SQL commands is available at
http://www.w3schools.com/sql. For example, to extract all of the data from a worksheet in an Excel file, select
* from [sheet1$] where sheet1 is the name of the worksheet.
Click on the [Run Query] button to execute the command. If the query is successful, a Database File Selection
window will be displayed. If the query is not successful, an error message will appear in the Output Console
window.
In the Database File Selection window, enter the name of the layer that will be created from the results of the
query in the Name of New Layer textbox.
Fig. 20.16: The eVis SQL query tab
1. SQL Query Text Window: A screen to type SQL queries.
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2. Run Query: Button to execute the query entered in the SQL Query Window.
3. Console Window: The console window where messages related to processing are displayed.
4. Help: Displays the online help.
5. OK: Closes the main Database Connection window.
and Y Coordinate
combo boxes to select the fields from the database that
Use the X Coordinate
stores the X (or longitude) and Y (or latitude) coordinates. Clicking on the [OK] button causes the vector layer
created from the SQL query to be displayed in the QGIS map window.
To save this vector file for future use, you can use the QGIS ‘Save as. . . ’ command that is accessed by rightclicking on the layer name in the QGIS map legend and then selecting ‘Save as. . . ’
Tip: Creating a vector layer from a Microsoft Excel Worksheet
When creating a vector layer from a Microsoft Excel Worksheet, you might see that unwanted zeros (“0”) have
been inserted in the attribute table rows beneath valid data. This can be caused by deleting the values for these
cells in Excel using the Backspace key. To correct this problem, you need to open the Excel file (you’ll need to
close QGIS if you are connected to the file, to allow you to edit the file) and then use Edit → Delete to remove the
blank rows from the file. To avoid this problem, you can simply delete several rows in the Excel Worksheet using
Edit → Delete before saving the file.
Running predefined queries
With predefined queries, you can select previously written queries stored in XML format in a file. This is particularly helpful if you are not familiar with SQL commands. Click on the Predefined Queries tab to display the
predefined query interface.
Open File
To load a set of predefined queries, click on the
icon. This opens the Open File window, which is used
to locate the file containing the SQL queries. When the queries are loaded, their titles as defined in the XML file
will appear in the drop-down menu located just below the
displayed in the text window under the drop-down menu.
Open File
icon. The full description of the query is
Select the query you want to run from the drop-down menu and then click on the SQL Query tab to see that the
query has been loaded into the query window. If it is the first time you are running a predefined query or are
switching databases, you need to be sure to connect to the database.
Click on the [Run Query] button in the SQL Query tab to execute the command. If the query is successful, a
Database File Selection window will be displayed. If the query is not successful, an error message will appear in
the Output Console window.
1. Open File: Launches the “Open File” file browser to search for the XML file holding the predefined queries.
2. Predefined Queries: A drop-down list with all of the queries defined by the predefined queries XML file.
3. Query description: A short description of the query. This description is from the predefined queries XML
file.
4. Console Window: The console window where messages related to processing are displayed.
5. Help: Displays the online help.
6. OK: Closes the main “Database Connection” window.
XML format for eVis predefined queries
The XML tags read by eVis
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Fig. 20.17: The eVis Predefined Queries tab
Tag
Description
query
Defines the beginning and end of a query statement.
shortdeA short description of the query that appears in the eVis drop-down menu.
scription
descripA more detailed description of the query displayed in the Predefined Query text window.
tion
database- The database type, defined in the Database Type drop-down menu in the Database Connection tab.
type
database- The port as defined in the Port text box in the Database Connection tab.
port
database- The database name as defined in the Database Name text box in the Database Connection tab.
name
databaseuser-The database username as defined in the Username text box in the Database Connection tab.
name
databasep- The database password as defined in the Password text box in the Database Connection tab.
assword
sqlstateThe SQL command.
ment
autocon- A flag (“true”” or “false”) to specify if the above tags should be used to automatically connect to
nect
the database without running the database connection routine in the Database Connection tab.
A complete sample XML file with three queries is displayed below:
<?xml version="1.0"?>
<doc>
<query>
<shortdescription>Import all photograph points</shortdescription>
<description>This command will import all of the data in the SQLite database to
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</description>
<databasetype>SQLITE</databasetype>
<databasehost />
<databaseport />
<databasename>C:\textbackslash Workshop/textbackslash
eVis\_Data\textbackslash PhotoPoints.db</databasename>
<databaseusername />
<databasepassword />
<sqlstatement>SELECT Attributes.*, Points.x, Points.y FROM Attributes LEFT JOIN
Points ON Points.rec_id=Attributes.point_ID</sqlstatement>
<autoconnect>false</autoconnect>
</query>
<query>
<shortdescription>Import photograph points "looking across Valley"</
˓→shortdescription>
<description>This command will import only points that have photographs
˓→"looking across
a valley" to QGIS</description>
<databasetype>SQLITE</databasetype>
<databasehost />
<databaseport />
<databasename>C:\Workshop\eVis_Data\PhotoPoints.db</databasename>
<databaseusername />
<databasepassword />
<sqlstatement>SELECT Attributes.*, Points.x, Points.y FROM Attributes LEFT JOIN
Points ON Points.rec_id=Attributes.point_ID where COMMENTS='Looking across
valley'</sqlstatement>
<autoconnect>false</autoconnect>
</query>
<query>
<shortdescription>Import photograph points that mention "limestone"</
˓→shortdescription>
<description>This command will import only points that have photographs that
˓→mention
"limestone" to QGIS</description>
<databasetype>SQLITE</databasetype>
<databasehost />
<databaseport />
<databasename>C:\Workshop\eVis_Data\PhotoPoints.db</databasename>
<databaseusername />
<databasepassword />
<sqlstatement>SELECT Attributes.*, Points.x, Points.y FROM Attributes LEFT JOIN
Points ON Points.rec_id=Attributes.point_ID where COMMENTS like '%limestone%'
</sqlstatement>
<autoconnect>false</autoconnect>
</query>
</doc>
20.7 GDAL Tools Plugin
20.7.1 What is GDAL Tools?
The GDAL Tools plugin offers a GUI to the collection of tools in the Geospatial Data Abstraction Library, http:
//gdal.osgeo.org . These are raster management tools to query, re-project, warp and merge a wide variety of raster
formats. Also included are tools to create a contour (vector) layer, or a shaded relief from a raster DEM, and to
make a VRT (Virtual Raster Tile in XML format) from a collection of one or more raster files. These tools are
available when the plugin is installed and activated.
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The GDAL Library
The GDAL library consists of a set of command line programs, each with a large list of options. Users comfortable
with running commands from a terminal may prefer the command line, with access to the full set of options. The
GDAL Tools plugin offers an easy interface to the tools, exposing only the most popular options.
20.7.2 List of GDAL tools
Fig. 20.18: The GDALTools menu list
Projections
Warp
(Reproject)
Assign
projection
Extract
projection
This utility is an image mosaicing, reprojection and warping utility. The program can reproject to
any supported projection, and can also apply GCPs stored with the image if the image is “raw” with
control information. For more information, you can read on the GDAL website http://www.gdal.org/
gdalwarp.html.
This tool allows you to assign projection to rasters that are already georeferenced but miss projection
information. Also with its help, it is possible to alter existing projection definitions. Both single file
and batch mode are supported. For more information, please visit the utility page at the GDAL site,
http://www.gdal.org/gdalwarp.html.
This utility helps you to extract projection information from an input file. If you want to extract
projection information from a whole directory, you can use the batch mode. It creates both .prj
and .wld files.
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Conversion
Rasterize
This program burns vector geometries (points, lines and polygons) into the raster band(s) of a raster
image. Vectors are read from OGR-supported vector formats. Note that the vector data must in the
same coordinate system as the raster data; on the fly reprojection is not provided. For more information
see http://www.gdal.org/gdal_rasterize.html.
This utility creates vector polygons for all connected regions of pixels in the raster sharing a common
Poly- pixel value. Each polygon is created with an attribute indicating the pixel value of that polygon. The
utility will create the output vector datasource if it does not already exist, defaulting to ESRI shapefile
gonize format. See also http://www.gdal.org/gdal_polygonize.html.
This utility can be used to convert raster data between different formats, potentially performing some
Trans- operations like subsetting, resampling, and rescaling pixels in the process. For more information you
late
can read on http://www.gdal.org/gdal_translate.html.
RGB
to
PCT
PCT
to
RGB
This utility will compute an optimal pseudocolor table for a given RGB image using a median cut
algorithm on a downsampled RGB histogram. Then it converts the image into a pseudocolored image
using the color table. This conversion utilizes Floyd-Steinberg dithering (error diffusion) to maximize
output image visual quality. The utility is also described at http://www.gdal.org/rgb2pct.html.
This utility will convert a pseudocolor band on the input file into an output RGB file of the desired
format. For more information, see http://www.gdal.org/pct2rgb.html.
Extraction
Contour
Clipper
450
This program generates a vector contour file from the input raster elevation model (DEM). On http:
//www.gdal.org/gdal_contour.html, you can find more information.
This utility allows you to clip (extract subset) rasters using selected extent or based on mask layer
bounds. More information can be found at http://www.gdal.org/gdal_translate.html.
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Analysis
This utility removes raster polygons smaller than a provided threshold size (in pixels) and replaces
them with the pixel value of the largest neighbor polygon. The result can be written back to the
existing raster band, or copied into a new file. For more information, see http://www.gdal.org/gdal_
sieve.html.
Sieve
This utility will scan an image and try to set all pixels that are nearly black (or nearly white) around
the edge to exactly black (or white). This is often used to “fix up” lossy compressed aerial photos
so that color pixels can be treated as transparent when mosaicing. See also http://www.gdal.org/
nearblack.html.
Near
Black
Fill
nodata
This utility fills selected raster regions (usually nodata areas) by interpolation from valid pixels
around the edges of the areas. On http://www.gdal.org/gdal_fillnodata.html, you can find more
information.
Proximity
This utility generates a raster proximity map indicating the distance from the center of each pixel
to the center of the nearest pixel identified as a target pixel. Target pixels are those in the source
raster for which the raster pixel value is in the set of target pixel values. For more information see
http://www.gdal.org/gdal_proximity.html.
Grid
(Interpolation)
DEM
(Terrain
models)
This utility creates a regular grid (raster) from the scattered data read from the OGR datasource.
Input data will be interpolated to fill grid nodes with values, and you can choose from various
interpolation methods. The utility is also described on the GDAL website, http://www.gdal.org/
gdal_grid.html.
Tools to analyze and visualize DEMs. It can create a shaded relief, a slope, an aspect, a color
relief, a Terrain Ruggedness Index, a Topographic Position Index and a roughness map from any
GDAL-supported elevation raster. For more information, see http://www.gdal.org/gdaldem.html.
Miscellaneous
Build
Virtual
Raster
(Catalog)
Merge
Information
This program builds a VRT (Virtual Dataset) that is a mosaic of the list of input GDAL datasets.
See also http://www.gdal.org/gdalbuildvrt.html.
This utility will automatically mosaic a set of images. All the images must be in the same coordinate system and have a matching number of bands, but they may be overlapping, and at different
resolutions. In areas of overlap, the last image will be copied over earlier ones. The utility is also
described at http://www.gdal.org/gdal_merge.html.
This utility lists various information about a GDAL-supported raster dataset. On http://www.gdal.
org/gdalinfo.html, you can find more information.
Build The gdaladdo utility can be used to build or rebuild overview images for most supported file
Overviews formats with one of several downsampling algorithms. For more information, see http://www.
gdal.org/gdaladdo.html.
Tile
Index
This utility builds a shapefile with a record for each input raster file, an attribute containing the
filename, and a polygon geometry outlining the raster. See also http://www.gdal.org/gdaltindex.
html.
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GDAL Tools Settings
Use this dialog to embed your GDAL variables.
20.8 Geometry Checker Plugin
Geometry Checker is a powerful core plugin to check and fix the geometry validity of a layer. The Geometry
Checker dialog show differents grouped settings in the first tab (Settings):
• Input vector layer: to select the layer to check. A
to the one selected.
Only selected features checkbox can filter the geometry
• Geometry validity: give to the user the choice between Self intersections, Duplicate nodes, Polygon with
less than 3 nodes.
• Allowed geometry types: to allow only some geometry types like point, multipoint, line, multiline, polygon
and multipolygon.
• Geometry properties displays
Polygons and multipolygons may not contain any holes and
objects must consist of more than one part.
Multipart
• Geometry conditions: user can add some condition to validate the geometries with a minimal segment
length, a minimum angle between segment, a minimal polygon area and sliver polygons detection.
• Topology checks: checks for duplicates, for features within other features, overlaps smaller than a number,
for gaps smaller than a number.
• Tolerance: you can define here the tolerance for the check.
• Output vector layer gives the choice to the user how get the result between modifiy the current layer and
create a new layer.
After you are happy with the configuration, you can click on the [Run] button.
The results appear in the second tab and as an overview layer of the errors in the canvas (its name is checker). A
table list the geometry check result with one error by row: the first row is an ID, the second the reason of the error,
then the coordinates of the error, a value (depending on the type of the error) and finally the resolution column
which indicates the resolution of the error. At the bottom of this table, you can export the error into a shapefile.
At the left, you have the number of the errors and the fixed errors.
The Geometry Checker Plugin can find the following errors:
• Self intersections: a polygon with a self intersection,
• Duplicate nodes: two duplicates nodes in a segment
• Holes: hole in a polygon,
• Segment length: a segment length lower than a threshold,
• Minimum angle: two segments with an angle lower than a threshold,
• Minimum area: polygon area lower than a treshold,
• Silver polygon: this error come from very small polygon (with small area) with a large perimeter,
• Duplicates features,
• Feature within feature,
• Overlaps: polygon overlapping,
• Gaps: gaps between polygons
The following figure shows the different checks made by the plugin.
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Fig. 20.19: The Geometry Checker Plugin
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Fig. 20.20: The Differents checks supported by the plugin
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You can select a row to see the localisation of the error. You can change this behaviour by selecting another action
between error (default), Feature, Don’t move, and
Highlight contour of selected features.
Below the zoom action when clicking on the table row, you can Show the selected features in attribute table, Fix
selected errors using default resolution and Fix selected errors, prompt for resolution method. In the latter, you
will see a window to choose the resolution’s method among which:
• Merge with neighboring polygon with longest shared edge,
• Merge with neighboring polygon with largest area,
• Merge with neighboring polygon identical attribute value,if any, or leave as it
• Delete feature
• No action
Tip: Fix multiple errors
You can fix multiple errors by selecting more than one row in the table with the CTRL + click action.
The default action could be changed with the last icon Error resolution settings. For some type of errors, you can
change the default action between some specific action or No action.
Finally, you can choose which attribute to use when merging features by attribute value.
20.9 Georeferencer Plugin
The Georeferencer Plugin is a tool for generating world files for rasters. It allows you to reference rasters to
geographic or projected coordinate systems by creating a new GeoTiff or by adding a world file to the existing
image. The basic approach to georeferencing a raster is to locate points on the raster for which you can accurately
determine coordinates.
Features
Icon
Purpose
Icon
Purpose
Open raster
Start georeferencing
Generate GDAL Script
Load GCP Points
Save GCP Points As
Transformation settings
Add Point
Delete Point
Move GCP Point
Pan
Zoom In
Zoom Out
Zoom To Layer
Zoom Last
Zoom Next
Link Georeferencer to QGIS
Link QGIS to Georeferencer
Full histogram stretch
Local histogram stretch
Table Georeferencer: Georeferencer Tools
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20.9.1 Usual procedure
As X and Y coordinates (DMS (dd mm ss.ss), DD (dd.dd) or projected coordinates (mmmm.mm)), which correspond with the selected point on the image, two alternative procedures can be used:
• The raster itself sometimes provides crosses with coordinates “written” on the image. In this case, you can
enter the coordinates manually.
• Using already georeferenced layers. This can be either vector or raster data that contain the same objects/features that you have on the image that you want to georeference and with the projection that you
want for your image. In this case, you can enter the coordinates by clicking on the reference dataset loaded
in the QGIS map canvas.
The usual procedure for georeferencing an image involves selecting multiple points on the raster, specifying their
coordinates, and choosing a relevant transformation type. Based on the input parameters and data, the plugin will
compute the world file parameters. The more coordinates you provide, the better the result will be.
The first step is to start QGIS, load the Georeferencer Plugin (see The Plugins Dialog) and click on Raster →
Georeferencer , which appears in the QGIS menu bar. The Georeferencer Plugin dialog appears as shown in
figure_georeferencer_dialog.
For this example, we are using a topo sheet of South Dakota from SDGS. It can later be visualized together with
the data from the GRASS spearfish60 location. You can download the topo sheet here: http://grass.osgeo.
org/sampledata/spearfish_toposheet.tar.gz.
Fig. 20.21: Georeferencer Plugin Dialog
Entering ground control points (GCPs)
1. To start georeferencing an unreferenced raster, we must load it using the
button. The raster will show
up in the main working area of the dialog. Once the raster is loaded, we can start to enter reference points.
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Add Point
2. Using the
button, add points to the main working area and enter their coordinates (see Figure
figure_georeferencer_add_points). For this procedure you have three options:
• Click on a point in the raster image and enter the X and Y coordinates manually.
From map canvas
button to add the X and Y
• Click on a point in the raster image and choose the
coordinates with the help of a georeferenced map already loaded in the QGIS map canvas.
• With the
button, you can move the GCPs in both windows, if they are at the wrong place.
3. Continue entering points. You should have at least four points, and the more coordinates you can provide,
the better the result will be. There are additional tools on the plugin dialog to zoom and pan the working
area in order to locate a relevant set of GCP points.
Fig. 20.22: Add points to the raster image
The points that are added to the map will be stored in a separate text file ([filename].points) usually
together with the raster image. This allows us to reopen the Georeferencer plugin at a later date and add new
points or delete existing ones to optimize the result. The points file contains values of the form: mapX, mapY,
pixelX, pixelY. You can use the
Load GCP points
and
Save GCP points as
buttons to manage the files.
Defining the transformation settings
After you have added your GCPs to the raster image, you need to define the transformation settings for the
georeferencing process.
Available Transformation algorithms
Depending on how many ground control points you have captured, you may want to use different transformation
algorithms. Choice of transformation algorithm is also dependent on the type and quality of input data and the
amount of geometric distortion that you are willing to introduce to the final result.
Currently, the following Transformation types are available:
• The Linear algorithm is used to create a world file and is different from the other algorithms, as it does
not actually transform the raster. This algorithm likely won’t be sufficient if you are dealing with scanned
material.
• The Helmert transformation performs simple scaling and rotation transformations.
• The Polynomial algorithms 1-3 are among the most widely used algorithms introduced to match source
and destination ground control points. The most widely used polynomial algorithm is the second-order
polynomial transformation, which allows some curvature. First-order polynomial transformation (affine)
preserves collinearity and allows scaling, translation and rotation only.
• The Thin Plate Spline (TPS) algorithm is a more modern georeferencing method, which is able to introduce local deformations in the data. This algorithm is useful when very low quality originals are being
georeferenced.
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Fig. 20.23: Defining the georeferencer transformation settings
• The Projective transformation is a linear rotation and translation of coordinates.
Define the Resampling method
The type of resampling you choose will likely depending on your input data and the ultimate objective of the
exercise. If you don’t want to change statistics of the image, you might want to choose ‘Nearest neighbour’,
whereas a ‘Cubic resampling’ will likely provide a more smoothed result.
It is possible to choose between five different resampling methods:
1. Nearest neighbour
2. Linear
3. Cubic
4. Cubic Spline
5. Lanczos
Define the transformation settings
There are several options that need to be defined for the georeferenced output raster.
• The
Create world file checkbox is only available if you decide to use the linear transformation type,
because this means that the raster image actually won’t be transformed. In this case, the Output raster field
is not activated, because only a new world file will be created.
• For all other transformation types, you have to define an Output raster. As default, a new file ([filename]_modified) will be created in the same folder together with the original raster image.
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• As a next step, you have to define the Target SRS (Spatial Reference System) for the georeferenced raster
(see Working with Projections).
• If you like, you can generate a pdf map and also a pdf report. The report includes information about the
used transformation parameters, an image of the residuals and a list with all GCPs and their RMS errors.
Set Target Resolution checkbox and define the pixel resolution of the
• Furthermore, you can activate the
output raster. Default horizontal and vertical resolution is 1.
• The
Use 0 for transparency when needed can be activated, if pixels with the value 0 shall be visualized
transparent. In our example toposheet, all white areas would be transparent.
• Finally,
Load in QGIS when done loads the output raster automatically into the QGIS map canvas when
the transformation is done.
Show and adapt raster properties
Clicking on the Raster properties option in the Settings menu opens the Layer properties dialog of the raster file
that you want to georeference.
Configure the georeferencer
• You can define whether you want to show GCP coordinates and/or IDs.
• As residual units, pixels and map units can be chosen.
• For the PDF report, a left and right margin can be defined and you can also set the paper size for the PDF
map.
• Finally, you can activate to
Show Georeferencer window docked.
Running the transformation
After all GCPs have been collected and all transformation settings are defined, just press the
button to create the new georeferenced raster.
Start georeferencing
20.10 MetaSearch Catalog Client
20.10.1 Introduction
MetaSearch is a QGIS plugin to interact with metadata catalog services, supporting the OGC Catalog Service for
the Web (CSW) standard.
MetaSearch provides an easy and intuitive approach and user-friendly interface to searching metadata catalogs
within QGIS.
20.10.2 Installation
MetaSearch is included by default with QGIS 2.0 and higher. All dependencies are included within MetaSearch.
Install MetaSearch from the QGIS plugin manager, or manually from http://plugins.qgis.org/plugins/MetaSearch.
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Fig. 20.24: Search and results of Services in Metasearch
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20.10.3 Working with Metadata Catalogs in QGIS
CSW (Catalog Service for the Web)
CSW (Catalog Service for the Web) is an OGC (Open Geospatial Consortium) specification, that defines common
interfaces to discover, browse and query metadata about data, services, and other potential resources.
Startup
icon or select Web → MetaSearch → MetaSearch via the QGIS main menu. The
To start MetaSearch, click
MetaSearch dialog will appear. The main GUI consists of three tabs: Services, Search and Settings.
Managing Catalog Services
Fig. 20.25: Managing Catalog Services
The Services tab allows the user to manage all available catalog services. MetaSearch provides a default list of
Catalog Services, which can be added by pressing [Add default services] button.
To all listed Catalog Service entries, click the dropdown select box.
To add a Catalog Service entry, click the [New] button, and enter a Name for the service, as well as the URL
(endpoint). Note that only the base URL is required (not a full GetCapabilities URL). If the CSW requires
authentication, enter the appropriate User name and Password credentials. Clicking [OK] will add the service to
the list of entries.
To edit an existing Catalog Service entry, select the entry you would like to edit and click the [Edit] button, and
modify the Name or URL values, then click [OK].
To delete a Catalog Service entry, select the entry you would like to delete and click the [Delete] button. You will
be asked to confirm deleting the entry.
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MetaSearch allows for loading and saving connections to an XML file. This is useful when you need to share
settings between applications. Below is an example of the XML file format.
<?xml version="1.0" encoding="UTF-8"?>
<qgsCSWConnections version="1.0">
<csw name="Data.gov CSW" url="https://catalog.data.gov/csw-all"/>
<csw name="Geonorge - National CSW service for Norway" url="http://www.
˓→geonorge.no/geonetwork/srv/eng/csw"/>
<csw name="Geoportale Nazionale - Servizio di ricerca Italiano" url="http://
˓→www.pcn.minambiente.it/geoportal/csw"/>
<csw name="LINZ Data Service" url="http://data.linz.govt.nz/feeds/csw"/>
<csw name="Nationaal Georegister (Nederland)" url="http://www.
˓→nationaalgeoregister.nl/geonetwork/srv/eng/csw"/>
<csw name="RNDT - Repertorio Nazionale dei Dati Territoriali - Servizio di
˓→ricerca" url="http://www.rndt.gov.it/RNDT/CSW"/>
<csw name="UK Location Catalogue Publishing Service" url="http://csw.data.gov.
˓→uk/geonetwork/srv/en/csw"/>
<csw name="UNEP/GRID-Geneva Metadata Catalog" url="http://metadata.grid.unep.
˓→ch:8080/geonetwork/srv/eng/csw"/>
</qgsCSWConnections>
To load a list of entries, click the [Load] button. A new window will appear; click the [Browse] button and
navigate to the XML file of entries you wish to load and click [Open]. The list of entries will be displayed. Select
the entries you wish to add from the list and click [Load].
Click the [Service info] button to displays information about the selected Catalog Service such as service identification, service provider and contact information. If you would like to view the raw XML response, click the
[GetCapabilities response] button. A separate window will open displaying Capabilities XML.
Searching Catalog Services
The Search tab allows the user to query Catalog Services for data and services, set various search parameters and
view results.
The following search parameters are available:
• Keywords: free text search keywords;
• From: the Catalog Service to perform the query against;
• Bounding box: the spatial area of interest to filter on defined by Xmax, Xmin, Ymax, and Ymin. Click [Set
global] to do a global search, click [Map extent] to do a search on the visible area only or manually enter
custom values as desired.
Clicking the [Search] button will search the selected Metadata Catalog. Search results are displayed in a list and
are sortable by clicking on the column title. You can navigate through search results with the directional buttons
below the search results. Clicking the [View search results as XML] button opens a window with the service
response in raw XML format.
Clicking a result will provides the following options:
• if the metadata record has an associated bounding box, a footprint of the bounding box will be displayed on
the map;
• double-clicking the record displays the record metadata with any associated access links. Clicking the links
opens the link in the user’s web browser;
• if the record is an OGC web service (WMS/WMTS, WFS, WCS), the appropriate [Add to
WMS/WMTS|WFS|WCS] buttons will be enabled for the user to add to QGIS. When clicking this button, MetaSearch will verify if this is a valid OWS. The OWS will then be added to the appropriate QGIS
connection list, and the appropriate WMS/WMTS|WFS|WCS connection dialog will then appear.
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Fig. 20.26: Searching catalog services
Fig. 20.27: Metadata record display
20.10. MetaSearch Catalog Client
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Fig. 20.28: Metasearch setting
Settings
You can fine tune MetaSearch with the following settings:
• Connection naming: when adding an OWS connection (WMS/WMTS|WFS|WCS), the connection is stored
with the various QGIS layer provider. Use this setting to set whether to use the name provided from
MetaSearch, whether to overwrite or to use a temporary name;
• Results paging: when searching metadata catalogs, the number of results to show per page. Default value is
10;
• Timeout: when searching metadata catalogs, the number of seconds for blocking connection attempt. Default value is 10.
20.11 Offline Editing Plugin
For data collection, it is a common situation to work with a laptop or a cell phone offline in the field. Upon returning to the network, the changes need to be synchronized with the master datasource (e.g., a PostGIS database). If
several persons are working simultaneously on the same datasets, it is difficult to merge the edits by hand, even if
people don’t change the same features.
Offline Editing
Plugin automates the synchronisation by copying the content of a datasource (usually PostGIS
The
or WFS-T) to a SpatiaLite database and storing the offline edits to dedicated tables. After being connected to the
network again, it is possible to apply the offline edits to the master dataset.
20.11.1 Using the plugin
• Open a project with some vector layers (e.g., from a PostGIS or WFS-T datasource).
• Go to Database → Offline Editing →
Convert to offline project and select the layers to save. The content
of the layers is saved to SpatiaLite tables.
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• You can check
Only synchronize selected features if a selection is present allowing the offline editing to
only save and work on a subset. It can be invaluable in case of large layers.
• Edit the layers offline.
• After being connected again, upload the changes using Database → Offline Editing →
Synchronize.
Fig. 20.29: Create an offline project from PostGIS or WFS layers
20.12 Topology Checker Plugin
Topology describes the relationships between points, lines and polygons that represent the features of a geographic
region. With the Topology Checker plugin, you can look over your vector files and check the topology with several
topology rules. These rules check with spatial relations whether your features ‘Equal’, ‘Contain’, ‘Cover’, are
‘CoveredBy’, ‘Cross’, are ‘Disjoint’, ‘Intersect’, ‘Overlap’, ‘Touch’ or are ‘Within’ each other. It depends on
your individual questions which topology rules you apply to your vector data (e.g., normally you won’t accept
overshoots in line layers, but if they depict dead-end streets you won’t remove them from your vector layer).
QGIS has a built-in topological editing feature, which is great for creating new features without errors. But existing
data errors and user-induced errors are hard to find. This plugin helps you find such errors through a list of rules.
It is very simple to create topology rules with the Topology Checker plugin.
On point layers the following rules are available:
• Must be covered by: Here you can choose a vector layer from your project. Points that aren’t covered by
the given vector layer occur in the ‘Error’ field.
• Must be covered by endpoints of: Here you can choose a line layer from your project.
• Must be inside: Here you can choose a polygon layer from your project. The points must be inside a
polygon. Otherwise, QGIS writes an ‘Error’ for the point.
• Must not have duplicates: Whenever a point is represented twice or more, it will occur in the ‘Error’ field.
• Must not have invalid geometries: Checks whether the geometries are valid.
• Must not have multi-part-geometries: All multi-part points are written into the ‘Error’ field.
On line layers, the following rules are available:
• End points must be covered by: Here you can select a point layer from your project.
20.12. Topology Checker Plugin
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Fig. 20.30: The Topology Checker Plugin
• Must not have dangles: This will show the overshoots in the line layer.
• Must not have duplicates: Whenever a line feature is represented twice or more, it will occur in the ‘Error’
field.
• Must not have invalid geometries: Checks whether the geometries are valid.
• Must not have multi-part geometries: Sometimes, a geometry is actually a collection of simple (singlepart) geometries. Such a geometry is called multi-part geometry. If it contains just one type of simple
geometry, we call it multi-point, multi-linestring or multi-polygon. All multi-part lines are written into the
‘Error’ field.
• Must not have pseudos: A line geometry’s endpoint should be connected to the endpoints of two other
geometries. If the endpoint is connected to only one other geometry’s endpoint, the endpoint is called a
pseudo node.
On polygon layers, the following rules are available:
• Must contain: Polygon layer must contain at least one point geometry from the second layer.
• Must not have duplicates: Polygons from the same layer must not have identical geometries. Whenever a
polygon feature is represented twice or more it will occur in the ‘Error’ field.
• Must not have gaps: Adjacent polygons should not form gaps between them. Administrative boundaries
could be mentioned as an example (US state polygons do not have any gaps between them. . . ).
• Must not have invalid geometries: Checks whether the geometries are valid. Some of the rules that define
a valid geometry are:
– Polygon rings must close.
– Rings that define holes should be inside rings that define exterior boundaries.
– Rings may not self-intersect (they may neither touch nor cross one another).
– Rings may not touch other rings, except at a point.
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• Must not have multi-part geometries: Sometimes, a geometry is actually a collection of simple (singlepart) geometries. Such a geometry is called multi-part geometry. If it contains just one type of simple
geometry, we call it multi-point, multi-linestring or multi-polygon. For example, a country consisting of
multiple islands can be represented as a multi-polygon.
• Must not overlap: Adjacent polygons should not share common area.
• Must not overlap with: Adjacent polygons from one layer should not share common area with polygons
from another layer.
20.12. Topology Checker Plugin
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CHAPTER
21
Help and Support
21.1 Mailing lists
QGIS is under active development and as such it won’t always work like you expect it to. The preferred way to
get help is by joining the qgis-users mailing list. Your questions will reach a broader audience and answers will
benefit others.
21.1.1 QGIS Users
This mailing list is used for discussion of QGIS in general, as well as specific questions regarding its installation
and use. You can subscribe to the qgis-users mailing list by visiting the following URL: http://lists.osgeo.org/
mailman/listinfo/qgis-user
21.1.2 QGIS Developers
If you are a developer facing problems of a more technical nature, you may want to join the qgis-developer mailing
list. This list is also a place where people can chime in and collect and discuss QGIS related UX (User Experience)
/ usability issues. It’s here: http://lists.osgeo.org/mailman/listinfo/qgis-developer
21.1.3 QGIS Community Team
This list deals with topics like documentation, context help, user guide, web sites, blog, mailing lists, forums, and
translation efforts. If you would like to work on the user guide as well, this list is a good starting point to ask your
questions. You can subscribe to this list at: http://lists.osgeo.org/mailman/listinfo/qgis-community-team
21.1.4 QGIS Translations
This list deals with the translation efforts. If you like to work on the translation of the website, manuals or the
graphical user interface (GUI), this list is a good starting point to ask your questions. You can subscribe to this list
at: http://lists.osgeo.org/mailman/listinfo/qgis-tr
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21.1.5 QGIS Project Steering Committee (PSC)
This list is used to discuss Steering Committee issues related to overall management and direction of QGIS. You
can subscribe to this list at: http://lists.osgeo.org/mailman/listinfo/qgis-psc
21.1.6 QGIS User groups
In order to locally promote QGIS and contribute to its development, some QGIS communities are organized into
QGIS User Groups. These groups are places to discuss local topics, organize regional or national user meetings,
organize sponsoring of features. . . The list of current user groups is available at http://qgis.org/en/site/forusers/
usergroups.html
You are welcome to subscribe to any of the lists. Please remember to contribute to the list by answering questions
and sharing your experiences.
21.2 IRC
We also maintain a presence on IRC - visit us by joining the #qgis channel on irc.freenode.net. Please wait for a
response to your question, as many folks on the channel are doing other things and it may take a while for them to
notice your question. If you missed a discussion on IRC, not a problem! We log all discussion, so you can easily
catch up. Just go to http://qgis.org/irclogs and read the IRC-logs.
Commercial support for QGIS is also available. Check the website http://qgis.org/en/commercial-support.html for
more information.
21.3 BugTracker
While the qgis-users mailing list is useful for general ‘How do I do XYZ in QGIS?’-type questions, you may wish
to notify us about bugs in QGIS. You can submit bug reports using the QGIS bug tracker at http://hub.qgis.org/
projects/quantum-gis/issues. When creating a new ticket for a bug, please provide an email address where we can
contact you for additional information.
Please bear in mind that your bug may not always enjoy the priority you might hope for (depending on its severity).
Some bugs may require significant developer effort to remedy, and the manpower is not always available for this.
Feature requests can be submitted as well using the same ticket system as for bugs. Please make sure to select the
type Feature.
If you have found a bug and fixed it yourself, you can submit either a Pull Request on the Github QGIS Project
(prefered) or a patch also. The lovely redmine ticketsystem at http://hub.qgis.org/projects/quantum-gis/issues has
this type as well. Check the Patch supplied checkbox and attach your patch before submitting your bug.
One of the developers will review it and apply it to QGIS. Please don’t be alarmed if your patch is not applied
straight away – developers may be tied up with other commitments.
Note that if you supply a Pull Request, your change would be more likely be merged into the source code!
21.4 Blog
The QGIS community also runs a weblog at http://planet.qgis.org/planet/, which has some interesting articles for
users and developers as well provided by other blogs in the community. You are invited to contribute your own
QGIS blog!
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21.5 Plugins
The website http://plugins.qgis.org provides the official QGIS plugins web portal. Here, you find a list of all stable
and experimental QGIS plugins available via the ‘Official QGIS Plugin Repository’.
21.6 Wiki
Lastly, we maintain a WIKI web site at http://hub.qgis.org/projects/quantum-gis/wiki where you can find a variety
of useful information relating to QGIS development, release plans, links to download sites, message-translation
hints and more. Check it out, there are some goodies inside!
21.5. Plugins
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CHAPTER
22
Contributors
QGIS is an open source project developed by a team of dedicated volunteers and organisations. We strive to be
a welcoming community for people of all race, creed, gender and walks of life. At any moment, you can get
involved.
22.1 Authors
Below are listed people who dedicate their time and energy to write, review, and update the whole QGIS documentation.
Tara Athan
Peter Ersts
Werner Macho
Otto Dassau
Alex Bruy
Tim Sutton
Astrid Emde
Andy Schmid
Andy Allan
Eric Goddard
Ilkka Rinne
João Gaspar
Ko Nagase
Manel Clos
Nick Bearman
Tom Chadwin
Paolo Corti
David Adler
Jaka Kranjc
Dick Groskamp
ajazepk
Ramon
Radim Blazek
Anne Ghisla
Denis Rouzaud
Brendan Morely
Anita Graser
Larissa Junek
Yves Jacolin
Arnaud Morvan
Matthias Kuhn
Frank Sokolic
Jacob Lanstorp
Joshua Arnott
Larry Shaffer
Mattheo Ghetta
Paul Blottière
Patrick Sunter
Hugo Mercier
Vincent Mora
Tom Kralidis
Mezene Worku
icephale
embelding
K. Koy
Stephan Holl
Tyler Mitchell
David Willis
Victor Olaya
Raymond Nijssen
Alexandre Neto
Akgar Gumbira
Chris Berkhout
Luca Casagrande
Ujaval Gandhi
Thomas Gratier
Luigi Pirelli
Bernhard Ströbl
Vincent Picavet
Nyall Dawson
Gavin Macaulay
Tudor Barascu
Zoltan Siki
Alexandre Busquets
Andrei
Godofredo Contreras
N. Horning
Claudia A. Engel
Jürgen E. Fischer
Marco Hugentobler
Richard Duivenvoorde
Alessandro Pasotti
Giovanni Allegri
Carson J.Q. Farmer
Harrissou Sant-anna
Jean-Roc Morreale
Marco Bernasocchi
Konstantinos Nikolaou
Luca Manganelli
Maximilian Krambach
Milo Van der Linden
Stefan Blumentrath
QGIS Koran Translator
Sebastian Dietrich
Dominic Keller
GiordanoPezzola
Martin Dobias
Magnus Homann
Lars Luthman
Yoichi Kayama
Gary E. Sherman
Andreas Neumann
Hien Tran-Quang
Diethard Jansen
Steven Cordwell
Saber Razmjooei
Salvatore Larosa
Marie Silvestre
Maning Sambale
Nathan Woodrow
René-Luc D’Hont
Paolo Cavallini
Nicholas Duggan
Stéphane Brunner
Uros Preloznik
Andre Mano
zstadler
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22.2 Translators
QGIS is a multi-language application and as is, also publishes a documentation translated into several languages.
Many other languages are being translated and would be released as soon as they reach a reasonable percentage
of translation. If you wish to help improving a language or request a new one, please see http://qgis.org/en/site/
getinvolved/index.html.
The current translations are made possible thanks to:
Lan- Contributors
guage
Ba- Emir Hartato, I Made Anombawa, Januar V. Simarmata, Muhammad Iqnaul Haq Siregar, Trias Aditya
hasian
Indonesian
Chi- Calvin Ngei, Zhang Jun, Richard Xie
nese
(Traditional)
Dutch Carlo van Rijswijk, Dick Groskamp, Diethard Jansen, Raymond Nijssen, Richard Duivenvoorde,
Willem Hoffman
FinnishMatti Mäntynen, Kari Mikkonen
French Arnaud Morvan, Augustin Roche, Didier Vanden Berghe, Dofabien, Etienne Trimaille, Harrissou Santanna, Jean-Roc Morreale, Jérémy Garniaux, Loïc Buscoz, Lsam, Marc-André Saia, Marie Silvestre,
Mathieu Bossaert, Mathieu Lattes, Mayeul Kauffmann, Médéric Ribreux, Mehdi Semchaoui, Michael
Douchin, Nicolas Boisteault, Nicolas Rochard, Pascal Obstetar, Robin Prest, Rod Bera, Stéphane Henriod, Stéphane Possamai, sylther, Sylvain Badey, Sylvain Maillard, Vincent Picavet, Xavier Tardieu,
Yann Leveille-Menez, yoda89
Gali- Xan Vieiro
cian
Ger- Jürgen E. Fischer, Otto Dassau, Stephan Holl, Werner Macho
man
Hindi Harish Kumar Solanki
Ital- Alessandro Fanna, Anne Ghisla, Flavio Rigolon, Giuliano Curti, Luca Casagrande, Luca Delucchi,
ian
Marco Braida, Matteo Ghetta, Maurizio Napolitano, Michele Beneventi, Michele Ferretti, Roberto Angeletti, Paolo Cavallini, Stefano Campus
Japanese
Baba Yoshihiko, Minoru Akagi, Norihiro Yamate, Takayuki Mizutani, Takayuki Nuimura, Yoichi
Kayama
Ko- OSGeo Korean Chapter
rean
Pol- Andrzej Świader,
˛
Borys Jurgiel, Ewelina Krawczak, Jakub Bobrowski, Mateusz Łoskot, Michał Kułach,
ish
Michał Smoczyk, Milena Nowotarska, Radosław Pasiok, Robert Szczepanek, Tomasz Paul
Por- Alexandre Neto, Duarte Carreira, Giovanni Manghi, João Gaspar, Joana Simões, Leandro Infantini,
tugueseNelson Silva, Pedro Palheiro, Pedro Pereira, Ricardo Sena
Por- Arthur Nanni, Felipe Sodré Barros, Leônidas Descovi Filho, Marcelo Soares Souza, Narcélio de Sá
tuguesePereira Filho, Sidney Schaberle Goveia
(Brasil)
Ro- Alex Bădescu, Bogdan Pacurar, Georgiana Ioanovici, Lonut Losifescu-Enescu, Sorin Călinică, Tudor
ma- Bărăscu
nian
Rus- Alexander Bruy, Artem Popov
sian
Span- Carlos Dávila, Diana Galindo, Edwin Amado, Gabriela Awad, Javier César Aldariz, Mayeul Kauffmann
ish
Ukrainian
Alexander Bruy
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23
Appendix
23.1 GNU General Public License
Version 2, June 1991
Copyright (C) 1989, 1991 Free Software Foundation, Inc. 59 Temple Place - Suite 330, Boston, MA 02111-1307,
USA
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not
allowed.
Preamble
The licenses for most software are designed to take away your freedom to share and change it. By contrast,
the GNU General Public License is intended to guarantee your freedom to share and change free software–to
make sure the software is free for all its users. This General Public License applies to most of the Free Software
Foundation’s software and to any other program whose authors commit to using it. (Some other Free Software
Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your
programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed
to make sure that you have the freedom to distribute copies of free software (and charge for this service if you
wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of
it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to
surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the
software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients
all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must
show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you
legal permission to copy, distribute and/or modify the software.
Also, for each author’s protection and ours, we want to make certain that everyone understands that there is no
warranty for this free software. If the software is modified by someone else and passed on, we want its recipients
to know that what they have is not the original, so that any problems introduced by others will not reflect on the
original authors’ reputations.
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Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To
prevent this, we have made it clear that any patent must be licensed for everyone’s free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow. TERMS AND CONDITIONS
FOR COPYING, DISTRIBUTION AND MODIFICATION
0. This License applies to any program or other work which contains a notice placed by the copyright holder
saying it may be distributed under the terms of this General Public License. The “Program”, below, refers to
any such program or work, and a “work based on the Program” means either the Program or any derivative
work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim
or with modifications and/or translated into another language. (Hereinafter, translation is included without
limitation in the term “modification”.) Each licensee is addressed as “you”.
Activities other than copying, distribution and modification are not covered by this License; they are outside
its scope. The act of running the Program is not restricted, and the output from the Program is covered only
if its contents constitute a work based on the Program (independent of having been made by running the
Program). Whether that is true depends on what the Program does.
1. You may copy and distribute verbatim copies of the Program’s source code as you receive it, in any medium,
provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice
and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any
warranty; and give any other recipients of the Program a copy of this License along with the Program.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty
protection in exchange for a fee.
2. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the
Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided
that you also meet all of these conditions:
(a) You must cause the modified files to carry prominent notices stating that you changed the files and the
date of any change.
(b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived
from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under
the terms of this License.
(c) If the modified program normally reads commands interactively when run, you must cause it, when
started running for such interactive use in the most ordinary way, to print or display an announcement
including an appropriate copyright notice and a notice that there is no warranty (or else, saying that
you provide a warranty) and that users may redistribute the program under these conditions, and telling
the user how to view a copy of this License. (Exception: if the Program itself is interactive but does
not normally print such an announcement, your work based on the Program is not required to print an
announcement.)
These requirements apply to the modified work as a whole. If identifiable sections of that work are not
derived from the Program, and can be reasonably considered independent and separate works in themselves,
then this License, and its terms, do not apply to those sections when you distribute them as separate works.
But when you distribute the same sections as part of a whole which is a work based on the Program, the
distribution of the whole must be on the terms of this License, whose permissions for other licensees extend
to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you;
rather, the intent is to exercise the right to control the distribution of derivative or collective works based on
the Program.
In addition, mere aggregation of another work not based on the Program with the Program (or with a work
based on the Program) on a volume of a storage or distribution medium does not bring the other work under
the scope of this License.
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
476
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(a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
(b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no
more than your cost of physically performing source distribution, a complete machine-readable copy
of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a
medium customarily used for software interchange; or,
(c) Accompany it with the information you received as to the offer to distribute corresponding source
code. (This alternative is allowed only for noncommercial distribution and only if you received the
program in object code or executable form with such an offer, in accord with Subsection b above.)
The source code for a work means the preferred form of the work for making modifications to it. For an
executable work, complete source code means all the source code for all modules it contains, plus any
associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is
normally distributed (in either source or binary form) with the major components (compiler, kernel, and
so on) of the operating system on which the executable runs, unless that component itself accompanies the
executable.
If distribution of executable or object code is made by offering access to copy from a designated place, then
offering equivalent access to copy the source code from the same place counts as distribution of the source
code, even though third parties are not compelled to copy the source along with the object code.
4. You may not copy, modify, sublicense, or distribute the Program except as expressly provided under this
License. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and will
automatically terminate your rights under this License. However, parties who have received copies, or
rights, from you under this License will not have their licenses terminated so long as such parties remain in
full compliance.
5. You are not required to accept this License, since you have not signed it. However, nothing else grants you
permission to modify or distribute the Program or its derivative works. These actions are prohibited by law
if you do not accept this License. Therefore, by modifying or distributing the Program (or any work based
on the Program), you indicate your acceptance of this License to do so, and all its terms and conditions for
copying, distributing or modifying the Program or works based on it.
6. Each time you redistribute the Program (or any work based on the Program), the recipient automatically
receives a license from the original licensor to copy, distribute or modify the Program subject to these terms
and conditions. You may not impose any further restrictions on the recipients’ exercise of the rights granted
herein. You are not responsible for enforcing compliance by third parties to this License.
7. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not
limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise)
that contradict the conditions of this License, they do not excuse you from the conditions of this License.
If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other
pertinent obligations, then as a consequence you may not distribute the Program at all. For example, if a
patent license would not permit royalty-free redistribution of the Program by all those who receive copies
directly or indirectly through you, then the only way you could satisfy both it and this License would be to
refrain entirely from distribution of the Program.
If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance
of the section is intended to apply and the section as a whole is intended to apply in other circumstances.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or
to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the
free software distribution system, which is implemented by public license practices. Many people have
made generous contributions to the wide range of software distributed through that system in reliance on
consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute
software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this
License.
23.1. GNU General Public License
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8. If the distribution and/or use of the Program is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Program under this License may add an
explicit geographical distribution limitation excluding those countries, so that distribution is permitted only
in or among countries not thus excluded. In such case, this License incorporates the limitation as if written
in the body of this License.
9. The Free Software Foundation may publish revised and/or new versions of the General Public License from
time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.
Each version is given a distinguishing version number. If the Program specifies a version number of this
License which applies to it and “any later version”, you have the option of following the terms and conditions
either of that version or of any later version published by the Free Software Foundation. If the Program
does not specify a version number of this License, you may choose any version ever published by the Free
Software Foundation.
10. If you wish to incorporate parts of the Program into other free programs whose distribution conditions are
different, write to the author to ask for permission. For software which is copyrighted by the Free Software
Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision
will be guided by the two goals of preserving the free status of all derivatives of our free software and of
promoting the sharing and reuse of software generally.
NO WARRANTY
11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR
THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE
THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND
PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL
ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO
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QGIS Qt exception for GPL
In addition, as a special exception, the QGIS Development Team gives permission to link the code of this program
with the Qt library, including but not limited to the following versions (both free and commercial): Qt/Noncommerical Windows, Qt/Windows, Qt/X11, Qt/Mac, and Qt/Embedded (or with modified versions of Qt that use
the same license as Qt), and distribute linked combinations including the two. You must obey the GNU General
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exception to your version of the file, but you are not obligated to do so. If you do not wish to do so, delete this
exception statement from your version.
23.2 GNU Free Documentation License
Version 1.3, 3 November 2008
Copyright 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc
http://fsf.org/
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Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not
allowed.
Preamble
The purpose of this License is to make a manual, textbook, or other functional and useful document “free” in the
sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying
it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a
way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free
in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free
software.
We have designed this License in order to use it for manuals for free software, because free software needs free
documentation: a free program should come with manuals providing the same freedoms that the software does.
But this License is not limited to software manuals; it can be used for any textual work, regardless of subject
matter or whether it is published as a printed book. We recommend this License principally for works whose
purpose is instruction or reference.
1. APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright
holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free
license, unlimited in duration, to use that work under the conditions stated herein. The Document, below, refers
to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the
license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied
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A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively
with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related
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ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant
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above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain
zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in
the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words,
and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic
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suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is
called Opaque.
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format,
LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML,
PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF
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The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold,
legibly, the material this License requires to appear in the title page. For works in formats which do not have any
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title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding
the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains
XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific
section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.)
To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled
XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the
Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as
regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.
2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided
that this License, the copyright notices, and the license notice saying this License applies to the Document are
reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not
use technical measures to obstruct or control the reading or further copying of the copies you make or distribute.
However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies
you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering
more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers
that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts
on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The
front cover must present the full title with all words of the title equally prominent and visible. You may add other
material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title
of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many
as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include
a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a
computer-network location from which the general network-using public has access to download using publicstandard network protocols a complete Transparent copy of the Document, free of added material. If you use the
latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to
ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the
last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large
number of copies, to give them a chance to provide you with an updated version of the Document.
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above,
provided that you release the Modified Version under precisely this License, with the Modified Version filling the
role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses
a copy of it. In addition, you must do these things in the Modified Version:
1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of
previous versions (which should, if there were any, be listed in the History section of the Document). You
may use the same title as a previous version if the original publisher of that version gives permission.
2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of
its principal authors, if it has fewer than five), unless they release you from this requirement.
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3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
4. Preserve all the copyright notices of the Document.
5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
6. Include, immediately after the copyright notices, a license notice giving the public permission to use the
Modified Version under the terms of this License, in the form shown in the Addendum below.
7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the
Document’s license notice.
8. Include an unaltered copy of this License.
9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year,
new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled
“History” in the Document, create one stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the
Document, and likewise the network locations given in the Document for previous versions it was based
on. These may be placed in the “History” section. You may omit a network location for a work that was
published at least four years before the Document itself, or if the original publisher of the version it refers
to gives permission.
11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section
numbers or the equivalent are not considered part of the section titles.
13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant
Section.
15. Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and
contain no material copied from the Document, you may at your option designate some or all of these sections
as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified
Version by various parties—for example, statements of peer review or that the text has been approved by an
organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover
Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one
of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you
are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from
the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for
publicity for or to assert or imply endorsement of any Modified Version.
5. COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined in
section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections
of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be
replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents,
make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author
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or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in
the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming
one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections
Entitled “Dedications”. You must delete all sections Entitled “Endorsements”.
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License, and
replace the individual copies of this License in the various documents with a single copy that is included in the
collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all
other respects.
You may extract a single document from such a collection, and distribute it individually under this License,
provided you insert a copy of this License into the extracted document, and follow this License in all other respects
regarding verbatim copying of that document.
7. AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in
or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the
compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not apply to the other works in the aggregate
which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document
is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket
the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the
terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright
holders, but you may include translations of some or all Invariant Sections in addition to the original versions of
these Invariant Sections. You may include a translation of this License, and all the license notices in the Document,
and any Warranty Disclaimers, provided that you also include the original English version of this License and the
original versions of those notices and disclaimers. In case of a disagreement between the translation and the
original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License.
Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your
rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated
(a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b)
permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days
after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies
you of the violation by some reasonable means, this is the first time you have received notice of violation of this
License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt
of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies
or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt
of a copy of some or all of the same material does not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
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The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from
time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address
new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular
numbered version of this License “or any later version” applies to it, you have the option of following the terms
and conditions either of that specified version or of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version number of this License, you may choose
any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a
proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance
of a version permanently authorizes you to choose that version for the Document.
11. RELICENSING
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes
copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that
anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained
in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons
Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as
future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published
under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the
MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at
any time before August 1, 2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the
following copyright and license notices just after the title page:
Copyright © YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with . . . Texts.” line with
this:
with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two
alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in
parallel under your choice of free software license, such as the GNU General Public License, to permit their use
in free software.
23.2. GNU Free Documentation License
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CHAPTER
24
Literature and Web References
GDAL-SOFTWARE-SUITE. Geospatial data abstraction library. http://www.gdal.org, 2013.
GRASS-PROJECT. Geographic ressource analysis support system. http://grass.osgeo.org , 2013.
NETELER, M., AND MITASOVA, H. Open source gis: A grass gis approach, 2008.
OGR-SOFTWARE-SUITE. Geospatial data abstraction library. http://www.gdal.org/ogr , 2013.
OPEN-GEOSPATIAL-CONSORTIUM. Web map service (1.1.1) implementation specification. http://portal.
opengeospatial.org, 2002.
OPEN-GEOSPATIAL-CONSORTIUM. Web map service (1.3.0) implementation specification. http://portal.
opengeospatial.org, 2004.
POSTGIS-PROJECT. Spatial support for postgresql. http://postgis.refractions.net/ , 2013.
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