Global Positioning System (GPS) Data Collection Guidelines

Global Positioning System (GPS) Data Collection Guidelines
Global Positioning System (GPS)
Data Collection Guidelines
[Mapping Grade GPS]
Suffolk County, New York
DRAFT
April, 2008
Purpose
The goal of this document is to provide a means of quality control and accuracy documentation of
Geographic Information System (GIS) data sets created with Global Positioning System (GPS)
technology. It is geared primarily for the mapping grade GPS user.
These GPS data collection guidelines seek to accomplish the following objectives:
(1) Establish methodology for collecting GPS data for use in a GIS;
(2) Provide guidelines for reporting metadata about GPS collected data and methods/means
used to collect such data;
(3) Supply GPS users with definitions of GPS terms and abbreviations; and
(4) Eliminate or reduce known and potential systematic errors.
This document was developed by the Suffolk County, Long Island GPS Sub-Committee; chaired by
M. Ross Baldwin. A large amount of material and formatting for this document was obtained and
used with permission from the GPS Standards Subcommittee within the Standards & Data
Coordination Work Group of the NYS GIS Coordination Program (www.nysgis.state.ny.us) and the
“VT GPS Guidelines” document, written by the Vermont Center for Geographic Information's
Technical Advisory Committee, led by Mike Brouillette.
(http://www.vcgi.org/techres/standards/partiii_section_l.doc)
*A special thanks to James Gormley for compiling the GPS Quick Reference Guide.
While these guidelines are generally intended to improve the quality of GPS-collected data,
following these guidelines does not guarantee that any suggested combination of hardware and
methods will insure a prescribed accuracy. A myriad of factors influence GPS data quality—many of
them not under the direct control of the user. Guidelines alone cannot substitute for experience
and judgment in the field. Specifications should balance the needs for accuracy against the
resources available for the project.
The user of these guidelines should understand that GPS technology is rapidly changing. Users of
this document require training and a base knowledge of GPS software and hardware. The GPS
technology is constantly evolving, necessitating the evolution of these guidelines. In order to
maintain the accuracy of these guidelines, this document will be reviewed and updated as
necessary.
In February 2006, the NYS GIS Coordination Program, through the NYS Office of Cyber Security &
Critical Infrastructure Coordination (CSCIC), presented a three hour workshop introducing GIS
practitioners to the basic concepts, functionality, accuracy issues and processes of data collection
via GPS, demonstrating the integration of GPS data into a GIS, and illustrating how positional error
within GPS data may affect the results of a GIS project. Additionally, a DVD of this workshop was
created and may be of interest to the readers of this document. This DVD is available upon request
from the NYS GIS Clearinghouse.
2
Survey, Professional Licensure and Use of GPS
The Global Positioning System (GPS) and Geographic Information Systems (GIS) have been a great
benefit to all levels of government. These two technologies have and will continue to change the
way governments manage land records, infrastructure, emergency response, and planning, to name
a few. Many of these GIS data layers are built and maintained by GIS consultants or government
employees.
Some decisions, regulations, ordinances, and law enforcement require government officials to base
their decision on information, data, or maps provided by State Licensed Professionals.
Licensed Land Surveyors commonly use Survey Grade GPS when performing boundary and
topographic surveys. Through the New York State Education Law, the State of New York governs
the Profession of Land Surveying, which this document will not address.
Users should familiarize themselves with and adhere to New York State Education Laws 7203 and
7209, which define the professions of engineering and land surveying as well as set guidelines. In
the interest of public health and safety, 7203 and 7209 set standards, respectively, by stating 2:
“The practice of the profession of land surveying is defined as practicing that branch of the
engineering profession and applied mathematics which includes the measuring and plotting of the
dimensions and areas of any portion of the earth, including all naturally placed and man- or
machine-made structures and objects thereon, the lengths and directions of boundary lines, the
contour of the surface and the application of rules and regulations in accordance with local
requirements incidental to subdivisions for the correct determination, description, conveying and
recording thereof or for the establishment or reestablishment thereof.”
AND
“No official of this state, or of any city, county, town or village therein, charged with the
enforcement of laws, ordinances or regulations shall accept or approve any plans or specifications
that are not stamped”.
More information about these New York State Education laws can be found online at
http://www.op.nysed.gov/pefaq.htm.
2
“NYS Education Law, Article 14,” 23 Jan. 2007 <http://www.op.nysed.gov/article145.htm>
3
TABLE OF CONTENTS
SECTION A – GUIDELINES
GPS QUICK REFERENCE GUIDE………………………………………………………………………………………………………….7
I. EXPLANATION OF GEOGRAPHIC INFORMATION SYSTEMS AND GLOBAL POSITIONING SYSTEMS…………….9
A. Geographic Information Systems…………………………………………………………………………..…………………...9
B. Global Positioning Systems………………………………………………………………………………………………9
C. Illustration of the Three GPS System Segments…………………………………………………………...10
II. CATEGORIES OF GPS RECEIVERS…………………………………………………….………………………….…………..11
A. Recreational Grade…………..………………………………………………………………………………….……..…11
B. Mapping Grade…………………………………………………………………………………………………………..……11
C. Survey or High Accuracy Grade………………………………………………………………………………..….…11
D. Categories of GPS Receivers Comparison Table………………………………………………………...…12
III. CHOOSING THE RIGHT TOOL FOR THE JOB…………………….……………………………………………………..13
A. Decision Tree…………………………………………………………………………………………………………..…...14
B. Other Characteristics to Consider…………………………………………………………………………..….….15
1.) Number of Channels…………….…………………………………………………………………..…..…..15
2.) Memory…………………………………………………………………………………………………………..…..15
3.) External Antenna….…………………………………………………………………………………….……….16
4.) GPS Power Source……….………………………………………………………………………….………....16
5.) Data Dictionary Design………………………………………………………………………………..……..16
6.) Critical Settings……………………………………………………………………………………………..……17
IV. DATA COLLECTION AND PROCESSING METHODOLOGY……………………………………………..……….….18
A. Mission Planning……………………………………………………………………………………………………..…..18
1.) Satellite Availability and Known Outages………………………………………………………....18
2.) Position Dilution of Precision (PDOP)……………………………………………………………..….18
3.) Local Obstructions of the Sky………………………………………………………………………..……18
B. GPS Receiver Configuration……………………………………………………………………………….……….18
1.) Position Dilution of Precision (PDOP)……………………………………………………….…………18
2.) Signal to Noise Ration (SNR) Mask…………………………………………………….……….……….19
3.) Elevation Mask Angle……………………………………………………………………………………..…..19
4.) Data Collection Rate……………………………………………………………………………………..…...19
5.) Datum…………………………………………………………………………………………………………….…...19
6.) Projection…………………………………………………………………………………………………….…...19
7.) Units of Measure……………………………………………………………………………………………..….20
C. GPS Data Download and Processing……………………………………………………………………..…….20
D. Quality Control……………………………………………………………………………………………………..…….21
E. Data Collection……………………………………………………………………………………………………..…….21
1.) GPS Receiver Antenna………………………………………………………………………………….….…21
2.) Prohibit Data Dictionary Editing………………………………………………………………………...21
3.) Data Download…………………………………………………………………………………………………...21
4.) Post-Processing……………………………………………………………………………………………….….21
5.) Base Station…………………………………………………………………………………………………….….22
V. GPS ACCURACY CONSIDERATIONS…………………………………………………………………………….……..……22
4
A. Sources of Error………………………………………………………………………………………………………….…..22
1.) Multipath……………………………………………………………………………………………………….……..22
2.) Atmospheric……………………………………………………………………………………………………….…22
3.) Distance from Base Station…………………………………………………………………………..…..…23
4.) Selective Availability………….……………………………………………………………………………....23
5.) Noise…………………………………………………………………………………………………….…………..….23
B. Default Settings that Affect GPS Data Accuracy…………………………………………..………….…..23
1.) Position Dilution of Precision (PDOP)………………………………………………….……………...24
2.) Elevation Mask Angle………………………………………………………………………..………………...24
3.) Number of Points Collected Versus Data Collection Rate…………….……………….…..24
4.) Data Collection Under Difficult Conditions………………………………………………….………24
C. Differential Correction to Improve GPS Data Accuracy………………………………………….….….24
1.) Post Processing Differential Correction……………………………………………….…….……...25
2.) Real-Time Differential Correction……………………………………………………….….….……..25
D. Quality Control and Reporting………………………………………………………………….….………………..26
1.) Validation and Quality Control………………………………………………………………….………..27
2.) Quality Control (QC)………………………………………………………………………………………… ..27
3.) Recommended Data Collection Methods……………………………………….……..…………..28
4.) Advanced Data Processing……………………………………………………………………….……….…33
E. Quality Assurance and Audit………………………………………………………………………………….……..35
1.) Quality Assurance and Accuracy Requirements…………………………………….…….…...35
2.) Quality Assurance………………………………………………………………………………………………..36
SECTION B – ACCURACY STANDARDS
I. INTRODUCTION………………………………………………………………………………………….….…………….……….…39
II. GENERAL CONCEPTS and DEFINITIONS……………………………………………………………………..….……....40
III. GPS ACCURACY STANDARDS…….……………………………………………………………………………………………..41
A. Re-Observation………………………………………………….………………….………………………….….……....42
B. Determining the NSSDA………………………………………….…………….….……………………………...……42
C. Base Station Accuracy…………………………………………………………….…….……………..………...……43
SECTION C – CONTENT SPECIFICATIONS
I. INTRODUCTION………………………………………………………………………………………………………….…………....45
II. TERMINOLOGY………………………………………………………………………………..…………………….……………..….45
III. GOALS………………………………………………………………………………..……………..………………………………..….46
IV. PRE_QUALIFICATION AND VALIDATION…………………………..…………………………………………………..….46
A. Total System………..………………………………………..………………………………………………………..…...46
B. Field Operator Training……………………………………………..………………………………………………....46
C. Data Processor/ Project Manager Training…………………….…………………………………….……....46
D. Contractor Validation…………………….………………………………………………………………………...…….46
V. VALIDATION SURVEY………………….………………………………….………………………………………….…………….47
VI. PRE-FIELDWORK PROCEDURE…………………………………….………………………………………….………..……..47
A. Proposal Meeting………………………………….…………….…………………………………………………..……..48
B. Auditing……………………………………………..……………………………………………………………………..…….48
C. Field Inspection….…………………………………….………………………………………………………………..…..48
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D. Reference Markers…………….………………………………………………………………………….…………..…..48
E. Map Ties……………………….…………………………………………………………………………………..…………...48
F. Legal Boundaries…………………………………………………………………………………………….…..….…....48
G. Required Survey Accuracies……………………………………………………………………….……..….…..….48
VII. FIELDWORK………………………………………………………………………………………………………………….……….48
A. Critical Rover Settings…………………………………………..…………………………………………….....…..48
B. Data Collection………………………………………………….…………….………….….……..………….………..49
VIII. GPS BASE STATION……………………………………………………………………………….……………...…....…….50
IX. PROCESSING AND QUALITY CONTROL…………………………………..………………………….………..…………50
X. PROJECT MANAGEMENT AND DELIVERABLES….………………………………………………….………....…….50
A. Project Report…………………………………………………………………………………………………………....…51
B. Hard Copy Plans……………………………………………………………………………………………………….…….51
C. GPS Data and Processing Deliverables……………………………………………………………………...….52
D. Data Ownership…………………………………………………………………………………………………….….…..52
E. Quality Assurance………………………………………………………………………………………………….…..….52
F. Data Management and Archiving…………………………………………………………………………….…....53
G. Digital Media……………………………………………………………………………………………………………...….53
XI. TECHNOLOGICAL/ PERSONNEL CHANGE………………………………………………………….……………..…....53
XII.METADATA GUIDELINES…………………………………………………………………………………………………..….….54
Appendix A – Glossary of Useful Terms……………………………………………………………………………….………55
Appendix B – Useful GPS and Related Websites………………………………………………………………….………63
Appendix C – Map of New York State Plane Zones…………………………………………………………….……….65
Appendix D – Map of NYSDOT CORS Stations………………………………………………………………….…………..66
Appendix E – Nat’l and Cooperative CORS Map of New York State………………………………….………..67
Appendix F – Wide Area Augmentation System (WAAS) Overview………………………………….………….68
Appendix G – United States Coast Guard Differential GPS Coverage of NYS…………………….……….69
Appendix H – Recommended Data Collection Practices………………………………………………….………….70
Appendix I – Sample Project Specifications…………………………………………………………………….………….78
Appendix J – Sample GPS Contractor Report………………………………………………………………………..…….85
Appendix K – Field Equipment List…………………………………………………………………………………….….…….86
Appendix L – Evaluating GPS Professionals………………………………………………….…………………….………..87
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GPS QUICK REFERENCE GUIDE
I. Mapping Grade GPS:
• Critical Settings
o Position Mode: Recommended manual 3D mode (requires 4 satellites)
o Minimum Satellites: This guideline recommends 4
o Elevation Mask: It is recommended that a mask of 15 degrees be applied (It may be
appropriate to increase the Elevation Mask when collecting data in valleys or urban areas)
o Signal to Noise Ration (SNR) Mask: Varies from GPS receiver Manufacture. Users should
consult the user manual to obtain the SNR Mask.
o Position Dilution of Precision (PDOP): the user should stop data collection when PDOP is
greater than 6
o Logging interval: Collection rate should be equal to or a multiple of the sampling rate of the
base station use in post-differential correction e.g., 1, 5, 10, 15, or 30 seconds
o Minimum Positions: For Point data this Guideline recommends that the user set the default
data collection rate to one second and the minimum number of positions to 30.
o Carrier Mode: allows a receiver to collect carrier phase data used for surveying purposes.
SHUT CARRIER MODE OFF.
•
Reference System:
o Datum: It is recommended that Datum be North American Datum from 1983 (AKA: NAD
83)
o Projection: New York State Plane
o Units of Measure: Feet
**The Suffolk County GIS Technical and User Committees adopted New York State Plane NAD 83
Feet
•
Mission Planning
o Users Should be aware of Known Satellite Outages, PDOP, and local Obstructions to the sky
and change the receiver settings and plan data collection times accordingly
•
Number of Channels
o The more channels that a receiver has the greater the potential accuracy for the data
collection points will be. This guideline recommends that the receiver have the capacity to
track 12 channels
•
Data Collection
o External Antenna: It is recommended that an external antenna be used when collecting data
in wooded or urban areas and anywhere where obstructions to the sky occur.
o Data Dictionary Design: It is recommended that a Data dictionary be used but not editable in
the field.
o Data Download: The data should be downloaded from the receiver to a computer or network
as soon as possible
o Post Processing: Recommended even when using a real time correction method
7
o Base Station: It is recommended that users use a single reference station throughout the
duration of the data collection project (For Projects that cover more than 30 miles it might be
appropriate to use more than one).
•
Quality Control
o Pre Collection Test: It may be necessary to collect “test” points for users who are unfamiliar
with the GPS Equipment.
o Use of Ancillary Data: Orthophotography, Internet mapping options, Street centerline data,
etc.
o Control Points: USGS Monuments, or any other feature with a known GPS location
o Users should review data collected to determine if collection procedures established during
Mission Planning were Followed
•
Important Variables that affect GPS Accuracy:
o Multipath error
o Atmospheric
o Distance From Base Station
o Selective Availability
o Noise (SNR)
o Receiver Default Setting
o Data Points Collected per feature
o Differential Correction
II. Recreation Grade GPS:
• Since most recreation grade GPS receivers are championed as an easy out of the box data collection
solution, the default settings set by the manufacturer will be optimal for most occasions.
8
SECTION A – GUIDELINES
I. EXPLANATION OF GEOGRAPHIC INFORMATION SYSTEMS AND GLOBAL POSITIONING SYSTEMS
A. Geographic Information System
In its simplest form, a Geographic Information System (GIS) is an electronic map used to display data
based on its geographic location; in its more complex form, it becomes a powerful analytical tool with
millions of pieces of data that are related geographically and can be displayed in a format that allows
the user to make the inter-relationships between the data visually understandable. 3
B. Global Positioning System
The Global Positioning System (GPS) consists of a constellation of 24 satellites that orbit the earth
twice a day (making one revolution approximately every 12 hours) at an altitude of approximately
124,000 miles. The GPS satellite navigation system was initiated by the U.S. Department of Defense in
the 1970's for military purposes. When the system is at full operational capacity, there are 24
operational satellites. This number changes periodically as satellites are commissioned (put into
operation) and decommissioned (removed from operation). At the time of this writing, 31 satellites
were in orbit. These satellites broadcast radio signals, containing satellite position and precise time
data, twenty-four hours a day. These signals enable anyone with a GPS receiver to determine a
geographic location.
The GPS system consists of three distinct segments: the space segment, the ground segment and the
user segment.4 The space segment, known as the Navigation Satellite Timing And Ranging (NAVSTAR)
constellation, consists of the GPS satellites, which transmit signals on two phase-modulated
frequencies (L1 - 1575.42 MHz and L2 - 1227.60 MHz). These transmissions are carefully controlled by
highly stable atomic clocks inside the satellites. The satellites also transmit a navigation message that
contains, among other things, orbital data for computing the positions of all satellites. The ground
segment, also called the control segment, consists of a Master Control Station located near Colorado
Springs, Colorado, and several monitoring stations located around the world. The purpose of the
control segment is to monitor satellite transmissions continuously, to predict the satellite ephemeris,
to calibrate satellite clocks, and to update the navigation message periodically. The user segment
simply stands for the total GPS user community. The user will typically observe and record the
transmissions of several satellites and will apply solution algorithms to obtain position, velocity, and
time.
Two signals are broadcast continuously by each satellite, one for use by the military, and the other for
civilian use. The latter is referred to as Standard Positioning Service. The basis of GPS technology is
precise information about time and position. To determine a horizontal location on earth, signals from
at least three satellites are required. A minimum of four satellite signals are needed for determination
of vertical position.
GPS receivers calculate the distance to each satellite by measuring the time interval between the
transmission and the reception of a satellite signal. Once the distance measurements of at least three
satellites are known, the method of trilateration can be used to determine the position of the GPS
receiver. GPS can be used worldwide, 24 hours a day and in all types of weather. While positional
accuracy can be very high, it does vary, depending on the type of GPS receiver, field techniques used,
post-processing of data, and error from various sources. 5 For further information, reference Section
A.V.A about SOURCES OF ERROR and Section A.IV about DATA COLLECTION & PROCESSING
METHODOLOGY.
3
“NYS Office for Technology – Policy,” 20 Dec. 2006 <http://www.oft.state.ny.us/policy/tp_9618.htm>
Alfred Leick , GPS Satellite Surveying, Second Edition (1995), 60
5
“North Carolina - Statewide Global Positioning System (GPS) Data Collection and Documentation Standards, Version 3,” 20 Dec.
2006, <http://cgia.cgia.state.nc.us/gicc/>
4
9
C. Illustration of the Three GPS System Segments 6
6
“GPS Control Segments.” 20 Dec. 2006, <http://www.mitrecaasd.org/proj_images/satnav/segment.gif>
10
II. CATEGORIES OF GPS RECEIVERS
A. Recreational Grade
Accuracy within five to twenty meters. These GPS receivers usually do not have the ability to "postprocess" collected data, but usually have the ability to perform real time correction using Wide Area
Augmentation System (WAAS). GPS receivers can be used to navigate to a specific area and/or compile
uncorrected GPS data; using associated third party software to convert the collected data directly into
GIS supported data formats.
B. Mapping Grade
Accuracy from sub-foot to five meters. These GPS receivers have the ability to log raw GPS data,
enabling these GPS-collected data to be post-processed utilizing desktop GPS software and allowing
locations to be refined or corrected to a higher level of precision than inherent in the raw data.7 This
category of GPS receiver also has the ability to communicate with a base station, store attributes of
features, use a data dictionary and upload data from the GPS device to a PC.
C. Survey or High Accuracy Grade
These include instruments with associated software that can achieve one-centimeter relative accuracy.
These are used by land surveyors primarily for boundary, topographic, geodetic surveys,
photogrammetry, and other activities requiring high accuracy. Specialized training is needed to use
this equipment.
7
Depending upon the model, corrections may occur as broadcast real time adjustments (WAAS or Coast Guard Beacon) or by post
processing.
11
D. Categories of GPS Receiver Comparison Table
RECREATIONAL GRADE
MAPPING GRADE
SURVEY GRADE
Primary Uses
• Navigation; hunting; fishing;
camping; backpacking; hiking; data
collection
• 5 to 20 meter
• Resource mapping; navigation;
GIS data collection
Horizontal Data Accuracy
• Sub-foot to 5 meter (real-time
or post-processing correction)
• resource mapping; site mapping;
land surveying; navigation; vertical
measurement
• Centimeter level (real-time OR postprocessed corrections, with a survey
control network)
Vertical Data Accuracy
• Not used to collect vertical data
• 2 to 15 meter (2 to 3 times
less accurate than horizontal
data)
• < 2 cm (real-time correction)
• < 1 cm (post-processed corrections
with a survey control network)
Differential Correction Options
• Most do not have post-processing
capabilities
• Real-time correction (WAAS) in most
GPS receivers
• Post-processing in all GPS
• Real-time in some GPS receivers
receivers
• Additional post-processing to
improve accuracy is in all GPS
• Most have real-time
receivers
capabilities (WAAS and/or
USCG beacon additional add
on)
Type of Features Collected
• points 8
• points, lines and polygons
• points, lines and polygons
Option to Load Custom Data Dictionary with Feature Attributes
• unavailable at this time
• all GPS receivers
• all GPS receivers
Option to Load Custom Coordinate Systems, Projections, Datums/Spheroids
• some GPS receivers
• all GPS receivers
• all GPS receivers
Training Requirements
• moderate
• advanced
Metadata (capability to generate metadata or extract metadata from GPS receiver type)
• minimal
• moderate
• advanced
Cost (circa 2006)
• $2,500 to $12,000
• $5,000 to $50,000
• $200 to $500 2636
• minimal
8
Additional software needed to generate lines and polygons
12
III. CHOOSING THE RIGHT TOOL FOR THE JOB
Based on the parameters established in mission planning, the user should choose a GPS receiver that meets or
exceeds those requirements. Resources (e.g. staff, hardware, software) must also be sufficient to support the
use and maintenance of the selected data collection tool. Therefore, choosing the right GPS receiver for a
specific project requires serious consideration of the following: 9
• Identify and use existing data collection procedures or standards.
• Anticipate use of the feature location and attribute data to be collected.
• Project data accuracy requirements for the data to be collected.
• Available resources to support data collection and processing activities.
• Type, number, and other characteristics of features to be located.
• Characteristics (e.g., rural vs. urban, remote vs. nearby) of the data collection site.
• Identify and use existing feature location or attribute data.
• Type of feature attribute data to be collected
• How the features to be located will be represented (i.e., as points, lines, or polygons)
9
“Vermont Center for Geographic Information VT GPS Guidelines,” 20 Dec. 2006
<http://www.vcgi.org/techres/standards/partiii_section_l.doc>
13
A. Decision Tree
The decision tree is intended to help users select an appropriate GPS receiver grade particular to their GPS
data collection project. This is only a general guide, however, and you must also consider several other
factors as noted above before making your final choice! 10
START
HERE
Project has determined data
relative accuracy requirements?
Assess
data accuracy
needs
Project requires data with better
than 8 inch horizontal accuracy
and 2 foot vertical accuracy?
YES
YES
NO
NO
Survey
Grade GPS
Project requires collection of point,
line or area feature data with better
than 10 meter accuracy?
YES
YES
NO
Project intends to load/use a
customized GPS data dictionary
during data collection?
NO
Mapping/Resource
Grade GPS with
Post-processing
YES
NO
YES
Project requires real-time
differential correction
capabilities?
NO
Recreational
Grade GPS
Mapping/Resource
Grade GPS with
Real-time
10
Project to collect and store more than
1,000 points before downloading data?
“WI-DNR. Comparing GPS Tools,” - <http://www.dnr.state.wi.us/maps/gis/gps.html>
14
B. Other Characteristics to Consider
In addition to the ability to set defaults and differentially correct data using a specific GPS receiver, users
should also consider the following additional receiver characteristics before choosing a GPS receiver for
your project. Nearly every new receiver surpasses the minimum configuration requirements noted below
for their class and this trend is likely to continue in the future. While survey, mapping/resource and
recreational-grade receivers share many of these characteristics, our suggestions pertain to
mapping/resource-grade receivers.
1.) Number Of Channels
GPS receivers track the signals from satellites via “channels”, with the signals from one satellite
occupying one channel on the receiver. A 3-channel GPS receiver tracks the signals from up to three
satellites at one time, while a 12-channel receiver tracks the signals from up to twelve satellites at one
time. The more channels a receiver has, the more likely that it will continue uninterrupted collection
of data if the parameters (e.g., PDOP) of one of the satellites fall out of optimal range. A GPS receiver
with twelve channels has a greater ability to track the “best” while continuing to seek out other
satellites with more optimal parameters. Therefore, this Guideline recommends that GPS receivers
have the ability to track 12 channels.
2.) Memory
The number of data points that a GPS receiver can collect and store (before you need to download the
data to a computer) differs greatly between recreational and mapping/resource systems. Recreational
grade receivers can only collect and store data for less than 1,000 points – and users do not usually
download these data for further processing or analysis. Therefore, memory requirements of
recreational grade receivers are of less concern. You must, however, consider how the field conditions
of your project may influence the memory requirements of your mapping/resource GPS receiver.
Larger data sets require more memory. Remote field locations may require larger files to be collected
between downloading opportunities. Attribute data requirements may take up considerable storage
space. Needs for higher accuracy usually means more data needs to be collected, increasing storage
needs. Many GPS units have memory slots to expand onboard memory.
™ How many features will be located?
More features may require more memory to minimize the number of data downloads you need
to perform.
™ How large are the line or area features to be located?
Long linear features (e.g., trails) or polygon features with very large areas (e.g., forest stand
boundaries) may require more memory to store all collected data. In addition, a GPS receiver
that lets you open and append data to existing files will minimize the number of total files
you need to create and compile for one feature.
™ How remote are the features to be located?
Remote features may require more memory in order to minimize the number of trips made to the
field to capture them. Without adequate memory the only alternative is to return to the office
numerous time to download data or bring a laptop into the field for downloading.
™ Will a customized data dictionary be loaded on the receiver?
The use of data dictionaries is highly recommended (unless using ArcPad), and they take up
memory!
15
™ What are your data accuracy requirements?
More memory may be needed to capture and store the larger volume of data needed to
support higher data accuracy requirements.
This Guideline recommends that your mapping/resource grade GPS receiver have a minimum 2Mb of
memory. This amount of memory should allow for the loading of a custom data dictionary and the
ability to collect data for 8 hours while using a one second sampling interval in all but the most
demanding situations. Additional memory is an option with most receivers.
3.) External Antenna
GPS satellite signals can be received from any direction. For best results the antenna must have a clear
view of the sky. Satellite signals do not penetrate metal surfaces, buildings, tree trunks, or similar
objects. In addition, signals are weakened when they penetrate tree canopies, glass, or plastic. GPS
receivers have an internal antenna that is sufficient for general use in clear sky areas away from
buildings. Most resource/mapping grade receivers also have the option of an external antenna. An
external antenna is very useful in situations where the internal antenna may be blocked by the user, an
obstruction or where a stable platform is desired. These are also useful mounted on top of a vehicle.
The internal antenna is disabled when an external antenna is used so that signals are not received from
both antennas. External antennas generally increase the amount of “signal gain” and the ability to
operate in demanding environments, e.g., tree canopy or narrow river valleys, at a minor cost of
additional battery drain.
Mounting an antenna on a pole mount raises the antenna above obstructions and limits multipath signal
degradation from reflected signals. The ground plane is established at the antenna height. An external
antenna mounted on a vehicle should be mounted on a metal surface to establish a ground plane rather
than on a plastic or fiberglass camper shell to limit multipath. It is important to properly secure the
external antenna’s cable to the GPS receivers the connection cannot become dislodged by an
obstruction or when walking through brush.
An external antenna is recommended when collecting data in wooded or urban areas where the sky is
partially obscured and when acquiring data with a vehicle.
4.) GPS Power Source
Battery capacity, charging systems and battery replacement should be considered. GPS receivers run
on electricity, so it is important to have a good battery supply available in the field. Important
parameters include: the ability to work in a range of temperatures, all day working capacity and
rapidly rechargeable. The ability to utilize a 12v adaptor of a vehicle socket will provide an endless
power when conducting mobile GPS work. There are a number of different battery types, e.g., lithium,
ni-cad that come in a variety of voltage and Wattage. One useful measure in comparing batteries is the
“Amp-hours” rating.
5.) Data Dictionary Design (ESRI ArcPad not applicable, uses ArcPad Forms)
A data dictionary is a menu of standard feature attributes (i.e., data elements) loaded on a GPS
receiver that is used to simplify and standardize data collection of geographic features in the field
when recording descriptive information. Individual geographic features are represented by multiple
coordinate pairs known as “fixes” that are captured according to the sampling interval of the receiver.
The data dictionary defines the fill requirements, default values, and valid codes/values (domain
values) for each attribute. This approach minimizes the effort of entering in descriptive text via the
16
keypad, prevents misspelled entries and improves data consistency, e.g., different fields operators
might otherwise assign different values to the same feature(s). Once a company or department defines
a data dictionary it can be used repeatedly to standardize data collection and ensure quality control of
attributes and their domain values. Some receivers are limited to a single dictionary while others can
store multiple ones. Other limitations worth assessing are: character maximum length for the feature
name, attribute name and menu attributes; maximum character length for a character string;
maximum character length for a user code and maximum character length for comments.
6.) Critical Settings
Traditionally, the user had full control over all of the “critical settings” that affect the quality of
captured data. Increasingly, these settings are being pre-defined by manufacturers in an attempt to
make receivers easier to use. While this may be desirable most of the time, it is useful to have the
choice to control them manually. Invariably you will find yourself one day in a deep river valley at dusk
coming to the conclusion that a point captured with a lower PDOP threshold is better than no point at
all. Critical settings include:
™
™
™
™
™
™
™
Logging interval – time between “positions”;
Minimum positions – minimum number of positions required to log point feature;
Minimum time – ensures acquisition of carrier phase information to calculate higher accuracy features
Position mode – driven by accuracy needs. Options are: “2D” (x,y), Manual 2D/3D, or 3D (x,y,z);
Elevation mask – prevents GPS receiver from using satellites not visible by the base station;
Signal-to-noise ratio (SNR) mask – prevents receiver from recording positions with low signal quality;
PDOP mask and switch - prevents receiver from logging inaccurate positions due to poor satellite
geometry.
17
IV. DATA COLLECTION AND PROCESSING METHODOLOGY
*Methodology refers to the techniques a user should apply prior to and while collecting data with a GPS
receiver. It should be noted that not all of these options are applicable to all recreational grade GPS receivers.
A. Mission Planning
For the purpose of this document, Mission Planning is a broad overview of planning a project to establish what
the purpose is, what the data will be used for and who will be using them. All these factors will help
determine the proper equipment and methods to be used.
1.) Satellite Availability & Known Outages
Before collecting data, the user should be aware of the theoretical satellite availability. Most GPS
software has the ability to provide a theoretical estimate of satellite availability at a certain
geographic location, on a certain day, at a specific point in time. This information is often
displayed in a variety of methods, including graphs, charts and diagrams, and skyplots, which
display the satellite constellation over a location.
The United States Coast Guard maintains a website that generates a digest of known or forecasted
GPS satellite outages. This digest is called the Notice Advisory to NAVSTAR Users (NANU) and lists
the times when specific GPS satellites will be unstable or not available for use. This information
can be used in the mission planning utility when considering which satellites will be available on a
specific day. For information about how to subscribe to the NANU email list, visit the following
webpage: http://www.navcen.uscg.gov/gps/gps_news_090905.htm
2.) Position Dilution of Precision (PDOP)
The user should plan their data collection at times when there is optimum satellite availability
(four or more) and when the satellites are in an appropriate configuration to produce an
acceptable (lower) PDOP value. Data collection can be planned to exclude poor (higher) PDOP
times. PDOP values should be reviewed daily as satellite geometry changes constantly. Most GPS
desktop software has the capability of providing graphics indicating the number of satellites
available over the course of a day at a specific location as well as the PDOP values.
3.) Local Obstructions of the Sky
The user should consider performing field reconnaissance in advance of data collection to identify
local obstructions of the sky, including urban canyon, forest canopy, etc., that can affect results.
B. GPS Receiver Configuration
It is recommended that the following values be set on the GPS receiver prior to field data collection. These
values are subject to the accuracy requirements of specific projects. The values below may be modified
depending on GPS receiver model. Additionally, the user should consult the manufacturers’ guidelines for
optimal GPS receiver configuration recommendations.
1.) Position Dilution of Precision (PDOP)
Most GPS receivers allow you to set a maximum acceptable Position Dilution of Precision (PDOP).
The PDOP is a statistical indicator of the geometry among the satellites being observed—it is an
important indicator of position accuracy. Since a GPS position is the calculated intersection of
measurements from multiple satellites, GPS data are more accurate if the satellites are evenly
distributed in all quadrants around and above the receiver. The ideal geometry of the satellites
which will produce the lowest PDOP is to have three satellites at 15 degrees above the horizon and
evenly distributed, separated horizontally by 120 degrees with a fourth satellite directly overhead.
Since the GPS system was designed to maximize coverage over temperate regions of the globe, this
theoretical ideal isn’t even possible in the current configuration of satellite orbits.
GPS receivers calculate PDOP from the distribution of usable satellites in the sky at the moment of
data collection. Receivers search for and use the combination of available satellites that will
18
produce the lowest “dilution of precision”, within the threshold setting specified by the user. This
Guideline recommends that you set your GPS receiver to stop collecting data when the PDOP is
over 6.
2.) Signal to Noise Ratio (SNR) Mask
Setting the value of the SNR mask higher will help minimize noise error. Varies from GPS receiver
manufacturer; each manufacturer has their own recommendations; user must refer to their specific
user manual.
3.) Elevation Mask Angle
As mentioned above, the distribution of satellites above the horizon is used to calculate PDOP.
Most GPS receivers let you set a minimum “elevation mask angle” to ensure that the GPS receiver
only tracks and uses satellites that are positioned a specified distance above the horizon. Setting
this value too low could allow the receiver to collect data from satellites not being tracked by the
base station having an adverse impact on post-processing efforts. Also, data from satellites that are
low on the horizon are “noisy” due to increased atmospheric refraction. The elevation mask setting
is a minimum threshold; it is very likely that local topography and obstacles blocking the horizon,
such as vegetation or buildings, are likely to constrain the “effective” minimum elevation to
something higher than the mask. This Guideline recommends that you set the minimum
elevation mask angle on your GPS receiver to 15º or greater.
4.) Data Collection Rate (Sync Rate)
The number of readings you collect for a feature affects the accuracy of GPS data. The user can
specify the minimum number of position fixes and the interval at which fixes are stored, based on
your project’s data accuracy requirements. There is an obvious relationship between the number of
points you collect and the rate at which you collect them. A collection rate of one fix per second
will yield 30 points in 30 seconds, whereas, it would take 150 seconds to record 30 points if the
rate is one per five seconds. In general, the more readings you record, the more accurate a
feature’s location will be with the caveat that GPS data accuracy does not significantly improve
after a “threshold” number of points are collected. In addition, the collection rate should be equal
to, or a multiple of, the sampling rate of the base station to be used in post-differential correction,
e.g., 1, 5, 10, 15 or 30 seconds. Refer to Table IV-1 Static Data Collection – Suggested Duration
and Number of Fixes for suggested collection rates and collection durations.
For point data, this Guideline recommends that you set the default data collection rate to
one second and the minimum number of position to 30. When collecting line or polygon
features the rate may vary between one and five seconds depending on your speed of
ground travel
5.) Datum
GPS receivers are designed to collect GPS positions relative to the WGS-84 datum, however the
user has the option of designating into which datum the data will be displayed. Users must have an
understanding of the datum in which the GIS project is developed.
*For most GIS applications, the WGS-84 datum is similar to the NAD-83 datum, however NAD-27
is significantly different from the NAD-83 datum. Most manufacturers allow the user the option
of displaying the data being collected in most datum’s. Various software exists that allow for
the transformation of data from one datum to another. Refer to Appendix B for more
information on datum transformation.
6.) Projection
It is recommended that data being collected with GPS be displayed on the GPS receiver in the New
York State Plane projection:
State Plane New York Long Island feet
19
Users should have an understanding of the projection the data are being collected in and the
projection in which the GIS project is in. GPS receivers are designed to collect data and
perform real-time correction in an unprojected geographic coordinate system
(latitude/longitude). Most manufacturers allow the user the option of displaying the data
being collected on the GPS receiver in most projections.
*Refer to Appendices C for maps of the State Plane Zone.
7.) Units of Measure
Users should be aware of the units of measure that are commonly used with each projection. The
State Plane projections can be published in US Survey Feet or meters. Users should also be aware
of the International Foot unit of measurement, which is different than the more commonly used US
Survey Feet.
Users should have an understanding of the units of measure in which the data can be displayed on
the GPS receiver. Some manufacturers allow the user the option of displaying the data being
collected in different units of measure (e.g. US Survey Feet, International Feet, Miles, Meters,
etc.).
When collecting data with a GPS receiver, the geographic location is represented as a coordinate
pair (e.g. 42.8123N, 75.8066W). The positional coordinate pair can be displayed in some common
formats:
Latitude/Longitude - Degrees/Minutes/Seconds (DMS)
A latitude or longitude might be written as 43º 5’ 20”, where the single quotation (’)
represents minutes and the double-quotation symbol (”) represents seconds.
Latitude/Longitude - Decimal Degrees (DD)
The same coordinate would be written as 43.088889º.
Latitude/Longitude - Degrees and decimal minutes
The same coordinate would be written as 43º 5.333333’.
UTM 18 extended North (meters)
The same coordinate would be written as (4740283N, 434057E).
State Plane New York Central (US feet)
The same pair would be written as (312608N, 313525E).
US National Grid
The same pair would be written as (18T WN 7125315437)
Conversion of a coordinate pair between any of these three formats can be performed with a
relatively easy formula found within existing tools. Additionally, calculators and mathematical
formulas on the Internet allow translation of one coordinate pair (i.e. latitude/longitude) in
any of these formats into another format for that same location. Refer to Appendix B for a list
of useful websites.
C. GPS Data Download and Processing
The data download process varies by GPS receiver manufacturer so the user should refer to their
specific user manual for instructions.
20
D. Quality Control
*Data should be reviewed to determine if procedures established during mission planning were
followed.
High resolution orthophotos, such as those available through the New York Statewide Digital
Orthoimagery Program (NYSDOP), can be used to determine if there are gross errors (i.e. does not meet
the accuracy standards defined in mission planning of a project) in the GPS data by comparing the GPS
data positions to the high resolution orthoimagery. It may be necessary to recollect data if the original
data do not meet project needs. NYSDOP has been producing orthoimagery since 2001 with highresolution orthoimagery available statewide outside New York City for viewing and downloading.11
More information about the New York State Digital Orthoimagery (DOI) Program can be found at the
following webpage:
http://www.nysgis.state.ny.us/gateway/orthoprogram/index.cfm
The SCTM digital parcel map should only be used to determine if there are gross errors in the GPS data
as well. The SCTM is best used for generalizations and as a reference.
After conducting quality control and if your positional requirements are not met, it may be necessary
to recollect the data.
E. Data Collection
1.) GPS Receiver Antenna
In order to minimize loss of GPS satellite lock, users should, whenever practicable, orient the GPS
antenna skyward; and in the case of handheld GPS receivers avoid signal blockage by their upper
body and head. In addition, when recording the location of tall features (e.g. trees, utility poles)
it is a good practice to approach the feature from the south, positioning the GPS receiver antenna
on the south side of the feature. This recommendation is due to the fact that, in the northern
hemisphere, GPS satellites are not present in the northern sky except at very high elevations above
the horizon (i.e. > 70 degrees).
2.) Prohibit Data Dictionary Editing
It is recommended to prohibit the editing of the data dictionary in the field in order to ensure
uniformity in the data attributes being collected.
3.) Data Download
It is recommended to download the collected data from the GPS receiver to a local computer as
soon as possible after returning from the field to minimize the risk of losing the data on the GPS
receiver due to battery failure, inability to store additional data or overwriting existing data.
4.) Post-Processing
It is recommended that users employ post-processed differential correction as part of their GPS
data management workflow as soon as practicable after downloading data from a field device.
Three key benefits to adhering to this approach are:
- Rapid identification of reference stations that are out of service or are experiencing
communication interruptions
- Avoidance of encountering a condition where reference station files are no longer available
because they have been deleted from the provider’s server
11
“NYS GIS Clearinghouse - Digital Orthoimagery Program,” 12 February 2007
<http://www.nysgis.state.ny.us/gateway/orthoprogram/index.cfm>
21
- Compliance with a standardized workflow procedure that delivers data in its final form
swiftly, allowing for archiving of raw field and intermediate data files, and promoting
streamlined and simplified file management
5.) Base Station
The user should determine the quality of the base station being used. It is recommended for the
novice GPS user that only NOAA/NGS published base stations be used.12 Advanced GPS users may
have the ability to establish their own base station and should consult the manufacturers’
guidelines for their specific hardware for instructions.
*It is recommended to utilize a single reference station for all project specific post-processed
differential correction activities, with the exception of projects that cover a large area (e.g.
several thousand square miles) or long (30 miles or more) “strand” or linear mapping projects.
This technique will promote data registration uniformity by inducing identical systematic errors
(if any) across the full breadth of your data sets. In addition, metadata documentation will be
simplified and differential correction parameters will be homogeneous across the entire
project data set.
**The most commonly used Base Stations for Long Islanders include NYCI, NYRH, CTDA (Conn.)
V. GPS ACCURACY CONSIDERATIONS
A. Sources of Error
*In order to effectively gather precise/accurate data, it is necessary to understand potential sources of error
that can affect GPS data quality
1.) Multipath
Errors caused by reflected GPS signals arriving at the GPS receiver, typically as a result of nearby
structures or other reflective surfaces (e.g. buildings, water). Signals traveling longer paths
produce higher (erroneous) pseudorange estimates and, consequently, positioning errors.
The user should be aware that multipath errors are not detectable or correctable with recreational
grade GPS receivers. Some mapping grade GPS receivers as well as most or all survey grade GPS
receivers have antennas and software capable of minimizing multipath signals.
2.) Atmospheric
GPS signals can experience some delays while traveling through the atmosphere. Common
atmospheric conditions that can affect GPS signals include tropospheric delays and ionospheric
delays.
Tropospheric delays have the capability of introducing a minimum of 1-meter variance. The
troposphere is the lower part (from ground level to 13 km) of the atmosphere that experiences the
changes in temperature, pressure, and humidity associated with weather changes. Complex
models of tropospheric delay require estimates or measurements of these parameters.13
Unmodeled ionospheric delays have the potential to introduce significant (i.e. >10 meter)
positional error. The ionosphere is the layer of the earth's atmosphere generally ranging from 50
km to 500 km above the earth's surface. During periods of heightened solar activity, charged
particles (ions) in the ionosphere impede GPS signal transmission. Specific phenomena that do
affect the GPS signal quality include periods of high solar activity (e.g. solar flares). The
12
13
“CORS Data,” 20 Dec. 2006 <http://www.ngs.noaa.gov/CORS/Data.html>
“Global Positioning System Overview,” 20 Dec. 2006 <http://www.colorado.edu/geography/gcraft/notes/gps/gps.html>
22
ionospheric model transmitted in the GPS signal compensates for approximately 50% of this delay.
The balance must be resolved through differential correction.14
Weather conditions, including cloud cover and precipitation, generally do not affect the GPS
receivers’ (hardware) capability of collecting accurate data. However, cold temperatures near
and below freezing could affect the GPS receiver LCD screen and battery life.
3.) Distance from Base Station
While differential correction will increase the quality of the data, accuracy is degraded slightly as
the distance from the base station increases. Users should use the nearest base station to where
the data is being collected. With the implementation of the NYS CORS Base Station Network (see
Appendix E) across the State, the density of base stations is increasing. This network should be
sufficient to provide differential correction for GIS users in most situations.
4.) Selective Availability (SA)
SA is the intentional degradation of the GPS signals by the Department of Defense (DOD) to limit
accuracy for non-U.S. military and government users. The potential error due to SA is between 30
to 100 meters.15 SA is presently turned off, but the DOD reserves the right to turn it back on at any
time and in specific geographic theaters.
5.) Noise
Noise error is the distortion of the satellite signal prior to reaching the GPS receiver and/or
additional signal “piggybacking” onto the GPS satellite signal. All three grades of GPS receivers are
capable of suffering from noise error. The amount of error due to noise cannot be determined.
™
™
™
™
™
™
™
*You can ensure that the quality of your data is high by understanding the numerous factors that can
affect GPS data quality, including:
Conditions in the ionosphere and atmosphere (e.g., solar flares)
Number of available satellites and their geometry and health
GPS receiver default settings (e.g., PDOP, mask angle)
Signal interference (e.g., multipath errors) by obstacles such as buildings and trees
Number of data points collected for a feature
How and if data are differentially corrected
Base station used for differential correction
By using appropriate data collection and processing techniques users can minimize much of the error
associated with these factors. Obviously, some factors are beyond user control, e.g., solar flares or
satellite characteristics. However, the right tool and its proper use can minimize these sources of error
and make your GPS data as accurate as possible.
B. Default Settings That Affect GPS Data Accuracy
Many GPS receivers let you set data collection constraints that disallow data collection unless certain
minimum operating thresholds are met. The following discussion introduces the most commonly
available constraints that are under the user’s control. The user should note that the recommendations
given below are intended to support accuracy in the 0.5 to 2 meter range. It should also be stated that
the trade-off between accuracy and productivity (and cost) is embodied in any choice of operating
constraints. The user should employ data collection constraints that meet the accuracy needs of the
project.
14
15
Alfred Leick , GPS Satellite Surveying, Second Edition (1995), 303
“Global Positioning System Overview,” 20 Dec. 2006 <http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#SA>
23
1.) Position Dilution of Precision (PDOP)
2.) Elevation Mask Angle
3.) Number of Points Collected Versus Data Collection Rate
4.) Data Collection Under Difficult Conditions
Topography, buildings, and vegetative canopy are among the most frequently encountered obstacles to GPS
signal reception. Signals can be blocked completely, the signal strength can be reduced (analogous to static on
a radio), or signals can bounce off nearby objects and contribute to position inaccuracies (multi-path). A full
discussion of this topic is beyond the scope of this Guideline. We offer some practical approaches to addressing
this condition here.
Experienced users recognize that GPS data collection conditions are seldom ideal. It is this same experience
that teaches these users to enter the field prepared for poor conditions. The following general strategy offers
some guidance, but its successful implementation relies heavily on the experience and judgment of the user.
The most successful strategy is to “be prepared”. In the case GPS data collection, this means entering the field
with knowledge of the conditions you are likely to encounter and knowledge of the “ideal” satellite times for
minimizing the impact of difficult conditions. Data collection on a north slope or in a steep stream valley may
dictate that GPS can only be collected at certain times of the day when a sufficient number of satellites are
available above the topographic “obstacles“. Most GPS software allows the user to predict the positions of all
the satellites at any time of the day and users can enter the field with this information, allowing them to make
decisions about when to attempt data collection or how long to wait at a particular location for favorable
satellite availability.
Vegetative canopy is more likely to reduce the strength of (rather than completely obstruct) the incoming
signal. If choosing the time of day is an option, plan to collect data when satellites are plentiful and high in the
sky. Alternately, you may be able to raise your antenna into or above the canopy for better signal reception or
plan your data collection for “leaf-off” conditions. Yet another option with some receivers is to collect an
“offset” position; that is, GPS data are collected some distance off the desired position, but a compass bearing
and estimate of distance to the actual point are also collected. Post-processing the position with the offset
information “projects” the collected data to the actual location from the offset location.
C. Differential Correction to Improve GPS Data Accuracy
Differential correction removes certain types of error from GPS data, and can occur back in the office
(post-processing) or as you are collecting data in the field (real-time). Post-processing these corrections is a
little more accurate than real-time differential correction because the individual fixes and the corresponding
base station corrections are perfectly time- synchronized, whereas, real-time corrections introduce a time
delay between the correction data and the position.
Both methods of correction work by comparing satellite signals received by the receiver with those received by
a base station, which is fixed over a highly accurate, surveyed point. Base station correction values are
calculated and then applied to the rover data to increase their accuracy to 5 meters or less, depending on the
GPS receiver grade.
Both post-processing and real-time differential correction require that the base station and receiver are able to
record data from the exact same satellites. In addition, the base station should be within 100 miles of the field
data collection site to maximize the effectiveness of the post-processing. When differentially correcting GPS
data you must decide which method will best support your project needs, and if your project resources are
24
adequate to support the selected technique. The major differences between the post-processing and real-time
are related to equipment cost and time sensitivity to accessing corrected data. The extra equipment needed to
attain real-time can add to cost of a system but comes with the advantage of access to the corrected data in
real-time. Generally speaking, unless you have a specific need for the enhanced location accuracy provided by
real-time differential processing in the field, it is easier and more economical to go the post-processing route
in Vermont. The topography and dense vegetative cover in Vermont can adversely impact radio and direct
satellite link signals. The characteristics of post-processing and real-time differential processing are described
in more detail below.
1.) Post-Processing Differential Correction
This type of differential correction occurs back in the office, after you have downloaded raw GPS
data from the receiver on to a computer. Special software (specific to the GPS receiver!) is used to
apply correction values calculated from base station data to the rover data. The ease of this
process has steadily improved over the years and is not difficult to learn.
In most cases, you can download free base station data from an Internet site operated by the
National Geodetic Survey (http://www.ngs.noaa.gov/CORS/). For Long Island, there are a number
of base stations to choose from. Please refer to Appendix D and E for a map and list of available
sites. A list of community base stations is available on Trimble’s website
(http://www.trimble.com/trs/findtrs.asp). You can also set up a temporary base station for a
specific project, but this requires additional effort and may involve security issues if the receiver is
to be unattended in a remote location. To ensure the best accuracy possible for field data ensure
that your collection rate, e.g., 1 fix per second is a factor of the sampling rate of the available
base station data. For example, if the closest base station has a 5 second sampling rate do not set
your collection rate to 2 or 3 seconds, rather, set it to 1, 5, 10, 15 or 30 seconds etc.
Most base stations do not store one second interval data for more than a month so do not wait to
acquire this data after a field collection effort!
GPS equipment with post-processing functionality is generally less expensive than systems with
real-time functionality, because less hardware is required (i.e., there is no need for a real-time
beacon receiver). However, today’s resource grade receivers often come with both capabilities,
and this is becoming the standard receiver configuration.
2.) Real-time Differential Correction
Some recreational and mapping/resource grade GPS receivers have real-time differential
correction functionality (also known as Differential GPS or DGPS). Real-time differential correction
occurs in the field, and requires another piece of equipment (either separate from or integrated
into the GPS receiver) to receive correction values from a GPS base station via radio signals or
direct satellite link, and automatically apply these data to adjust GPS rover data as they are being
collected. Systems with a built-in satellite link provide real-time capabilities anywhere in the
world.
The real-time corrections are based on an extrapolation of the derived base station corrections
computed at some time in the past. This extrapolation is a result of the small time lag between the
time the satellite information is collected and stored by the base station; a correction is computed
and finally transmitted to the GPS receiver. The time lag between the simultaneous reception of
satellite signals by the base station and GPS receiver and the receipt of the correction transmission
to the receiver is known as RTCM-Age (Radio Technical Commission for Maritime Services). Most
units have the ability to set the limit on RTCM-Age that will be used by the receiver to calculate
the real-time position. With the removal of Selective Availability the RTCM-Age can be longer than
before without adverse results, e.g., a meter every few minutes. The recommended settings are as
follows, relative to the desired target accuracy.
25
Target Accuracy
(95%) using RealTime GPS
1m
2m
5m
10m
Suggested
Maximum RTCMAge
15 seconds
30 seconds
60 seconds
90 seconds
Table IV-1: Suggested Maximum RTCM Correction Age Settings
Another type of real-time differential correction is the Wide Area Augmentation System (WAAS),
developed by the Federal Aviation Administration to aid the avionic application of GPS. Though many
recreational grade receivers are now WAAS capable, the signal is highly susceptible to blockage from
topographic relief and vegetation and is not as accurate as post processing. To avoid questioning if your
data has been enhanced through the use of WAAS always post process your data for the best results!
See APPENDIX F - WIDE AREA AUGMENTATION SYSTEM (WAAS) OVERVIEW for more information.
Three types of receivers are used to receive base station correction data:
™ external real-time radio link receiver
™ real-time radio link receiver built into the GPS receiver
™ direct satellite link built into the GPS receiver (i.e., correction data are transmitted from the base
station up to a communications satellite and then back down to the receiver)
In order to help secure accurate results, it is important that the base station transmitting the radio
signals carrying the correction values be within 100 miles of the field data collection site. A map of
Nationwide Differential GPS (NDGPS) sites that broadcast corrections can be found at
http://www.navcen.uscg.gov/dgps/coverage/EastCoast.htm. Radio signals carrying correction data can
be received from base stations more than 100 miles from the field data collection site, but results are
inconsistent and use of these correction data are not recommended for real-time differential
correction. Post-processing is also recommended when base station radio signals are blocked by
terrain.
D. Quality Control and Reporting
Quality Control (QC) and Quality Assurance (QA) procedures ensure reliability in GPS survey results and instill
confidence in the data. Whereas QC procedures are undertaken by the GPS Contractor to ensure accuracy and
completeness of the data produced throughout the data collection effort, QA procedures are the responsibility of
the Contracting Agency to ensure the GPS data are accurately imported into existing map databases once
received. QC procedures are discussed here. QA and auditing procedures are outlined in Section A.V.E - Quality
Assurance and Audit. Once again, it is important to stress that QA/QC specifications should be considered very
carefully and balanced against the needs of the project. They will almost always add something to the cost.
*It is always good practice to follow Quality Control and Quality Assurance procedures internally as well.
26
1.) Validation and Quality Control
Requiring Contractors to submit a small “trial run”, or validation survey, in order to pre-qualify for
responding to a GPS survey contract is a logical place to start controlling the process of creating high
quality, reliable data. These surveys provide insight into a Contractors technical capabilities and ability
to assess a project and plan the data collection and processing efforts accordingly. If poor results are
received in a Contractors validation survey, it may have a bearing on how the actual project will be
conducted. Reviewing Contractor credentials to determine their success in conducting past GPS work is
highly recommended.
2.) Quality Control (QC)
The primary QC method is to ensure that parameters associated with field data capture were followed.
Many of the procedures outlined in Sections A.V.C - Differential Correction to Improve GPS Data
Accuracy above and A.V.D.3 - Recommended Data Collection Methods below detail field procedures,
processing methods and specifications that help control GPS data quality.
Additional, specific QC procedures that help ensure GPS survey data is as reliable and accurate as
possible are detailed below:
16
i.
PDOP Masks
Not all GPS receivers have settings that enforce that no data be collected when Position Dilution of
Precision (PDOP) values are too high. Ideally, PDOP values are logged for each position fix for
verification. When the receiver isn’t capable of this, PDOPs can be computed afterwards with most
manufacturers’ software. Whenever possible, it is suggested that the following QC parameters be
output: solution standard deviations, residuals, variance factors, etc. The capabilities of
commercial software that offer these outputs vary by manufacturer.
•
2D vs 3D
Most mapping/resource grade GPS receivers allow the user to set data collection to be either twodimensional (2-D) or three-dimensional (3-D) and correspondingly, how many satellites are
required. 2-D positions need three satellites and 3-D positions need at least four. 3-D positions are
more reliably accurate than 2-D ones and it is recommended that only 3-D positions be collected.
Occasionally, difficult site locations and conditions may limit satellite availability and force a 2-D
position. When the rover files are exported from the desktop GPS software it is highly
recommended that the option to export the position type attribute be enabled to allow for ready
identification of 2-D or 3-D positions, or alternatively, only 3-D, corrected positions should be
accepted.
•
Digital Imagery Comparison
An alternative to re-observing points, when accuracy validation isn’t a project requirement, is the
use of the New York State Digital Orthophotography Program. By planning ahead and capturing field
points that are readily identifiable on the “orthos”, e.g., road intersections, distinct driveways etc.
it is possible to compare the field point onscreen with the imagery to gain a general sense of the
field data accuracy without re-observation. Although the “orthos” are quite accurate, (1:5000
source scale) remember that accuracy is a relative term and that the imagery you’re using as a
frame of reference to compare field points does contain a certain amount of error. According to
the U.S. National Map Accuracy Standards16, the horizontal accuracy for 90% of points at the 1:5000
scale is approx. 2.5m (8ft). Due to the unpredictable nature of accuracy, this means that a point
lining up perfectly with the corresponding point on the image can still be off by the error of the
source imagery, i.e., 2.5m.
http://www.oh.nrcs.usda.gov/technical/gis/natl_map_accuracy.html
27
•
Re-Observation
The best method of assessing the accuracy of a GPS survey is by re-observing a portion of the
original positions using the same receiver and settings. This topic is fully discussed in Section
B.III.A. Re-Observation. Re-observing points is a good way to verify that your collection effort is
on the right track, however, it is only absolutely necessary if you are trying to prove your data
meets a certain accuracy.
•
Use of other ancillary data (parcels, benchmarks, streets, etc)
•
Benchmarking to Established Monuments
A benchmark is an established and documented field location with known coordinates. One can
occupy a benchmark, collect GPS data and compare the collected position to the “published”
position. However, the accuracy noted for one GPS point against a benchmark, regardless of a
benchmarks coordinate accuracy, does not apply to other points in a typical GPS data collection
survey. Instead, the utility of this comparison may be limited to simply ensuring the receiver’s
critical settings have proper values and that it isn’t malfunctioning. Benchmarking should never be
a substitute method for re-observation to estimate the accuracy of a survey.
3.) Recommended Data Collection Methods
The three main types of features in Geographic Information Systems (GIS), e.g., points, lines (arcs),
and polygons (areas), are all based on individual points or vertices. The definition of a line or polygon
feature is affected by the proximity of the points to each other under certain conditions. Just how
close the points should be is discussed in more detail below. Most mapping/resource grade receivers
and their software are capable of capturing all of these features while recreational grade receivers are
not.
GPS data can be collected in one of two ways, first by remaining stationary over a point or while
moving “dynamically” over a line or edge of a polygon feature. These data collection methods are
called “static” or “dynamic” modes, respectively. Points can be the result of a single positional fix or
an average from many positional fixes, each taken at intervals from one second (the minimum) to the
highest setting allowed by the receiver (a maximum of 30 seconds is practical). The combined impact
of the number of fixes and sampling interval on the accuracy of an averaged point is less pronounced
for point features than for line or polygon features captured dynamically. While capturing features
dynamically is an efficient, acceptable means of data capture it requires consideration of additional
factors in order to maintain the desired accuracy.
This section defines data collection methods and suggested field methods and GPS receiver settings to achieve
target accuracies.
i. Static Point Features
“Static point features are normally surveyed by grouping a number of individual position fixes
to produce an averaged single position. Examples of static point features are: a project
location, culvert, bridge, cabin etc. A static point feature has a start and an end time, and
usually includes attributes describing the feature. The post-processing software will average all
individual position fixes to compute a single position for the feature and attach any attributes
for export to a GIS or mapping system.
The largest errors in Differential GPS (DGPS) positions are usually due to multipath and signal
attenuation caused by nearby objects such as foliage, reflecting surfaces, etc. While the
antenna is moving, these errors tend to be random (more or less), but significant systematic
errors can occur at a stationary antenna. Multipath on L1 pseudoranges occurs in cycles of 6-10
minutes (theoretically). If the antenna is kept over a point for a full multipath cycle, the errors
28
should average out and accuracies of a few meters may be attainable under forest canopy.
However, requiring a 10-minute occupation time at point features may not be practical, or
necessary if the project’s accuracy target is lower. It is important that enough data is collected
to be able to detect systematic multipath at static point features. In most cases, 45 – 60
seconds of observations is sufficient for an experienced” user post-processing the data “to
detect multipath trends in a point feature. Note that this time period is enough to usually
detect multipath effects, however, it may not be enough to ensure accurate and reliable
feature coordinates from the remaining fixes once the multipathed fixes are deleted. In this
case the feature would have to be re-surveyed in the field.
This averaging improves positional accuracy and minimizes random measurement “noise” and
multipath effects. In theory, accuracy continues to improve as more data is averaged, however
there is a point of diminishing returns after a number of minutes of recording. It is
recommended that at least 30 fixes be averaged for every static point observed,
regardless of the project’s accuracy.
Both the number of individual position fixes and the length of occupation will affect the
accuracy for a point feature. There are two minimum conditions that must be met. The
operator must stay for at least the minimum time and have at least the minimum number of
position fixes recorded. Under marginal observing conditions, the operator may have to stay for
a longer time to meet the minimum fix requirement.”17
The table below details the minimum number of fixes and sampling rate recommended to
achieve the desired relative target accuracy detailed in Section B: Accuracy Standards.
Relative target accuracies listed presume a mapping/resource grade GPS receiver collecting
data under good site conditions, e.g., no obstructions, and favorable critical values, e.g.,
PDOP, SNR, small baseline error etc. and where all data is post-differentially corrected or
acquired via “real-time” correction.
Note! This table is only a guideline and no field tests were conducted to determine these values. Due to
the large selection of mapping/resource grade GPS receivers on the market it would be virtually impossible
to assess them all.
Relative
Target Accuracy
< 1.0 m
1.0 m
2.0 m
5.0 m
10.0 m
20.0 m
Suggested
Data Collection Duration
15 minutes (900s)
10 minutes (600s)
8 minutes (480s)
5 minutes (300s)
1 minute (60s)
.5 minutes (30s)
Suggested
Number of
Fixes
180
120
96
60
60
30
Sampling
Interval
5s
5s
5s
5s
1s
1s
Table IV-2 Static Data Collection – Suggested Duration and Number of Fixes
ii. Linear Features - Dynamic Mode
Line features are formed from a number of individual GPS position fixes and similar to point
features they have a start and end time and associated attributes. The two modes of collecting
linear features are dynamic traverses and point-to-point traverses.
17
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-35
29
“Dynamic Traverses are analogous to “stream-mode” digitizing of a line. The Field Operator
guides the antenna along the linear feature to be mapped while collecting GPS position fixes at
a specified time interval. This time interval will be chosen based on the resulting distance
between position fixes, which includes consideration of the traveling speed, feature
complexity, and tracking environment. It is important that position fixes be recorded at all
significant deflections in the linear feature. Static point features can be added to record
features along the line (e.g. a culvert along a stream survey). The individual position fixes are
connected to form the linear feature. The line can be smoothed and generalized later in
mapping / GIS software.
Resource surveys can be done on foot by a Field Operator wearing a GPS backpack, from the air
via helicopter or fixed-wing aircraft, and by vehicle (truck, quad, snowmobile, bike, boat, etc).
These surveys can be very productive, but are only suitable if the feature is easy to identify
and the vehicle can accurately guide the antenna over the feature at all times. These surveys
must also conform to the fix spacing limits set by the contracting entity (e.g. a position fix
every 25m). Also, the speed of the vehicle may affect how accurately the feature can be
followed. The speed limits defined in the following sections are based on the speed that can
safely be flown in a helicopter (from interviews with pilots familiar with GPS mapping). During
some road surveys there may be safety reasons to increase the vehicle speed limit (e.g. so as
not to impede vehicles on an active road), but for most surveys, 50 km/h (30 mph) is a
practical upper limit.
During dynamic linear positioning the data recording rate should be set according to the fix
spacing desired which is related to the vehicle speed. For example, if a road is to be surveyed
at 10m fix spacing and the vehicle speed is 35 km/hr (~20 mph), then the data collector must
be capable of recording one fix per second. Note that some GPS systems claims a one-second
recording rate, but can only sustain this when tracking less than 5 satellites.
The following table shows examples of various fix spacing for different traveling speeds and recording
rates.”18
Example Modes
Of Transportation
Speed
(m/s)
Walking
1.4m/s (5km/h or 3mph)
Bike
4.2m/s (15km/h or 10mph)
Vehicle – slow
8.3m/s (30km/h or 18mph)
Vehicle – fast
17m/s (60km/h or 35mph)
Data Collection Rate (s)
And corresponding
Point Separation (m)
@1.0s separation = 1.4m
@5.0s separation = 7.0m
@1.0s separation = 4.1m
@5.0s separation = 21m
@1.0s separation = 8.3m
@5.0s separation = 42m
@1.0s separation = 17m
@5.0s separation = 84m
Table IV-3 Dynamic Traversing - Speed & Data Rate vs. Point Separation19
iii. Linear Features - Point-to-Point Mode
Point-to-Point Traverses entail capturing individual points, i.e., a “traverse point”, that
collectively defines linear features. No fixes are logged between points and once averaged the
points are converted into a linear feature by either the GPS desktop or CAD / GIS software. The
18
19
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-36
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-41.
30
operator should acquire an overview of the features to be captured in order to log key
inflection points in the features. Point-to-point traverse may not be more accurate than
dynamic traverses under forest canopy but are more practical under certain circumstances.
Practicality may prevent a dynamic traverse, e.g., a river or other impassible topographic
features and the accuracy target may render delineation of minute details unnecessary.
Another advantage is the ability to use offsets when logging these points to overcome obstacles
or improve satellite reception, e.g., reducing multipath interference from forest canopy when
delineating a tree stand. Offsets are described below.
iv. Linear Features – Hybrid-mode
Individual “nestled” point features can be recorded while a separate feature is being logged by
a dynamic traverse, e.g., a spring or cabin along a trail. This is extremely useful in improving
the efficiency of the capture effort where multiple features can be captured in a single effort
but when the point feature has an accurate location or may be readily identifiable on a digital
Orthophotography. This can provide an additional QC checkpoint for the linear feature
especially in situations where forest canopy may be affecting the target accuracies.
v. Polygon Features
Polygon (area) features are essentially linear features that close, e.g., connect at their
endpoints. These can be collected explicitly as polygons or as linear features that are later
processed into polygons using CAD/GIS software. For simple features in open areas with good
satellite reception it may be more straightforward to collect the features as polygons.
For more complicated or large features where reception may be an issue, logging linear
features can be more versatile logistically for two reasons; A) A single line segment
representing the polygon can be saved midway along the traverse to capture other linear
features in the vicinity; and B) Collecting multiple segments guards against losing the entire
feature due to loss of signal, lack of storage space or battery power. In either case linear
features are saved whereas a polygon feature will close between the original position and last
available position.
Whichever approach is chosen it is highly recommended that one approach be adopted and followed for
consistency throughout a project.
vi. GPS Events
An additional method for logging point features is the “GPS Event”, “Nested Point”, or
“quickmark”. Using the “time stamp” ingrained in every position fix, the quickmark is
interpolated from stored GPS fixes taken before and after the mark. These are not substitutes
for static point features because they are only based on individual fixes, not averaged
positions. They are useful in defining general reference points or when the GPS antenna can’t
remain stationary over a point feature, i.e., during collection efforts while using a car. They
will not work if signals to the antenna are blocked at the instant the mark is taken.
To gain a sense of how accurate the event times must be in relation to the speed of travel the
following tables depicts different accuracies at different times and assumes the quickmark be
accurate to one-half of the target accuracy. “For example, if the accuracy specification is 10m
and the traveling speed is 50km/h (14m/s), Event times must be accurate to one half of 10m
divided by the speed (i.e. 5m divided by 14m/s = 0.36 seconds).”20
20
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-38
31
Desired Network
Accuracy
1.0 m
2.0 m
5.0 m
10.0 m
Receiver Speed
5km/h (1.4m/s)
30km/h (8.3m/s)
60km/h (17m/s)
100km/h (28m/s)
5km/h (1.4m/s)
30km/h (8.3m/s)
60km/h (17m/s)
100km/h (28m/s)
5km/h (1.4m/s)
30km/h (8.3m/s)
60km/h (17m/s)
100km/h (28m/s)
5km/h (1.4m/s)
30km/h (8.3m/s)
60km/h (17m/s)
100km/h (28m/s)
Required GPS Timing
Accuracy
0.36 seconds
0.06 seconds
0.03 seconds
0.02 seconds
0.72 seconds
0.12 seconds
0.04 seconds
0.04 seconds
1.80 seconds
0.30 seconds
0.15 seconds
0.09 seconds
3.60 seconds
0.60 seconds
0.30 seconds
0.18 seconds
Table IV-4 Desired Point Accuracy vs. Speed & Timing Accuracy21
vii. Point and Line Offsets
Offsets allow the capture of points without having to directly occupy them. Using a bearing and
distance measure the GPS receiver applies and adjustment to its present location to derive the
offset. The accuracy of the offset is subject to the accuracy of the bearing, the distance
measure and reference GPS position but if the antenna is no longer under canopy as a result of
the offset, accuracy can improve. Some receivers are sophisticated enough to allow for a
digital laser range finder that calculates both distance and bearing to the feature and
automatically feeds this information into the GPS receiver to calculate the offset. Offsets are
desirable under field conditions where physical access or satellite obstacles, e.g., forest
canopy and safety concerns prevent direct occupation of the feature. It can also be more
efficient to log features this way, e.g., logging fire hydrants from a vehicle mounted GPS
antenna.
The power of offsets comes with the responsibility to manage them properly in order to avoid the
introduction of error. All manually entered measurements that help compute offsets must be done
correctly and a thorough understanding of magnetic and true azimuths, inclination angles, and slope and
horizontal distances is required to ensure accuracy.
Receivers supporting offsets usually allow input measurements to be reviewed and edited to remedy any
incorrect entries. For points acquired by receivers without offset capabilities, offsets can still be
calculated if measurements are recorded and used in concert with CAD/GIS software.
viii. Point Offsets
The following procedures are recommended when using point offsets:
™ Only use a single Azimuth measurement for the entire project, i.e., magnetic or true north. For
projects covering large geographical areas multiple magnetic declination values may be required.
The value(s) used should be documented.
™
All azimuth measurements should be made relative to the GPS antenna.
21
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-39
32
Measuring the azimuth value from both the offset location and from the actual feature location
helps to improve the accuracy of the value.
™
The accuracy of distance measurements directly affects an offsets overall accuracy. Distances
measured on an incline must be adjusted from slope to horizontal distance. For receivers that
accept an inclination angle the horizontal distance is automatically calculated.
™
All compasses are affected by natural and man-made attractions so all efforts should be made
to prevent these sources of magnetic distortion from influencing azimuth readings.
™
The magnetic declination value used in the survey should be present in the project report along
with methods employed to measure distance, direction and inclination. Before offset coordinates
are calculated the magnetic declination value must be applied. Setting the declination in the field
compass allows a direct reading of true azimuth but it is also possible to apply the declination to
magnetic azimuth values after the fact. The best source of magnetic declination in the United
States is the National Geophysical Data Center’s Geomagnetism home page and values can be
computed using their on-line Magnetic Declination Calculator. The accuracy of the predicted
magnetic declination is variable and local anomalies can exist.
The following table associates the effects of compass precision and offset distance. Both analogue and
digital compasses are affected by magnetic declination and local variations.
Compass
Instrumentation
Standard Compass
Compass
Precision
Declination &
Variation Uncertainty
2.0°
1.0°
1.0°
1.0°
0.3° - 0.5°
1.0°
e.g. Silva Ranger (15T)
Precise Compass
e.g. Suunto KB-14D
Digital Compass
e.g. MapStar, Laser
Atlanta
Offset
Distance
25m
50m
100m
25m
50m
100m
25m
50m
100m
Offset Point
Uncertainty
(approximate)
1.0m
2.0m
3.9m
0.6m
1.2m
2.5m
0.6m
1.1m
2.3m
Table IV-5 Offset Accuracy vs. Instrumentation Precision & Offset Distance22
ix. Linear Offsets
For linear offsets digitized in a dynamic traverse, maintaining a constant offset distance from
the feature is essential, particularly when collecting data from a vehicle when both speed of
travel and practical safety concerns can affect the offset distance. Keeping the offset distance
small, i.e., less than 5m, in dynamic traverses can help minimize error. When linear offsets are
digitized in a static traverse each offset can be a managed individually and does not necessarily
have to be a constant value.
4.) Advanced Data Processing
This section is provided for users wishing to go the extra mile in validating the accuracy of their data
and when required by an audit. When more than four satellites are available for determining a
22
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-41
33
positional fix extra, i.e., “redundant” information is available to the receiver that yields what is called
an “over-determined” solution. Redundant information can make the data more accurate because
there are more satellites to choose from in calculating the position and it also provides additional
statistical information. This information comes in two forms, e.g., solution variances and observation
residuals and can be used for quality control assessment with desktop software capable of processing
it. At present, most desktop software shipped with mapping/resource grade GPS units aren’t capable of
processing this information but this may change in the future.
i. Filtering
Some manufacturers of GPS systems utilize a variety of techniques for interpolating, filtering
and estimating GPS data in their software. Details on these techniques are too involved for this
document and they will simply be referred to collectively as “filtering”. For these
manufacturers the functionality is always implemented in the desktop software but not always
in the receiver. This determines whether the filtering can be applied dynamically in the field or
only during post-processing. Some receivers have a corresponding receiver setting allowing the
user to pick different modes of data collection, e.g., walking, driving, flying etc., while other
provide no such controls to the user. Filters and their settings used should be noted in the
project report, when applicable.
Filtering works by assessing points that surround the point being re-computed, e.g., previous and
subsequently logged points. Points that are too far apart for the time elapsed to have been
acquired by a pedestrian (remember everything is “time stamped” in GPS) can be identified as
incorrect, i.e., an outlier, and deleted.
ii. Data Editing, Smoothing and Generalizing
Post-differentially corrected or real-time differential data is considered to be “original
corrected” GPS data and should be retained in the project archive previous to any edits to
provide an opportunity to review the level of noise in the GPS traverse as well as any major
errors. In cases where critical parameters are changed or special processing controls applied in
either post-processing efforts or part way through a survey (avoid this whenever possible) to
produce originally corrected data, e.g., a different elevation mask or outlier deletion criteria,
they should be noted in the project report.
Most point, linear and polygon features are edited or generalized in some way to remove
apparent errors from individual points caused by a poor acquisition environment or user error.
Editing individual points collected as either static point features or in point-to-point traverses
(to form a linear feature) can be done automatically by desktop GPS or CAD/GIS software
where individual fixes meeting certain criteria are deleted and the position is recomputed. One
of these criteria is the standard deviation value.
The goal of most edits to linear of polygon features is to find the best representative line or
“best-fit” line using the GPS positions as a guide. These lines are created in a variety of ways:
1) Manually drawing a line over the GPS position fixes in CAD/GIS software commonly referred
to as “heads-up digitizing”; 2) Sequentially connecting each positional fix to form the line; or
3) Deleting outliers. Some of these edits can be done automatically in GIS software.
Regardless of the method there is a certain amount of subjectivity involved by the data
processor and this makes it all the more important that they have adequate experience and
training to complete the job successfully. Asking a green technician to determine which fixes
are outliers, to interpret the “best-fit” line for complex features or those with “noisy” data is
simply not reasonable or acceptable as the majority of errors in traverses are due to
insufficient interpretation. Ultimately, if the data is too complicated to interpret with an
acceptable level of confidence, it must be reacquired.
34
E. Quality Assurance and Audit
Whereas Quality Control procedures are undertaken by the GPS Contractor to ensure accuracy and
completeness of the final products, Quality Assurance (QA) procedures are the responsibility of the Contracting
Agency to ensure the final products are properly integrated into their existing map databases. The QA
procedures may be detailed in a contract but are primarily intended for the Agency’s benefit. It is
recommended that Audit procedures be outlined in the contract to inform the Contractor how the delivered
data will be assessed. The following sections are intended to provide an overview of QA concepts and auditing
data submitted by the Contractor and are highly recommended for use by the Agency to ensure data integrity
and accurate integration of the data into existing database. When data sets of extreme value or sensitivity,
e.g., emergency services, are involved these QA procedures may not be detailed enough and further research
should be conducted. Note that only relative accuracy is covered by these guidelines. See Section B: Accuracy
Standards.
1.) Quality Assurance & Accuracy Requirements
The process of Quality Assurance (QA) entails integrating data acquired from the Contractor into
existing database and ensuring that they are complete, correct, and meet the target accuracies
detailed in the contract. Failing to implement QA processes can create doubt as to data integrity and
may make users justifiably reluctant to invest large amounts of time and energy based on unknown
source data. While it has always been the responsibility GIS users to understand data limitations it is
also the responsibility of both the Contractor and Agency to ensure integrity in the data they create.
Target accuracies that the data must meet and how these values are reported is detailed in Section B:
Accuracy Standards. Reviewing this section will help interpret the following sections.
As all features are ultimately comprised of point features, it is possible to apply standard statistical
methods to each individual position averaged from numerous fixes and have them output from the
desktop GPS software. Including the raw GPS data as a requirement in the data deliverables allows
these values to be recreated if not originally generated by the Contractor. If more than the minimum
number of fixes was collected for a feature it is possible to remove a number of outliers in order to
recreate the averaged point and improve the points relative accuracy, e.g., reduce the standard
deviation. An averaged point with a low spread of individual fixes and therefore a low standard
deviation does not guarantee an increase in absolute accuracy. See the General Concepts and
Definitions in Section B: Accuracy Standards for a discussion on relative vs. absolute accuracy.
i.
Assessing Linear Features
The method for assessing linear and polygon features is identical. Linear features captured via
static or dynamic traverses usually have their individual point fixes edited to remove
“blunders”, i.e., obvious errors or outlier points, to produce a smooth or generalized “best-fit”
line.
Visually comparing the “best-fit” line with the original GPS position fixes onscreen provides an
insight into data quality by allowing the differences between these individual fixes and the
final line to be explicitly viewed. Printing these data out at even a smaller scale for review is
impractical due to the time and volume of paper it would require.
The easiest way to do this is to create a buffer equal to the relative target accuracy around the
best-fit line and view in concert with the raw data in a GIS.
35
Fixes far from the final line are likely the result of poor satellite geometry, forest canopy or
multipath errors introduced into the data. If only the minimum number of fixes were taken for
each point and any show up as outliers the feature may need to be recaptured.
ii. Assessing Point Features
Assessing the quality assurance of point features is also done primarily using visual techniques,
though it is entirely possible to automate the process by creating custom programs. Creating a
buffer on the post-processed feature, equal to the target accuracy can be used for comparison
against the original position fixes.
2.) Quality Assurance
Auditing and quality checking the work submitted by the Contractor is the single largest component to Quality
Assurance (QA). Making sure the data received is accurate and complete before merging with existing Agency
data is the first order of business and can be covered by different levels of auditing. The level and number of
audits depends on available project resources, scope and mission critical nature of the data. The three levels
of audit presented here are: 1) Quality Check Audit, 2) Detailed Audit, and 3) Complete Audit.
While the Contractor should clearly understand how their data is going to be audited they should not be given
any information that allows them to predetermine what points or line segments will be reviewed to avoid
biasing the results. Review features should be selected randomly while still representing the project as a
whole.
The ascending levels of audit entail an increasing level of detail while at the same time testing a smaller
percentage of the GPS data, e.g., 15%, 5%, and 1% for the respective audits, e.g., Quality Check Audits,
Detailed Audits, and Complete Audits.
i.
Quality Check Audit
The purpose of this audit is to verify that all deliverables detailed in the contract are
submitted in full, adhere to the relative accuracy targets and digital data specifications and
that field data was collected following the appropriate protocols. It is highly recommended
that this type of audit be conducted on all projects as a primary means of verifying project
objectives have been successfully completed. This basic check can be achieved by reading the
project report, ensuring the completeness of all digital data and its relative accuracy through
visually and/or quantitatively checking 15% of all data. This audit is geared so that technicians
and those with minimal GPS experience can conduct the audit successfully.
Checking relative data accuracy by following procedures outlined in the “Assessing Linear
Features” and “Assessing Point Features” noted above can provide a check on relative accuracy
through comparison of the raw, individual position fixes and the final interpreted lines or
averaged points. This visual review can also reveal things like the distance between position
fixes and number of position fixes per point feature. The observed distance between fixes can
be compared to the reported method of collection and Section A.V.D.3.ii- Table IV-3
Dynamic Traversing - Speed & Data Rate vs. Point Separation to check for continuity
between the report and the data.
The procedures below outline a Quality Check Audit23:
™ Centralize all submitted data and materials.
™ Create a review directory in the existing project directory, e.g., “QAQC”.
™ Copy all submitted digital files to the directory.
23
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. D-79
36
™ Review project report.
o review dates of milestones (i.e. field survey, post processing, mapping).
o review equipment, personnel, etc.
o specifically note data capture parameters (i.e. elevation masks, DOP limits, data collection
duration, etc.).
o note any anomalies.
™ Review field notes.
o note any anomalies that may not have been caught in mapping.
o review established reference markers, map ties, etc.
™ Review digital files visually.
o overall view looking for large blunders.
o verify relative accuracy standards for point and line features.
o verify spacing of reference markers, etc.
o verify spacing or number of position fixes on line and point features.
o verify offsets and supplemental traverses.
o verify map datum and translations.
™ Review digital files using automated methods if available.
™ Review hard copy output for completeness and presentation.
™ Verify that other returns are complete (particularly digital files).
ii. Detailed Audit
The next level of QA is the detailed audit and it essentially equals the level of detail the
Contractor should have invested in Quality Control checks before submitting the data. In
addition to the procedures outlined above in the Quality Check audit, the detailed audit
includes the re-processing of the raw data and a review of the parameters used in data
collection efforts and Quality Assurance procedures.
This level of audit requires a thorough understanding of GPS concepts and practical experience
is essential. If a separate consultant is hired to perform these audits they must be unaffiliated
with the Contractor originally conducting the survey. This requires both GPS post-processing
and CAD/GIS software be available and they should have the capacity to assess quality control
measures such as solution variances (standard deviations) and observation residuals in the data.
The GPS receiver and software used must be capable of storing pseudorange data to permit the
generation of these measures.
The re-processing of the data should follow procedures outlined in sections IV.2 Differential
Correction to Improve GPS Data Accuracy and IV.3.D Advanced Data Processing for checking
15% of all data. The original GPS base station data should be reused unless the Contractor
setup their own field reference base station, in which case the nearest CORS base station data
should be used in re-processing.
iii. Complete Audit
The highest level of audit is the re-survey of a small portion of the Contractor’s work. The
original contractor should not do the work in order to ensure objectivity of the test. The resurvey test should not include features originally re-observed by the Contractor in order to
expand the scope of points re-observed and to provide a separate value of relative accuracy.
This value should compare to the original relative accuracy determination and can prove useful
in official situations where courts or appeal boards etc. are involved.
As with the Detailed Audit work must be done by qualified personnel or independent
consultants and both GPS post-processing and CAD/GIS software be available.
37
iv. Other Audit Procedures
Other possibilities for audits include using equipment and skilled in-house personnel available
to the Agency contracting the work as either a substitute or compliment to the Detailed or
Complete Audits. The Quality Check Audit should always be conducted. This is advantageous
when the Agency lacks certain GPS equipment and experience but has traditional surveying
resources and skills that can successfully fulfill the audit requirements in certain situations,
i.e., when project is in proximity to a known benchmark or reference. Tools such as theodolites
and (digital) laser range finders can locate features with a very high degree of absolute
accuracy though this approach may be limited to more open areas free of obstacles and forest
canopy. Traditional methods can also accurately determine areas for polygon features. While
these methods can register the boundaries to the VSC with a tie-in to a known horizontal
benchmark, this isn’t necessary for comparing the area values.
*Subject to data target accuracy other techniques are also possible such as overlying the GPS data with
digital data of a known accuracy for visual comparison, e.g., digital Orthophotography from the Vermont
Mapping Program.
38
SECTION B – ACCURACY STANDARDS
I. INTRODUCTION
This section provides a means to classify the estimated accuracy of different GPS data capture efforts. It is a resource
for entities contracting out GPS data collection, responding Contractors or individual users alike. It is supported by the
Guidelines section and provides a common reference for use in classifying different surveys by data precision and
relative accuracy. In turn, these common accuracy classes support the Specifications section that details how a target
accuracy can be achieved.
The level of accuracy desired for each feature type has a direct impact on the time and cost associated with achieving
that accuracy so it is recommended when contracting out GPS data collection that the desired accuracy be carefully
considered in the scoping of the project.
Accuracy standards specify the relative accuracy of positions while specifications detail how the standards can be
met and what rules to follow to meet them. This section specifies positional accuracy while Section C – Content
Specifications details how they can be met. Relative accuracy differs from absolute accuracy in that it is a measure
against a relative position, e.g., a previous GPS point, vs. one that has been established through traditional surveying
methods or surveying grade GPS that “ties into” an established National Spatial Reference System (NSRS) benchmark.
The National Geodetic Survey manages the NSRS. In order to determine the true accuracy for a point taken with a
resource/mapping grade GPS receiver, each point would have to be surveyed using traditional survey methods and that
is beyond the scope of this document. While it is possible to acquire a GPS point on an established benchmark ever
changing satellite geometry, weather conditions and terrain factors negate associating a derived absolute accuracy to
other points taken in a field collection effort.
The accuracy table below provides a common reference, or positional relative accuracy standard, for use in classifying
different surveys by data precision and relative accuracy. This table applies to both vertical and horizontal relative
accuracy so more than one level may be selected for a particular project, especially when the fact that vertical values
are generally about half as accurate as horizontal values captured by resource/mapping grade receivers. With this
table users can choose the relative accuracy requirements they aspire to achieve and subsequent data users are
provided with a good sense of how accurate the data is and how to use it appropriately.
ACCURACY CLASS
ACCURACY CODE
CLASS RANGE
5 decimeter
1
0.201 - 0.50 meters
1 meter
2
0.51 - 1.00 meters
2 meter
3
1.01 - 2.00 meters
5 meter
4
2.01 - 5.00 meters
10 meter
5
5.01 - 10.00 meters
20 meter
6
10.01 – 20.00 meters
Table B- 1 Relative Accuracy Classification Standards
39
II. GENERAL CONCEPTS AND DEFINITIONS
To understand positional accuracy one must look at its individual components: 1) Accuracy and 2) Precision. Whereas,
accuracy is connected to the quality of a result, precision is connected to the quality of the operation used to obtain
the result. For example, a measuring tape that has been crimped or stretch may measure a table top consistently too
short or too long making it report low accuracy (an incorrect absolute measurement) but if the same value was
returned each time then the process of measuring the table can be defined as one of high precision (see Figure B-1
below). Likewise, an x,y coordinate captured for a point by a GPS receiver reporting the same number repeatedly, but
the coordinate isn’t the same as the absolute coordinate for the point is defined as high precision but not highly
accurate.
Accuracy is defined as the proximity of a horizontal coordinate or an elevation to the “true value”. The closer the
approximate value is to the true value the higher its accuracy (see Figure B-3 below). Ultimately, only relative
accuracy can be estimated because the true value or absolute value of a feature requires traditional surveying methods
to acquire.
Statistically speaking, precision measures the tendency of a set of numbers to cluster around the mean of those same
numbers without regard to the true value (Figure B-1). Any method that results in a number close to this mean, e.g., a
GPS point, would be of higher precision.
Common ways to measure precision are via the standard deviation and the root-mean-square (RMS) methods. Many GPS
applications contain an option for outputting the data with a standard deviation value for each feature. These methods
produce a value that estimates the spread or dispersion of individual point fixes around their mean (averaged) or
expected value, reflecting the random error in the individual fixes. These values are useful estimates of precision so
long as the data is unaffected by biases due to blunders or uncorrected systematic effects. By combining precision with
reliability (Quality Control procedures) or precision in the absence of bias, the distance between true and relative
accuracy can be minimized to produce the best possible result.
Figure B-2: Accurate
but not precise
Figure B-1:Precise
but not accurate
F
40
Figure B-3: Precise
AND accurate
III.
GPS ACCURACY STANDARDS
Before continuing acquire the following resources: 1) Geospatial Positioning Accuracy Standards, Part 3: National
Standard for Spatial Data Accuracy (http://www.fgdc.gov/standards/documents/standards/accuracy/chapter3.pdf);
and 2) "Positional Accuracy Handbook: Using the National Standard for Spatial Data Accuracy to Measure and Report
Geographic Data Quality". (http://www.mnplan.state.mn.us/pdf/1999/lmic/nssda_o.pdf).
The different relative accuracy classes listed in Table B- 1 Relative Accuracy Classification Standards provide a
common frame of reference for assessing the reliability of results in a single class of features and also between
different features. In order to instill confidence in these classes it is necessary for the reported values to be validated
by a process that is reproducible. While even recreational receivers have the capacity to store values that provide a
measure of precision, e.g., the root-mean-square (RMS) or Standard Deviation (1 sigma) methods, these values may be
unreliable means of accuracy. When a location is averaged from individual fixes over a short period, i.e., 30-seconds,
the same troposphere effects and other sources of systematic error can affect all of them and impact the precision
values. Alternatives to relying on these single value point measures include comparing a GPS position to a known
benchmark or re-observing points at a later date when sources of systematic errors are bound to be different.
While it is possible to compare a GPS position with a known benchmark to derive an accuracy measurement, that
measurement does not hold true for all points taken with the receiver at all times and under all conditions. It only
holds true for that individual point, at the time it was taken and not for any subsequent point(s) acquired, e.g., taken
five minutes later in a narrow river valley with a northerly aspect and under canopy. Factors affecting the accuracy of
a GPS position, e.g., satellite geometry, troposphere effects, weather, topography etc., are constantly changing and
short of using traditional surveying methods to survey each point using reference benchmarks, there is no way to derive
the true accuracy for the entire field effort.
A more realistic way to estimate the horizontal and/or vertical accuracy attainable by an individual GPS receiver is to
employ a validation procedure where test results are compared against independent control coordinates. Essentially,
random points are selected from the field data and compared against an independent data set. The horizontal
coordinates or vertical values for each set of points are subtracted from the independent points with the results
yielding a consistent reporting value, the National Standard for Spatial Data Accuracy (NSSDA) statistic. While this
statistic has a supporting national standard behind it and represents a credible, consistent approach to the issue of
relative accuracy it is important to understand its limitations. The reported relative accuracy values is subject to the
dynamic nature of GPS error that preclude the accuracy associated with a single point to be fully representative of
other points in the same survey. This is especially true under forest canopy or areas of high topographic relief and less
true in open areas of gentle terrain. Ultimately, the effort of re-observing positions only provides an indication of
accuracy for the entire survey.
This approach is detailed in a set of standards created by the Federal Geographic Data Committee24 (FGDChttp://www.fgdc.gov/fgdc/fgdc.html) in part three of the five part Geospatial Positioning Accuracy Standards, titled
the National Spatial Standards for Data Accuracy (NSSDA) (http://www.fgdc.gov/standards/status/sub1_3.html). The
NSSDA is a reporting standard that describes the usability of data in terms of quality and accuracy, using a consistent
terminology that allows for direct comparison between data sets. Fortunately, these standards have been condensed
into a usable document by the Minnesota Land Management Information Center (MLMIC) titled the "Positional Accuracy
Handbook: Using the National Standard for Spatial Data Accuracy to Measure and Report Geographic Data Quality" (see
links at beginning of this section). This provides a step-by-step set of procedures that results in an accuracy statement
for the data being tested and is a central resource for applying this accuracy standard.
24
The FGDC develops standards to implement the National Spatial Data Infrastructure (NSDI - http://www.fgdc.gov/nsdi/nsdi.html)
41
A. Re-Observation
The NSSDA standard requires that a minimum of 20 check-points be tested independently of the original GPS
survey. These points should be distributed throughout the geographic area of interest and be representative of
the type of error likely to occur in the dataset, e.g., points under canopy, in narrow ravines, north facing
locations, etc. The testing of 20 points allows for one point to fail the target accuracy threshold while allowing
the remainder to be within the 95% confidence level of the target accuracy. While the ideal for this standard is
to acquire the independent data set separately from the test data and that it be three times more accurate,
the practical solution presented in this standard is to re-observe 20 representative points using either the same
GPS receiver, or one capable of higher accuracy, if available, and the same critical settings but collecting more
individual fixes.
To maximize the independence of the re-observed locations they should be taken at least one hour later than
the originals and if possible, conducted by a different individual.
For the sake of clarity, the Contracting Agency should request a project report that contains either a table or
spreadsheet detailing both the original and repeat measurements with a summary showing the percentage of
re-observed points that were within the relative accuracy test level. To meet a relative accuracy target, 95% of
the re-observed points must be within the square root of twice the relative accuracy target squared, e.g.:
Example:
Relative accuracy target:
5m
R e p e a t M e a s u re m e n t T e s t L e v e l = (2 x 5 2 ) = 7 .1 m
QC Test: 95% of the radial distances between separate, averaged observations at the
same point must be less than 7.1m to meet the relative accuracy target of 5m.
Radial distances measure the direct line distance between the original and re-observed averaged points, e.g.,
the mean location of the individual fixes acquired for each point.
For “network survey’s”, i.e., individual points that define a linear features like a road, whole segments of the
network should be re-observed (preferably run in the opposite direction from the original survey). The two
segments can be compared graphically and the separation measured to determine the relative accuracy level.
B. Determining the NSSDA
The NSSDA uses the root-mean-square error (RMSE) method to estimate positional accuracy. RMSE is calculated
by squaring differences between the original and re-observed, independent coordinate values and then
squaring the average value of the differences. Subsequently, the NSSDA statistic “is determined by multiplying
the RMSE by a value that represents the standard error of the mean at the 95 percent confidence level: 1.7308
when calculating horizontal accuracy, and 1.9600 when calculating vertical accuracy.”25
The NSSDA accuracy statistic is reported in ground distances at the 95% confidence level. Accuracy reported at
the 95% confidence level means that 95% of the positions in the dataset will have an error with respect to true
ground position that is equal to or smaller than the reported accuracy value.”
25
MLMIC. Positional Accuracy Handbook. pp. 8.
42
Reporting
This value can be reported in one of two ways, refer to either the FGDC NSSDA standard or the MLMIC’s
Positional Accuracy Handbook26 for more details:
1) “Tested _____ (meters, feet) (horizontal, vertical) accuracy at 95% confidence level”; and
2) “Compiled to meet _____ (meters, feet) (horizontal, vertical) accuracy at 95% confidence level”.
Refer to the Positional Accuracy Handbook for questions on these steps. To generate the NSSDA Statistic
conduct the following steps:
1. Acquire the following resources if you haven't already: A) Geospatial Positioning Accuracy Standards,
Part 3: National Standard for Spatial Data Accuracy
(http://www.fgdc.gov/standards/documents/standards/accuracy/chapter3.pdf); and B) "Positional
Accuracy Handbook: Using the National Standard for Spatial Data Accuracy to Measure and Report
Geographic Data Quality" (http://www.mnplan.state.mn.us/pdf/1999/lmic/nssda_o.pdf).
2. Determine if the test involves horizontal accuracy, vertical accuracy or both.
3. Select a set of test points from the data set being evaluated representative of both the geographical
extent and likely sources of error, e.g., topography, canopy etc.
4. Select an independent data set of higher accuracy, if possible, that corresponds to the data set being
tested.
5. Collect measurements from identical points from each of those two sources.
6. Calculate a positional accuracy statistic using either the horizontal or vertical accuracy statistic
worksheet. (Accuracy statistic worksheets may be downloaded off the Internet from MLMIC's positional
accuracy web page (http://www.mnplan.state.mn.us/press/accurate.html) and clicking on "Accuracy
Statistic Worksheets".)
7. Add any Base Station errors that apply to the statistic before reporting it, e.g., base station accuracy
or baseline error (see below).
8. Prepare an accuracy statement in a standardized report form.
9. Include that report in a comprehensive description of the data set called metadata.
C. Base Station Accuracy
The accuracy of all GPS field data benefits from post-differentially correcting the points with base station files.
There are two minor elements of error involved with base stations that affect the accuracy of postdifferentially corrected data; 1) baseline error; and 2) positional accuracy of the base station. The latter error
is negligible, as all base stations within or in close proximity to Vermont have been surveyed to a high level of
accuracy. These are part of the Continuously Operating GPS Reference Station (CORS) network, managed by
the National Geodetic Survey. Nationally, CORS stations are considered to have a horizontal accuracy of 2 cm,
and a vertical (ellipsoid height) accuracy of 4 cm. Base station positional accuracy becomes a more appreciable
factor when a mobile GPS base station is established in the field, however, this document does not cover the
use of these stations.
Baseline error is a function of an individual units rating, the distance between the GPS receiver in the field and
the base station and has a direct affect upon data accuracy. Though not all receivers provide specification
sheets that detail this issue, all claims of receiver accuracy must have the baseline error added to derive true
receiver accuracy. This value is measured in parts per million (ppm) and generally speaking less expensive
receivers generally have a higher baseline error component than their more sophisticated cousins
See the CORS map (http://www.ngs.noaa.gov/CORS/cors-data.html) for base station locations nationwide.
26
MLMIC. Positional Accuracy Handbook. pp. 7.
43
Summary
To meet the relative target accuracy required by the project it is necessary to re-observe a portion of the original
positions, calculate the NSSDA statistic and add the baseline error to produce the complete reporting accuracy.
The only CORS base station with a one second sampling interval on Long Island is located at MacArthur Airport
(ZNY1) and permits the best flexibility for field work in that it allows the GPS receiver to use any sampling interval
setting (as they are all divisible by “1”). For those of you in Western Long Island and New York City the New Jersey
Institute of Technology (NJI2) may be a viable site.
The CORS stations in the rest of Long Island and surrounding states are desirable where baseline errors from ZNY1
and NJI2 are larger but these stations have a sampling interval of five seconds requiring field collection efforts to
either use sampling intervals in multipliers of five, e.g., 5, 10, 15 seconds etc. or to contend with interpolated
results when using a one second sampling interval. Matching the GPS receiver sampling interval with that of the
base station is the best way to maximize the advantage of post-differential correction and avoid the correctional
base station values from being interpolated in non-multiplier increments. As base stations do sometimes “go down”
and lose periods of base data, throwing off the best laid plans, one option is to always use a one second sampling
interval providing: 1) your unit has ample memory; and 2) any project requirements for occupation time (vs. fixes)
are still met. In other words, no one will contend more fixes acquired for a feature so long as the occupation time
was adhered to.
44
SECTION C – CONTRACTOR SPECIFICATIONS (To aid in contracting out GPS service requests)
I. INTRODUCTION
To aid individuals, public non-profit entities and private sector companies in contracting out or responding to GPS
service requests, the Specifications section, with support from both the guidelines and accuracy standards can be used
to form the technical section of a GPS survey contract. Review of the Guidelines section is recommended prior to using
the Specifications. These specifications contain the rules that convey how the data accuracy standards can be met and
facilitate the standardization of data collection procedures and quality control. As previously mentioned, the
Guidelines provide general background information while the Accuracy Standards establish common target accuracy
classes.
These specifications are presented as a resource for contracting agencies and Contractors alike to facilitate the
collection and integration of high quality GPS data into a variety of data layers where targets for horizontal accuracy
are between .5 – 20m and for vertical accuracy between 1-20m. Reporting for these accuracies is at the 95% confidence
level.
II. TERMINOLOGY
The following definitions and abbreviations are used in this section:
Agency
Contractor
Contract
Administrator
Data Processor
DGPS
Dynamic-mode
Field Operator
Geoid
GPS
GPS Event
GPS Reference
Station
NAD27
NAD83
NAVD88
Static-mode
Supplemental
Traverse
UTM
Agency, Department, Division or other entity administering the Contract.
Corporation, firm, or individual that provides works or services to the Agency under
terms and conditions of a contract.
Agency representative who has authority for issuing and managing the contract and
for receiving the items or services delivered by the Contractor.
A trained employee of the Contractor who performs the calculations to convert raw
field GPS data into processed maps / databases using DGPS procedures and QC
checking / editing.
Differential GPS (i.e. pseudorange code positioning differentially corrected either
post-mission or real-time).
Collection of GPS data while traveling along a linear feature to be surveyed (e.g. a
road or watercourse).
An employee of the Contractor who performs the field portion of the data
collection.
The equipotential surface approximating Mean Sea Level.
Global Positioning System as operated by the United States Department of Defense
(US DoD). Also called NAVSTAR.
A GPS Event is a single position instead of a group of positions averaged to a single
position (i.e. Static survey). Events are typically used when the antenna cannot, or
need not, be stationary over a point.
A GPS receiver located at a known location collecting data continuously to be used
for correcting field data (either in real-time or post-mission). Also known as a base
station.
North American Datum of 1927, based on the Clarke 1866 ellipsoid.
North American Datum of 1983, based on the Geodetic Reference System 1980
(GRS80) ellipsoid.
The North American Vertical Datum of 1988; vertical control datum established in
1991
Collection of GPS data at a discrete point while remaining stationary.
Supplemental Traverses are conventional traverses (e.g. compass and tape) that are
integrated with GPS surveys.
Universal Transverse Mercator projection (map projection system).
45
The statements in this document have been structured according to two levels of compliance:
required
recommended
Used to describe tasks that are deemed necessary and are good practice. Exceptions
are possible, but only after careful consideration by the Contracting Agency.
Used to describe tasks that are deemed desirable and good practice, but are left to
the discretion of the Contracting Agency. In some cases, cost is a large factor in
recommended tasks vs. requiring them even if they are desirable.
III. GOALS
™ To establish achievable levels of accuracy by task, and to classify the surveys to be performed by end
specifications aimed at achieving target accuracies.
™ To provide a technical document for individuals, agencies or the private section to use in contracting GPS
related services.
™ To provide users with a consistent set of methods that can be used at the individual or agency level that allows
results to be easily integrated.
™ To qualify a GPS Contractors’ equipment, methods, and employees to ensure target accuracies are achievable
under various conditions.
IV. PRE-QUALIFICATION AND VALIDATION
A.
Total System
It is required that any Contractor expecting to undertake GPS data collection be prepared to fulfill the
requirements of the full “System”, including: GPS hardware and software for field and office; field and GPS
Reference Station receivers (when applicable); and reporting techniques. All parts of the System are to be capable
of meeting the contractual specifications below.
B.
Field Operator Training
It is recommended that Field Operator(s) be qualified through a GPS training course provided by an established and
reputable company, agency or organization.
C.
Data Processor/Project Manager Training
It is required that Data Processor/Project Manager(s) have an established track record in the planning,
management and implementation of GPS projects. It is recommended that the background include the capture,
processing and management of GPS data.
D.
Contractor Validation
For large or extremely important GPS efforts it is required that the GPS System used prove its ability to meet the
accuracy targets through a validation survey. Subsequent to determining validation accuracies and the conditions
under which they were achieved, the results should apply to all subsequent fieldwork. For a large enough project
the validation exercise could simultaneously provide an additional means of assessing a Contractors proposal.
However, this approach should be considered carefully as it will add an extra component to proposal estimates and
may put downward pressure on proposal submittals. See Section C.V Validation Surveys.
46
V. VALIDATION SURVEYS
Due to the nature of GPS technology there is no easy way to detect outright blunders or to balance random errors
homogenously throughout a survey. A skilled operator can certainly stack the odds in their favor for reducing blunders
and errors but short of tying each GPS point to a known benchmark through traditional survey methods, there is no way
to precisely assess accuracy. While using tradition methods to survey each point would negate the cost and efficiency
advantages of using GPS, there is a middle ground solution that can be very useful in pre-qualifying GPS Contractors if
the project is big enough, important enough and worth the extra costs incurred.
A sample Contractor GPS Contractor Report resides in Appendix J of this document to assist Contractors in complying
with a validation survey, if required. The report contains the minimum information required but Contractors may
provide additional analytical information, if practical to do so, on additional page(s). If a validation survey is not
required for pre-qualification by the Contracting Agency it is still required for the actual fieldwork and must
accompany the deliverables. Regardless, the parameters outlined in the specifications must be followed for both prequalification and contracted fieldwork.
A validation survey simply compares the coordinate values of points acquired by the Contractor with known values. The
Contracting Agency can establish a “Test Range” with either point, line, and area features located to simulate field
conditions, e.g., canopy, steep terrain etc. where Contractors acquire location that are compared to highly accurate
feature coordinates acquired using traditional methods. If practical, it is recommended that at least two of these
points be benchmarks from the National Spatial Reference System (NSRS). Horizontal and vertical values must be
tested with horizontal or vertical benchmarks, respectively. Needless to say the Test Range requires qualified
personnel to establish and evaluate in order to ensure that the trials are fair and scientifically defensible. When
evaluating test results don’t neglect practical, non-accuracy related considerations, e.g., if the target accuracy is 5m,
then the small firm with less expensive equipment acquired 3.5m accuracy vs. the large firm with 1.5m accuracy may
be able to do the job more economically and just as well.
Once a Contractor’s system has been validated for a certain accuracy level, they may be exempt from future validation
requirements if key components and conditions affecting their GPS system are unchanged; 1) Key Personnel (Project
Manager, Data Processor); 2) Type of rover hardware; 3) Processing software (type and version number); 4)
Observational parameters such as DOPs, SNR, and elevation masks; 5) Separation distances between Reference Station
and rover; and 6) Number of epochs (fixes) averaged at static points.
Some practical considerations when deciding to utilize a validation survey requirement include:
™ It would be unreasonable to require this for a small project unless the features being collected are
extremely important and accuracy is a premium.
™ Firms located further away from the test site may be at a competitive disadvantage than companies
located nearby.
™ The test should take no longer than one day to complete including reasonable, round trip driving times.
™ No one can operate a business at a loss so it is unreasonable to expect the cost of this extra level of
effort will not be reflected somewhere in a Contractors proposal. Those that don’t charge it up front
may simply charge a higher hourly rate.
™ Contractor results should be within both the horizontal and/or vertical target accuracies, if applicable.
VI. PRE-FIELDWORK PROCEDURES
A.
Proposal Meeting
It is recommended the Contract Administrator conduct a meeting upon release of a project Request for Proposals
(RFP) to clearly define the feature(s) to be surveyed, to identify “High-Significance” from “Standard-Significance”
points (if applicable), project extent and guidelines for interpretation of special features. In addition, this meeting
will provide a clear definition of deliverables, services, work quality, payment schedule, and other relevant
contract issues to minimize confusion of the nature and quantity of work expected. It is also important to establish
the Quality Control and Assurance schedule at this point.
47
B.
Auditing
It is recommended the RFP clearly detail the Auditing process, including the frequency and methods of the
data/field inspections, as well as, the use of independent GPS or other surveys to be used in assessing accuracy
compliance with the contract.
C.
Field Inspection
Subsequent to project award, it is recommended the Contract Administrator conduct a field inspection with the
Contractor to reiterate details regarding the nature and scope of work detailed in the contract.
D.
Reference Markers
When physical reference markers are required to detail project specifics, it is required that the interval and type
of markers be stated in the contract, making use of any pre-existing Agency guidelines or requirements.
E.
Map Ties
It is recommended that all projects include a sufficient number of map ties to allow for accurate geo-positioning
and reliability checks. Map Ties should be readily visible from the air, e.g., the Vermont Mapping Program’s digital
Orthophotography. Good candidates include stream junctions, road intersections, baseball diamonds or other
publicly accessible, readily visible features. The signed contract should detail the number, location and nature of
the tie points.
F.
Legal Boundaries
GPS technology cannot be used to legally define parcel boundaries in New York unless the operator is a licensed
land surveyor as defined by New York Statute. This in no way precludes boundaries from being captured with a GPS
receiver by anyone, subject to permission by the land owner, but the results simply can’t be used in any legal
proceedings unless they are certified.
G.
Required Survey Accuracies
Target accuracies (at the 95% confidence level) for the project are:
m
m
Interpretative Horizontal Accuracy =
Interpretative Vertical Accuracy =
(Class =
(Class =
)
)
Refer to Section B.1.Table B- 1 Relative Accuracy Classification Standards for determining the Class code to insert in the above.
The target accuracy is defined by having at least 95% of the individual position fixes within the above-specified accuracies of the
true position of the point. For a GPS traverses done in dynamic linear mode, at least 95% of the individual GPS position fixes
must be within the specified accuracies from the line’s true position.
VII. FIELDWORK
A.
Critical Rover Settings
™ The receiver will be set to only record observations using a minimum of four (4) satellites, e.g., “over
determinate 3D” mode.
™ The minimum satellite elevation angle/mask for the field GPS receiver is 15 degrees above the horizon.
™ It is required the maximum Signal-to-Noise Ratio be _________.
™ It is required that the DOP not exceed the following values:
DOP Component
Geometrical DOP (GDOP)
Positional DOP (PDOP)
Horizontal DOP (HDOP)
Vertical DOP (VDOP)**
Maximum DOP Value Allowed*
48
*Not all DOP values are required to be completed.
**VDOP limits are only required when accurate elevations are required
B.
Data Collection
™ During Static (point-mode) surveys, it is required that the feature be occupied according to the minimum
values below, or the values used during the Validation survey, which ever is higher.
Point Significance
Minimum Occupation
Time (sec)
Minimum Number
of Fixes
Standard-Significance
Point
High-Significance
Point
™ It is required that position fixes being mapped statically for linear features (i.e. static or point-to-point
traverses) not be greater than _______meters apart. Capture the traverse points according to the specs
outlined for Standard Significance Points.
™ It is required that position fixes being mapped dynamically for linear features in a dynamic traverse not be
greater than ______ meters apart.
™ It is required that both ends of a dynamic traverses be captured to the specs outlined for High-Significance
points. These can be referred to as either the Point of Commencement (PoC) or the Point of Termination
(PoT).
™ It is required that any deviations in an otherwise straight line, point-to-point traverse must be mapped
regardless of the minimum separation between points detailed above. This also applies to significant
vertical breaks if elevations are required.
™ Interpolated points – e.g., GPS Events are recommended to be accurate within ______ seconds.
™ Point offsets - The following is required to be recorded:
™ (see Section A: IV.3.C.8.Table IV-5 Offset Accuracy vs. Instrumentation Precision & Offset Distance for
related information):
™ The vertical angle from the GPS antenna to the feature. Many compasses also include an inclinometer for
this purpose.
™ If not automatically set, magnetic declination must be factored into any compass readings before
computing offset coordinates. See the magnetic declination calculator
(http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp) at the National Geophysical Data Center.
™ The maximum distance allowed for a point offsets is ______ meters.
™ Bearings accuracy must be at least ______ degrees
™ Distance accuracy must be at least ______ meters.
™ Linear offsets – The following is required:
™ The horizontal distance and the true bearing to the direction of travel.
™ The maximum horizontal distance allowable is ______ meters.
™
™
™
™
For supplemental traverses it is required that:
The PoC and PoT physically marked end points must be High-Significance GPS static points.
The distance traversed is to be less than _______ meters.
The traverse close between the end points by ________________ of the linear distance traversed.
49
™ The traverse must be balanced between the end points by an acceptable method (i.e., compass rule
adjustment).
™ If applicable, physical reference markers must be established at an interval of ______ meters along linear
features. Enter “N/A” if this doesn’t apply. If the Contracting Agency has standards for reference markers
they will be used unless other standards are agreed to.
™ It is required that physical reference markers have static point features collected as STANDARD / HIGH
(circle one) Significance points.
™ The maximum allowable SNR mask CAN / CANNOT (circle one) be relaxed during a linear traversing.
VIII. GPS BASE STATION
It is highly recommended that users employ a CORS station either on Long Island, or one of the CORS stations
in the two neighboring states. Temporary GPS Reference Stations established by the Contractor are not
covered in the scope of the GPS guidelines document.
™ It is required that the baseline distance between the CORS stations and the field receivers be reported in miles
______. If the project area covers a large geographic extent (greater than 10 miles in either direction) then
this value should be broken down to minimum and maximum baseline distances. If the baseline distance is
greater than that the distance present during Validation, and the validation accuracy was border line to the
target accuracy then the Contractor must detail how the target accuracy will be met with the increase in
baseline error.
™ It is recommended that the minimum elevation angle/mask of the GPS Base Station be 10 degrees. This is the
default setting for CORS stations.
™ If real-time corrections are used, it is required that the Total Correction Age of the rover GPS system not
exceed __15__ seconds. The larger the delay between the base station files used to correct the real time
position, the larger the error introduced.
IX. PROCESSING AND QUALITY CONTROL
™ All GPS positions are to be corrected by standard differential GPS methods (pseudorange or navigation
corrections). If navigation corrections are used, the same GPS satellites must be used by the GPS Reference
Station and the receiver for all corrected positions.
™ If the GPS receiver and/or post-mission software provides the option for dynamic filtering, it is recommended
the filters be set to reflect the speed of the operator or vehicle, and the software versions and filter settings
are to be noted in the project returns.
™ It is required that the Contractor implement a Quality Control (QC), or reliability assessment, program in order
to show compliance to specified guidelines or standards (i.e. positional accuracy, content accuracy,
completeness, data format adherence, and data integrity assurance).
™ It is required that the Contractor be prepared to entirely re-survey those areas that do not meet the
compliance standard at their own cost.
X. PROJECT MANAGEMENT and DELIVERABLES
Effectively managing the volume of data produced in a GPS project is critical for ensuring its future usability,
especially when a majority of the data represents raw, intermediary or supporting data. It is recommended that the
50
Contracting Agency require all raw, intermediary or supporting digital data be retained by the Contractor and included
on digital media in the deliverables.
This section details deliverable specifics present in the GPS contract including content, file format and media. It also
describes requirements for managing and archiving data. In the absence of special requirements by the Contracting
Agency these guidelines should be followed as closely as possible.
A.
PROJECT REPORT
The elements of the recommended project report are identical to those detailed in the GPS Contractor
Validation Report created during the pre-qualification survey. If a validation survey was not required by the
Contracting Agency, the Contractor may still prefer to use the SAMPLE GPS CONTRACTOR REPORT located in
Appendix J to fulfill these report requirements.
It is recommended that the Contractor submit a project report including the following information:
™ “A brief description of the project work (i.e. purpose, target accuracy, location, etc.).
™ A brief description of the Contract particulars, including the Contracting Agency that commissioned the
work; the Contract Coordinator; a project name (if available) and a project identifier.
™ A listing of all personnel (Contractor and Subcontractors) involved in the project detailing their
particular duties and background (i.e. their educational background; formal GPS training details
(courses with dates); their experience on similar projects, etc.). This could be a copy of what was
provided with the pre-qualification package.
™ A key map showing the project area and a description of any GPS Reference Stations used.
™ A description of the GPS Reference Stations used.
™ If using a temporary GPS Reference Station the issue of validating the GPS Reference Station will also
have
to be resolved (i.e. a GPS reference Station validation will have to be submitted).
™ A schedule of events showing key dates (contract award, field data acquisition, data processing, and
submission of the results, etc.).
™ A list of all hardware and software used on the project; including but not limited to:
o GPS hardware (i.e. models, receivers numbers, data loggers, antennas, firmware versions, etc.);
o GPS software (i.e. name, version number, settings, etc.);
o mapping software (i.e. name, version number, settings, etc.); and
o utility software (i.e. name, version number, settings, etc.).
™ A summary of the project including planning, field data collection methods and parameters (i.e. GPS
receiver settings/defaults), data processing methods and parameters (i.e. post-processing
settings/defaults), any project problems, anomalies, deviations, etc.
™ A summary of the results, including repeatability test details.
™ An explanation of the deliverables (digital and hard copy) including formats, naming conventions,
compression utilities, media, etc.
™ A copy of all field notes (digital or hard copy).
™ A list of all features that have been mapped or surveyed.”27
B. HARD COPY PLANS
If the Contracting Agency requires a final hard copy map then the media, scale, datum etc. must conform to
Agency cartographic standards, if applicable, as outlined in the contract and presented with other deliverables.
Providing the Contractor with a “map template” is the easiest way to achieve this.
The following map components are suggested:
™ General project information in text boxes: project title; project number/identifier; Contracting Agency
name; Contractor name; and date of survey.
™ Datum, projection and units of measure, e.g., NAD83 ft.
27
British Columbia Standards, Specifications and Guidelines for Resource Surveys Using GPS Technology. pp. C-8
51
™
™
™
™
Scale bar
North arrow with either or both True North and Magnetic North.
Graticules, if requested, e.g., 1,000 or 10,000 intervals.
Source information for non-GPS data, e.g., Roads or Surface Water data.
It is required that the accuracy of GPS acquired data be stated on the map.
C. GPS DATA AND PROCESSING DELIVERABLES
It is required that all raw rover files, originally corrected and interpreted (originally corrected with edits) GPS
data and base station sampling files be kept for archive and Quality Assurance (QA) purposes in their original
format. The raw GPS data is required to be stored in the manufacturer’s original, proprietary format. It is
acceptable to supply the one-hour block Base Station files merged for the time extent of the daily rover data files.
The originally corrected GPS data is raw data post-differentially corrected with base station sampling files prior to
any averaging, generalizing, filtering or editing, e.g., interpreted GPS data.
Data collected with customized data dictionaries that have GIS feature and attribute information may not be
supported by the current RINEX format. In this situation, the manufacturer’s proprietary format is required to
preserve the integrity of the data.
It is required that digital data be submitted on the storage media and format required by the Contracting Agency.
Table X-1 below details the data required for submittal by the Contractor. See the respective Guidelines sections
for details on these different data.
Deliverables
GPS Base Station Data
Raw Field GPS Data
Original Corrected GPS Data
Final Interpreted GPS Data
Format
DAT, SSF, or RINEX
DAT, SSF, or RINEX
Datum
WGS84
WGS84
NAD83
NAD83
Notes
Merged if possible
Originally downloaded
Unedited
Edited
Table X-1: Digital Deliverables
If the Agency requires any other local datum, the methods used to transform the data are to be explicitly described
in the project report and approved by the Agency.
D.
DATA OWNERSHIP
All project related data and submitted deliverables are the property of the Contracting Agency and access to
project data prior to delivery, by the Contract Manager is required to be honored upon request. All the documents
submitted to state, regional or local government entities will be subject to the disclosure provisions of state
statutes governing the access to public records.
E.
QUALITY ASSURANCE
All data submitted by the Contractor shall be validated by the Contracting Agency following guidelines in Section
A.V.E Quality Assurance and Audit before integration with existing databases.
52
F. DATA MANAGEMENT AND ARCHIVING
It is highly recommended that the Contracting Agency archived the GPS base station data, raw field GPS data,
original corrected GPS data and final interpreted GPS data in a consistent and organized manner to ensure ready
access by the Agency itself or any project partners in case of questions about the features or their accuracy. Each
Contracting Agency office must establish their own system for managing and archiving the deliverables. This is
essential as the deliverables can present a large volume of data that can be difficult to use reliably and effectively
if they are not stored in an organized manner.
G. DIGITAL MEDIA
The GPS deliverables and their archive should be stored on stable media, e.g., CD-ROM, DVD, backed up hard
drives etc. It is recommended the Contracting Agency integrate specific project information into an existing data
retrieval system of consider devising one that, at a minimum, affords quick access to basic project information,
e.g., project name, Contracting Agency, Contractor, map reference, file names, formats, significant dates,
physical storage location, etc.
The Contracting Agency will be responsible for transferring the data to archive quality media.
XI. TECHNOLOGICAL/PERSONNEL CHANGE
™
If significant changes occur to the Contractor’s GPS system components (i.e., hardware, firmware,
software, methodology, etc.) or personnel during an active contract, it is recommended the Contractor
consult with the Contract Administrator. A decision will be made as to whether the Contractor GPS System
Validation and/or personnel qualification be reevaluated.
It is required that the Contractor and the Contract Administrator ensures that the most current versions of the
GPS Data Collection Guidelines for Suffolk County, NY are used.
53
XII. METATADATA GUIDELINES
Simply defined, metadata is “data about data”, or information that describes the characteristics of a GIS data
set. In describing a GIS data set, metadata usually provides information about its content and origins; it may
also be used to track the updates, corrections or changes to a data set. In addition, metadata should also
contain distribution information, which explains how a potential user can acquire the data set.
Metadata, created and updated according to the Federal Geographic Data Committee (FGDC)28 standards is
important and valuable. It is required that metadata accompany all data collected with GPS as it:
• Maintains the value of the data set over time;
• Preserves the data description (e.g. origin, format, use, purpose.)
• Allows users to search for and use existing geospatial data and contributed to an NSDI
Clearinghouse (such as the NYS GIS Clearinghouse).
28
“Content Standard for Digital Geospatial Metadata,” 20 Dec. 2006 <http://www.fgdc.gov/standards/projects/FGDCstandards-projects/metadata/base-metadata>
54
Appendix A – Glossary of Useful Terms
Accuracy
The degree of conformity with a standard or accepted value. Accuracy relates to the quality of the result, and is
distinguished from precision which relates to the quality of the operation by which the result is obtained.
Autonomous Positioning
The least precise form of positioning that a GPS receiver can produce. The position fix is calculated in real time from
satellite data alone. Autonomous positions are generally accurate to within 10 meters.
Base station
A base station is comprised of a GPS antenna and GPS receiver positioned at a known location specifically to collect
data for differential correction. The purpose of the base station is to provide reference data for performing
differential correction on data collected in the field. Base data need to be collected at the same time as you collect
data with a GPS rover receiver. A base station can be a permanent installation that collects base data for provision to
multiple users, or a GPS rover receiver that you temporarily locate on known coordinates for the duration of a specific
project or datalogging session.
BlueTooth
A wireless technology capable of using short-range radio technology for Internet and mobile devices, aimed at
simplifying communications among them. Some GPS receivers use Bluetooth to communicate with the datalogger.
Carrier Phase
The difference between the carrier signal generated by the internal oscillator of a roving GPS receiver and the carrier
signal emitted from a particular GPS satellite.
Coarse/Acquisition (C/A) Code
A pseudorandom noise code (PRN) modulated onto a L1 signal which helps the GPS receiver to compute the distance
from each satellite. Specifically, the difference between the pseudorandom number code generated by the GPS rover
software and the pseudorandom number code coming in from the satellite is used to quickly compute the distance to a
satellite and therefore calculate your position.
CORS (Continuously Operating Reference) Station
A network of GPS base stations coordinated by the National Geodetic Survey for the purpose of providing GPS reference
data to permit end users to perform post-processed differential correction of GPS data collected with roving GPS
receivers. Reference data are typically acquired via direct download from the Internet.
Data Dictionary / Feature Library
A term used to describe the schema, or structure, that defines the relationship between features and their descriptive
attributes that will be located in the field with a professional GPS receiver. This description typically includes feature
name(s), data type classification (point, line, or polygon), attribute names, attribute types, and attribute values. After
being created on a PC, a data dictionary is transferred to a GPS datalogger and used when collecting data in the field.
Data Message
A message included in the GPS signal, which reports a satellite’s location, clock correction, and health. It includes
information on other satellites’ health and their approximate positions.
Datum
A mathematical model of the earth’s surface. World geodetic datums are typically defined by the size and shape of an
ellipsoid and the relationship between the center of the ellipsoid and the center of the earth. Because the earth is not
a perfect ellipsoid, any single datum will provide a better model in some locations than others. Therefore, various
datums have been established to suit particular regions. For example, maps in the United States are often based on
the North American datum of 1927 (NAD-27) or 1983 (NAD-83). All GPS coordinates are based on the WGS-84 datum
surface.
55
Datum Transformation
A mathematical calculation that converts the coordinates of a position in one datum to coordinates in terms of another
datum. Two types of datum transformations are supported by most professional grade GPS data collection and
management software: three parameter and seven parameter. A datum transformation is used when the GPS results
are required in terms of a local datum.
Declination
See magnetic declination.
Differential Correction
The process of correcting GPS data collected on a rover with data collected simultaneously at a base station. Because
it is on a known location, any errors in data collected at the base station can be measured, and the necessary
corrections applied to the rover data. Differential correction can be done in real time, or after the data has been
collected by post processing.
Dilution of Precision (DOP)
An indicator of the quality of a GPS position, which takes account of each satellite's location relative to the other
satellites in the constellation, and their geometry in relation to the GPS receiver. A low DOP value indicates a higher
probability of accuracy.
Standard DOPs for GPS applications are:
PDOP – Position (three coordinates)
HDOP – Horizontal (two horizontal coordinates)
VDOP – Vertical (height only)
TDOP – Time (clock offset only)
Dual-frequency (GPS) Receiver
A type of GPS receiver that uses both L1 and L2 signals from GPS satellites. A dual-frequency GPS receiver can compute
more precise position fixes over longer distances and under more adverse conditions by compensating for ionospheric
delays.
Earth Centered, Earth Fixed (ECEF)
A Cartesian coordinate system used by the WGS-84 reference frame. The center of the system is at the earth’s center
of mass. The z axis is coincident with the mean rotational axis of the earth, the x axis passes through 0×N and 0×E, the
y axis is perpendicular to the plane of the x and z axes.
EGNOS (European Geostationary Navigation Overlay Service)
A satellite-based augmentation system (SBAS) that provides a differential correction service for GPS users in Europe.
EGNOS is the European equivalent of WAAS, which is available in the United States.
Elevation Mask
The angle above and relative to the horizon, below which your GPS rover will not track satellites. It is normally set to
15º to avoid interference problems caused by buildings and trees and multipath errors and avoid the rover GPS receiver
using a GPS satellite that the base station is not tracking.
Ellipsoid
An ellipsoid is the three-dimensional shape that is used as the basis for mathematically modeling the earth’s surface.
The ellipsoid is defined by the lengths of the minor and major axes. The earth’s minor axis is the polar axis and the
major axis is the equatorial axis.
Ephemeris
The current satellite position predictions that are transmitted from a GPS satellite in the NAVDATA message.
Epoch
The measurement interval of a GPS receiver.
56
Geoid
A mathematical surface of constant gravitational potential that approximates sea level (See Mean Sea Level, below).
Or, the equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level.
Image Source: National Geodetic Survey
Global Positioning System (GPS)
The generic term used to describe the satellite-based timing and positioning system operated by the United States
Department of Defense (DoD).
Grid North
The meridian of any particular grid that is referenced to true north.
Height Above Ellipsoid (HAE)
Distance (h) above the reference ellipsoid. HAE is always measured orthogonal to the ellipsoidal surface. Three
dimensional GPS positions reference HAE. Recreational grade GPS receivers calculate approximate orthometric height
(elevation) for the user.
57
Image Source: National Geodetic Survey
Horizon
The line at which the earth and sky seem to meet for any particular observer.
Horizontal Dilution of Precision (HDOP)
See DOP.
L1
The primary L-band carrier used by GPS satellites to transmit satellite data. The frequency is 1575.42 MHz. It is
modulated by C/A code, P-code and a 50 bit/second navigation message.
L2
The secondary L-band carrier used by GPS satellites to transmit satellite data. The frequency is 1227.6 MHz. It is
modulated by P-code and a 50 bit/second navigation message.
Latitude
An angular measurement made from the center of the earth to north or south of the equator. It comprises the
north/south component of the latitude/longitude coordinate system, which is used in GPS data collection.
Traditionally, north is considered positive, and south is considered negative. Example: 43º south of the equator may be
expressed as either unsigned (-43º) or signed (43º S)
Longitude
An angular measurement made from the center of the earth to the east or west of the Greenwich meridian (London,
England). It comprises the east/west component of the latitude/longitude coordinate system, which is used in GPS data
collection. Traditionally, east is considered positive, and west is considered negative. Example: 74º west of the
Greenwich meridian may be expressed as either unsigned (-74º) or signed (74º W)
Magnetic Declination
The local angular difference between magnetic and true north. Declination is expressed as a positive or negative
angle, and varies by location and over time. In New York State, declination values range from approximately -10
degrees in western Chautauqua County to -15 degrees in northeastern Clinton County.
58
Image Source: National Oceanic and Atmospheric Administration (NOAA)
Magnetic North
The direction of the north-seeking end of a magnetic compass needle, not subject to transient or local disturbance
(Definitions of Surveying Terms Prepared by a joint committee of the
American Congress on Surveying and Mapping and the American Society of Civil Engineers 1978)
Map Projection
A defined method of transforming positions defined on an ellipsoid to a map grid; for example, the Transverse Mercator
and Parallel Lambert projections.
Mean Sea Level (MSL)
The average height of the surface of the sea at a tide station for all stages of the tide over a 19-year period, usually
determined from hourly height readings measured from a fixed predetermined reference level.
Metadata
Simply defined, metadata is “data about data”, or which information which describes the characteristics of a GIS data
set. In describing a GIS data set, metadata usually provides information about its content and origins; it may also be
used to track the updates, corrections or changes to a data set. In addition, metadata should also contain distribution
information, which explains how a potential user can acquire the data set.
59
Minimum Elevation
See Elevation Mask
Multipath
Interference, similar to ghosts on a television screen, which occurs when GPS signals arrive at an antenna after
traversing different paths. The signal traversing the longer path will yield a larger pseudorange estimate and increase
positional error. Multipath occurs when GPS signals reflect off a surface before reaching the GPS antenna.
NAVDATA
The Navigation Message broadcast by each GPS satellite on both the L1 and L2 transmitters. This message contains
system time, clock correction parameters, ionospheric delay model parameters, and the satellite vehicle’s ephemeris
and health. A GPS receiver uses this information to process GPS signals and thus obtain user position and velocity.
NAVigation Satellite Timing And Ranging (NAVSTAR) System
The formal name given to the United States Department of Defense’s navigation and timing system comprised of GPS
satellites, monitoring stations, and Master Control Station.
P-Code
The precise code transmitted by the GPS satellites. Each satellite has a unique code that is modulated onto both the L1
and L2 carrier waves. The P-code is replaced by a Y-code when Anti-Spoofing is active.
PDOP Mask
The highest level of PDOP that will allow the GPS receiver to compute a fix. For example, if the PDOP Mask is set to
(6), the GPS receiver will not record a location when the PDOP exceeds (6).
Position Dilution of Precision (PDOP)
A unitless figure of merit expressing the relationship between the error in user position and the error in satellite
position. Values considered good for positioning are small, such as 3. Values greater than 7 are considered poor. PDOP
is related to horizontal and vertical DOP by the following formula: PDOP² = HDOP² + VDOP². See also DOP.
Postprocessing (Differential Correction)
The processing of satellite data after it has been collected in order to eliminate error. This involves using PC software
to compare data from the rover to data collected at the base station. Because the base station is on a known location,
systematic errors can be determined and removed from the rover data.
Precision
A measure of the repeatability or uniformity of a measurement. Precision relates to the quality of the operation by
which the result is obtained, and is distinguished from accuracy which relates to the quality of the result. In order to
comply with a specific standard, accuracy results must meet the minimum while complying with the precision required.
Obtaining suitable accuracy results without complying with the precision is not acceptable to meet the standards.
Pseudorandom Noise or Number (PRN)
A signal that carries a code that appears to be randomly distributed like noise, but can be exactly reproduced. PRN
codes have a low auto-correlation value for all delays or lags, except when they are exactly coincident. Each NAVSTAR
satellite has its own unique PRN code.
Radio Technical Commission for Maritime Services (RTCM)
A commission established to define a differential data link for real-time differential correction of roving GPS receivers.
There are two types of RTCM differential correction messages. Most modern GPS receivers use the newer Type 2.2
RTCM protocol.
Real Time (Differential Correction)
The processing of satellite data as it is being collected in order to eliminate error. This involves using software to
compare data from the rover to data collected at the base station. Because the base station is a known location,
systematic errors can be determined and removed from the rover data as it is being logged. This correction is not
instantaneous and adequate time on station should be planned for accurate readings. Users should consult the
60
manufacturers’ guidelines for their specific hardware for recommended time on station. Two free systems offering
real time differential correction capabilities include the United States Coast Guard (USCG) beacon system and the
WAAS system. The USCG beacon system has a greater accuracy than WAAS and is more reliable. See Time on Station.
Reference Station
See Base station.
Root Mean Square (RMS)
An expression of the accuracy of a point measurement. It is the radius of the error circle, within which approximately
68% of position fixes are to be found. RMS is typically expressed in distance units of feet or meters.
Rover/Roving (GPS) Receiver
Any mobile GPS receiver and data collector used for determining location in the field. A roving GPS receiver’s position
can be differentially corrected relative to a stationary base GPS receiver.
RTK (Real-Time Kinematic)
A real-time differential GPS method that uses carrier phase measurements for greater accuracy. RTK measurements
typically yield relative horizontal accuracy of approximately one centimeter.
SBAS (Satellite Based Augmentation System)
The generic term that refers to differential GPS applied to a wide area, such as an entire continent. WAAS and EGNOS
are examples of SBAS networks, and are comprised of a series of reference stations that generate GPS corrections
which are broadcast to GPS rovers via geostationary satellites.
Selective Availability (SA)
The artificial and deliberate degradation of GPS satellite signals by the United States Department of Defense. Selective
Availability was implemented in order to enhance national security, but was turned off on May 10, 2000 due to the
presence of several sources of various differential correction (DGPS) messages, which rendered SA obsolete. The SA
bias on each satellite signal is different, and so the resulting position solution is a function of the combined SA bias
from each satellite used in the navigation solution. Because SA is a changing bias with low frequency terms in excess
of a few hours, position solutions or individual satellite vehicle pseudo-ranges cannot be effectively averaged over
periods shorter than a few hours. Differential corrections must be updated at a rate less than the correlation time of
SA (and other bias errors). 29
Signal-to-Noise Ratio (SNR)
The signal strength of a satellite is a measure of the information content of the signal, relative to the signal’s noise.
The typical SNR of a satellite at 30° elevation is between 47 and 50 dBHz. The quality of a GPS position is degraded if
the SNR of one or more satellites in the constellation falls below 39. This value is used to determine whether the
signal strength of a satellite is sufficient for that satellite to be used by the GPS receiver. If a satellite’s SNR is below
the configured minimum SNR, that satellite is not used to compute positions.
SV
Satellite Vehicle or Space Vehicle, referring to the actual GPS satellite.
Time Dilution of Precision (TDOP)
See DOP.
Time on Station
The amount of time needed to be at a location in order to accurately collect an X,Y value per the project
requirements.
29
“Global Positioning System Overview,” 20 Dec. 2006 <http://www.colorado.edu/geography/gcraft/notes/gps/gps.html#SA>
61
True North
A term used to define 1) an astronomic meridian; 2) a geodetic meridian; 3) the direction of north from magnetic north
corrected for declination; 4) the meridional direction assumed in a survey description; 5) the cardinal directions run in
the Public Land Survey. Since the term is subject to several interpretations it should not be used (Definitions of
Surveying Terms Prepared by a joint committee of theAmerican Congress on Surveying and Mapping and the American
Society of Civil Engineers 1978)
Vertical Dilution of Precision (VDOP)
See DOP.
VRS (Virtual Reference Station)
A VRS system consists of GPS hardware, software, and communication links. It uses data from a network of base
stations to provide corrections to each rover that are more accurate than corrections from a single base station. To
start using VRS corrections, the rover sends its position to the VRS server. The VRS server uses the base station data to
model systematic errors (such as ionospheric noise) at the rover position. It then sends RTCM correction messages back
to the rover.
WAAS (Wide Area Augmentation System)
WAAS was established by the Federal Aviation Administration (FAA) for flight and approach navigation for civil aviation.
WAAS improves the accuracy and availability of the basic GPS signals over its coverage area, which includes the
continental United States and outlying parts of Canada and Mexico. The WAAS system provides correction data for
visible satellites. Corrections are computed from ground station observations and then uploaded to two geostationary
satellites. This data is then broadcast on the L1 frequency, and is tracked using a channel on the GPS receiver, exactly
like a GPS satellite.
Waypoint
A geographical point that, unlike a feature, holds no attribute information beyond a name and location. Typically,
waypoints are used to denote objects or locations of primary interest, such as a survey mark. Waypoints are most often
used for navigation.
WGS-84
World Geodetic System (1984), the mathematical ellipsoid used by GPS since 1984. See also Ellipsoid.
62
Appendix B - Useful GPS and Related Websites
GPS TRAINING AND INFORMATION RESOURCES
1.) General GPS Information
• Trimble GPS Support and Updates (Terra Sync, Pathfinder Office, GPS hardware, etc.) www.trimble.com\support
• ESRI Support for ArcPad/ GPS Analyst Extension - www.support.esri.com
• ArcPad Blog – http://arcpadteam.blogspot.com
• Long Island GIS – www.ligis.org
• National Geodetic Survey (NGS) – Continuosly Operating Reference Stations (CORS) –
http://www.ngs.noaa.gov/CORS/
• Historical Maps - www.historicmapworks.com
• Library of Congress Maps - http://memory.loc.gov/ammem/gmdhtml/
• Glossary of Terms - http://www.novatel.com/about_gps/glossary.htm
• GPS Information - http://gpsinformation.net/
• United State Coast Guard Navigation Center - http://www.navcen.uscg.gov
• US Naval Observatory (USNO) GPS Operations - http://tycho.usno.navy.mil/gps.html
• Positioning, Navigation, and Timing - http://www.pnt.gov
2.) GPS Publications
• GPSWorld Online Magazine – http://www.gpsworld.com/
• Point of Beginning - http://www.pobonline.com/
• GPS User Magazine - http://www.gpsuser.com/
• GPS Reviews - http://www.gpsreview.net/
• GPS Technology Reviews - http://gpstekreviews.com/
• GPS Gadgets - http://gps.engadget.com/
• Inside GNSS - http://www.insidegnss.com/
• The Problems with NAD27 - http://www.dot.pima.gov/gis/data/about/nad27problem.htm
• GIS/GPS Best Practices - http://www.esri.com/library/bestpractices/using-gis-with-gps.pdf
3.) Tutorials
• Trimble’s Interactive On-line Tutorial - http://www.trimble.com/gps/index.shtml
• ESRI ArcPad Free Training http://training.esri.com/gateway/index.cfm?fa=search.results&searchterm=ArcPad&software
type=All+Software&trainingformat=1%2C2
• ESRI GPS Analyst Free Training http://training.esri.com/gateway/index.cfm?fa=search.results&searchterm=GPS+Analyst&so
ftwaretype=All+Software&trainingformat=1%2C2&search=search
4.) Government GPS Sites
• F.A.A. – GPS Satellite Product Team – http://gps.faa.gov/
• National Spatial Reference System (NSRS) – Access info. To locate indiv. benchmarks http://www.ngs.noaa.gov/cgi-bin/datasheet.prl
• NGS/NOAA GPS Site – http://www.ngs.noaa.gov/orbits/
• US Coast Guard - http://www.navcen.uscg.gov/gps/default.htm
63
•
USGS Geographic Names Information System - http://geonames.usgs.gov/
5.) GPS Receiver Manufacturers
• Comprehensive List of all manufacturers - http://gauss.gge.unb.ca/manufact.htm
• Trimble – www.trimble.com
• Magellan – www.magellangps.com
• Garmin – www.garmin.com
• Leica – www.leica-geosystems.com
• Northstar – www.northstarnav.com
• Lowrance – www.lowrance.com
• Topcon – www.topcon.com
• Corvalis Microtechnology - www.cmtinc.com
• Tripod Data Systems - www.tdsway.com
6.) Standards
• Datum and Coordinate Standards http://www.nysgis.state.ny.us/coordinationprogram/workgroups/wg_1/related/standards/datum.ht
m
• Metadata Standard: FGDC Content Standards and Digital Geospatial Metadata –
http://www.fgdc.gov/standards/projects/FGDC-standards-projects/metadata/base-metadata
• Four Character County Code Standard http://www.nysgis.state.ny.us/coordinationprogram/workgroups/wg_1/related/spcodes/4cntycode.h
tml
• Federal Standards – www.fgdc.gov/standards
7.) Coordinate Translation
http://jeeep.com/details/coord/
http://www.terraserver.com/tools/degrees_converter.asp
http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html
http://life.csu.edu.au/geo/dms.html
64
Appendix C – Map of New York State Plane Zones
- 61 -636
65
61
Appendix D -
NYSNET Map
Image Source: New York State Department of Transportation (NYSDOT)
11/7/2007
4
Appendix E
.
N.Y.S. CORS Sites
Continuously Operating Reference Stations
CORS Sites
Network
#
*
2
%
#
I
2
%
#
*
#
*
#
*
#
I
1 Second, NATIONAL
5 Second, COOP
#
I
5 Second, NATIONAL
NYPD
#
*
10 Second, COOP
#
I
Unavailable, NATIONAL
#
*
KNGS
#
I
NYPB
PSC1
#
*
#
I
NYWT
#
I
I
#
I#
#
I
LOZ1
15 Second, NATIONAL
30 Second, NATIONAL
NYML
VTUV
NYET
NYLV
OSPA
PWEL
*#
#
*#
I
2
%
#
I
#
I
YOU2
NYMC
#
I#
I
TOA1 NYLP
BFNY
NYNS
#
I
#
*
2
%
SMTS
NYPF
NYHF
NYMX
#
I
SYCN
NYHB
#
I
#
I
NYFD
#
I
NYSM
#
I
NYFS
#
*
NYDV
#
I
MVCC
2#
%
I
NYHM
#
I
#
I
NYFV
#
I
%
2
NYCL
NYON
NYCP
#
I
PACP
HDF2
NYST
NYAB
NYEC
#
I
#
I
#
I#
*
NYRM
#
I
NYBH
#
I
NYHC
#
I
#
I
NYHS
NYKT
NYNP
#
I
NYLC CTBR
I
#
I#
NYMD
#
I
CTGU
CTDA
LAMT RVDI
I %
I
2#
#
I#
NYVH
NJMT
NJI2
#
I I
# #
I
#
I
#
#
I I
NYQN
NYRH
MOR2
NJTP
NJDY
#
I
Long Island CORS Sites
#
I
#
I
#
I
I
2#
%
RVDI
LAMT
NYVH
CTDA
#
I
#
I
SHK6
NYQN
#
I
*
#
I#
ZNY1
#
I
#
*
SHK6
CTGR
#
*
NYCI
NJI2
CTGU
CTGR
MNP2
#
I
*
# #
#
I*
ZNY1 NYCI
#
I
MNP2
NYRH
MOR2
67
#
*
APPENDIX F - WIDE AREA AUGMENTATION SYSTEM (WAAS) OVERVIEW
This entire appendix is based on the Wisconsin Department of Natural Resources. WIDE AREA AUGMENTATION
SYSTEM (WAAS), document with some minor reformatting.
I.
WHAT IS WAAS?
The Federal Aviation Administration developed the Wide Area Augmentation System (WAAS) to improve its basic
aviation global positioning system (GPS) service to meet accuracy, availability and integrity requirements critical to
flight navigation and safety. WAAS consists of two geostationary communication satellites and a network of 25
wide-area ground reference stations (WRSs). Each WRS has a surveyed location, and receives signals from GPS
satellites to determine if any data errors exist. The WRS then sends a GPS correction message to a master station
that computes correction algorithms and transmits them to the two WAAS satellites. The WAAS satellites broadcast
the correction data on the same frequency that GPS satellites use to transmit their data. WAAS-capable units
receive both GPS data and WAAS corrections, and differentially correct the data in real-time. For more
information about WAAS, see http://GPS.faa.gov/Programs/WAAS/waas.htm.
II.
HOW DOES WAAS AFFECT GPS
WAAS was developed to support real-time navigation - not mapping activities. Most WAAS- capable GPS receivers are
recreational grade. Users should consider the following issues when deciding if and how to use a WAAS-capable GPS
receiver.
1.) WAAS AVAILABILITY:
WAAS supports aviation uses in which obstacles and terrain do not block WAAS satellites on the horizon (satellite
#35 over the Atlantic Ocean and satellite #47 over the Pacific). In Vermont, WAAS-capable GPS receivers may be
highly sensitive to terrain and obstacles blocking the horizon. These receivers also take about 10-30 minutes to
acquire WAAS signals the first time, then about 1-2 minutes for subsequent uses.
2.) REAL-TIME DIFFERENTIAL CORRECTION METHOD:
For real-time differential correction, WAAS-capable recreational GPS receivers are less expensive and bulky than
recreational units with “beacon-on-the-belt”™35 (BoB) receivers. Depending on the site, however, a GPS with BoB
may be less susceptible to obstacles and terrain interference, because ground-based beacons are physically closer
and are located in several different directions around the data collection site.
3.) RECREATIONAL VS. MAPPING GRADE RECEIVER:
WAAS has the potential to improve the horizontal and vertical accuracy of recreational grade GPS data to
approximately 7 meters. Differentially corrected mapping grade GPS data are still more accurate. Mapping grade
receivers also have better data logging capabilities, such as allowing users to: (1) load customized data
dictionaries, (2) capture lines and areas in addition to point data, (3) collect points along line and area features,
and (4) export data directly in a GIS compatible format.
35
Trimble registered trademark
- 64 -
68
64
Appendix G – United States Coast Guard Differential GPS Coverage of New York State 30
30
USCG Navigation Center DGPC Coverage Page – New York,” 20 Dec. 2006
<http://www.navcen.uscg.gov/dgps/coverage/NYork.htm>
69
APPENDIX H - RECOMMENDED DATA COLLECTION PRACTICES
This entire appendix is based on a section of the Georgia Department of Transportation; GPS Data Collection Guideline and
Standards: A Manual for Georgia Service Delivery Regions and Regional Development Centers, document with some minor
reformatting.
“GPS Collection methods used to capture roads, sidewalks, and trails can vary depending on a variety of factors. Collection of
road centerlines, under the data standard outlined in this manual, requires the use of a motorized vehicle (car/truck) capable of
highway travel. Collection methods for sidewalks and trails can vary. Depending on environmental factors, congestion and
accessibility, sidewalks and trails can be collected using foot, bicycle or motorized vehicle travel. Despite these differences,
common best practices do exist for the collection of road, sidewalk and trail centerline collection. The following are data
collection tips.
General Collection Tips
I. Centerline Collection from Beginning to End
When an intersection does not exist at the end of a road or trail, collect to the far end of the centerline (i.e., a road cul-de-sac
or dead end to a trail). See Figure 1 for examples.
When a cul-de-sac has an island or curbed circle in the center of the cul-de-sac, collect the centerline completely around the
island. See Figure 2 for examples.
70
II.
Under-Runs vs. Over-Runs
An under-run is created data collection stops prior to reaching an endpoint (i.e., road or trail intersection). Ideally, data
collection should clearly start and stop at centerline intersections in order to prevent future confusion for staff attempting to
integrate the data into the existing USGS DLG-F data set.
An over-run is created when data collection continues past the intended stop (i.e., road or trail intersection). Over-runs create
less confusion during the data integration phase and are more easily edited out of the data set.
If it is not possible to collect data clearly from endpoint (intersection) to endpoint (intersection), then the intentional errors
created by over-runs are preferred to the unintentional ones created by under-runs. GDOT will accept over-run errors within a
range of 50 to 100 feet. See Figure 3 for a diagram showing the correct procedure for collection over-runs.
71
III.
Turning vs. Head-On Approach
Collect centerline features using a head-on approach. As you approach a road, trail or sidewalk for collection, do not start
collection until you are aligned in a straightforward fashion with the feature.
Do not start collecting data while you are approaching a feature. This will not accurately represent the feature being
collected and will present problems for staff trying to integrate the data in to the USGS DLG-F data set. Figure 4 shows
the correct way to approach a feature for collection.
72
IV. Obstructions in the Collection Path
If a road, sidewalk or trail cannot be safely traveled, it is not considered accessible to the public and therefore is not
eligible for collection under this data standard. However, some centerlines may have objects like tree limbs, built-up
water, dead animals, or fallen rocks blocking the collection pathway.
A significant obstruction may require enough of a deviant movement to avoid the obstacle so as to cause an inaccurate
data capture of the road. Hitting the pause button, avoiding the obstacle, and then resuming collection can avoid this.
The Pause feature is best used on straight a ways. Pausing GPS collection simply suspends data capture until an
obstruction is passed. If several turns are made during the pause in data collection, the GPS unit will simply connect the
dots from the point of last collection to the point at which the GPS unit is resumed. Figure 5 shows examples of how to
correctly apply the Pause function.
If contingent situations arise and data collection must be temporarily suspended, the Pause function allows data
collectors to take a break that can be resumed at a later time. Be careful to remember the general location where the
pause function was executed. Forgetting the location of a pause may cause undesirable collection results if you
incorrectly start collection in a different location.
V. Loss of Signal
Use the distinctive audible capabilities of the GPS data logger to ascertain when signal is lost and regained. In areas of high
multi-pathing (dense tree canopy, mountainous areas, urban areas, etc…) signal may be lost frequently.
During signal loss, attempt to slow the collection pace significantly- or even come to a complete stop- if conditions dictate
that it is safe to do so in order to wait for signal to return.
Signal loss on long straight a ways is less of a problem than signal loss on curvilinear centerline paths. Therefore, it is
advisable to slow the collection pace around curves if signal strength is weak. If loss of signal occurs more than several
73
hundred feet, the shape of the feature being collected may become distorted and will require recapture at a later date/time.
See Figure 6 for an example of shape distortion due to signal loss on a curve.
In the office, signal loss is easily detected. If the centerline appears coarse, jaunty or looks incorrect against the
background image or data layer, examine the vertices. Since the positions are being collected between one and five
seconds, long gaps between the vertices will be a strong indicator of loss of signal. In such a case, re-collection of the
centerline may be necessary.
VI.
Divided Highways (Road Centerline Collection)
A divided highway contains a median in the center that separates different directions of travel. Collect the centerlines of
both sides of a divided highway. Each side of a divided highway is treated as a one-way road.
If a divided highway is represented by a single centerline in the existing USGS DLG-F data set, SDR data collectors are
required to GPS capture both lanes of the divided highway as dictated by Section A above.
74
VII.
Offsets
An offset is a known distance set away from the antennae location of the GPS unit that is used to collect data in areas of
difficult accessibility. Offsets are either used at the time of data collection (instant offset) or prior to data collection
(constant offset). Most often, instant offsets will be applied to the collection of long sidewalk centerlines when using
vehicular travel. Also, instant offsets will be used while collecting road centerlines. Do not use offsets to capture trails,
as they are usually more accessible.
During road data collection, use offsets to capture the true centerline for roads with an even number of lanes. Do not use
an offset on roads with an odd number of lanes, as you will be able to drive the true centerline. Figure 7 describes the
correct way to collect centerlines on multi-lane roads.
If it is necessary to apply an offset during the collection of long sidewalks using vehicular travel, drive in the lane nearest
the sidewalk and apply an offset distance and direction that most accurately captures the true centerline of the sidewalk.
Exercise caution when applying offsets during data collection. Be certain to apply the correct side, measurement and
units for the offset prior to data collection (see Figure 8). If unnoticed during data collection, offset errors can render an
entire data collection effort useless. And, offset errors are hard to ascertain during the post-processing phase. If you
suspect an offset error at the post-processing phase, use DOQQ or other aerial imagery to perform quality control.
If offset errors are detected during the post-processing phase, it may be possible to use PathFinder Office to correct any
mistakes made due to miscalculation of distance or direction. However, if too many offset errors are present and a clear
method cannot be established to correct the mistakes, re-collect the centerlines.
75
VIII. Segmenting
The segmentation option is used to change one or many attributes that differentiate along any given road, sidewalk or
trail centerline. For example, if the centerline lane width changes along a specific path during data collection, use the
segmentation function to signify a new record to the attribute table. For example, if a paved (bituminous surface) road is
being collected and the surface type suddenly changes to that of gravel or stone, apply the segmentation button at the
exact location where the two surface types meet. This allows changes along a singular feature to be accurately reflected
while allowing for the continuous collection of the feature.
Linear features like road, sidewalk and trail centerlines are dynamic and change often. Data collectors should use the
segmentation feature of the GPS unit to reflect these changes. During the post-processing phase, be cautious of data that
shows little differentiation along given linear features.
76
IX. Repeating
The repeat function allows data collectors to copy feature attributes from the most recently collected data feature to the
one currently being collected. This function may improve efficiency in data collection if many roads, sidewalks and/or
trails are to be collected that share like features attributes.
77
APPENDIX I - SAMPLE PROJECT SPECIFICATIONS
This entire appendix is based on the British Columbia Standards, Specifications and Guidelines for Resource Surveys
Using GPS Technology, Appendix A, with some minor reformatting.
I.
APPLICATION
The Content Specifications facilitate standardization and quality control for geo-spatial data acquired via GPS
technology for Agencies contracting out GPS data collection. This document is provided for use by Contracting
Agencies without a pre-established specifications geared to GPS data collection using differential GPS
techniques with resource/mapping grade receivers and having target accuracy requirements from 1m to 20m
horizontal accuracy classes (at 95% confidence) and the 5m to 20m vertical accuracy classes (at 95%
confidence). The actual target accuracies required for the project or application are to be entered below.
The Content Specifications are supported by two documents: the Accuracy Standards and the Guidelines:
A. Accuracy Standards
Document outlining target accuracy categories in a standardized and uniform manner. Using the
Content Specifications document, one may specify the target accuracies to be achieved based on the
standardized categories established within the Accuracy Standards document.
B. Guidelines
The Guidelines support document provides relevant background information in order to complete those
areas of the Content Specifications that vary project by project. This Specification document, when
completed using the Guidelines, will form the technical section of a GPS survey contract.
II. INTERPRETATION
These Content Specifications may be interpreted with the help of the accompanying Guidelines document. In
order to interpret the Content Specifications correctly, the reader must have prior familiarity with GPS
operations. The Guidelines are intended to assist users in this regard.
In this document, the following definitions and abbreviations shall be used:
Agency
Contractor
Contract
Administrator
Data Processor
DGPS
Dynamic-mode
Field Operator
Geoid
GPS
GPS Event
GPS Reference
Agency, Department, Section or other entity administering the Contract.
Corporation, firm, or individual that provides works or services to the Agency
under terms and conditions of a contract.
Agency representative who has authority for issuing and managing the contract
and for receiving the items or services delivered by the Contractor.
A trained employee of the Contractor who performs the calculations to
convert raw field GPS data into processed maps / databases using DGPS
procedures and QC checking / editing.
Differential GPS (i.e. pseudorange code positioning differentially corrected
either post-mission or real-time).
Collection of GPS data while traveling along a linear feature to be surveyed
(e.g. a road or watercourse).
An employee of the Contractor who performs the field portion of the data
collection.
The equipotential surface approximating Mean Sea Level. Consult GDBC for
provincial standard geoid model.
Global Positioning System as operated by the United States Department of
Defense (US DoD). Also called NAVSTAR.
A GPS Event is a single position instead of a group of positions averaged to a
single position (i.e. Static survey). Events are typically used when the
antenna cannot, or need not, be stationary over a point.
A GPS receiver located at a known location collecting data continuously to be
78
Station
NAD27
NAD83
NADV88
Static-mode
Supplemental
Traverse
UTM
used for correcting field data (either in real-time or post-mission). Also
known as a basestation.
North American Datum of 1927, based on the Clarke 1866 ellipsoid.
North American Datum of 1983, based on the Geodetic Reference System 1980
(GRS80) ellipsoid and as defined by the GRS in British Columbia.
North American Vertical Datum of 1988
Collection of GPS data at a discrete point while remaining stationary.
Supplemental Traverses are conventional traverses (e.g. compass and tape)
that are integrated with GPS surveys.
Universal Transverse Mercator projection (map projection system).
The statements in this document have been structured according to two levels of compliance:
required
Used to describe tasks that are deemed necessary and are good practice.
Exceptions are possible, but only after careful consideration by the contracting
Agency.
recommended
Used to describe tasks that are deemed desirable and good practice, but are
left to the discretion of the contracting Agency.
III. GOALS
1. To establish realistic, reasonable levels of accuracy by task assignment, and to classify the surveys to be
performed by end specifications aimed at achieving target accuracies.
2. To provide a capacity for integrating requirements across Vermont and to standardize those requirements
where common standards are applicable.
3. To qualify GPS Systems (i.e. equipment, processing methods, and personnel) by a Contractor GPS System
Validation survey to establish the accuracies achievable under various conditions.
IV. PRE-QUALIFICATION AND VALIDATION
1. Total System - It is required that any Contractor expecting to undertake GPS data collection be prepared
to fulfill the requirements of the full “System”, including: GPS hardware and software for field and office;
field and GPS Reference Station receivers; and reporting techniques. All parts of the System are to be
capable of meeting the contractual specifications below.
2. Field Operator Training – It is required that Field Operator(s) have a demonstrated proficiency in GPS data
collection methods or, if the operator is in training, be accompanied by an individual meeting this
requirement.
3. Data Processor/Project Manager Training – It is required that Data Processor/Project Manager(s) have
demonstrated proficiency in the planning, management and execution of GPS projects - this includes the
processing and management of GPS data.
4. It is required that any GPS System used be proven to meet the accuracy requirements through a GPS
Contractor System Validation survey as outlined in Section C Content Specification-IV. For accuracy levels
established during the validation and the conditions under which they were established, it is
recommended they apply for all subsequent projects.
79
V. PRE-FILEDWORK PROCEDURES
1. It is recommended the Contract Administrator conduct a pre-fieldwork conference for all potential and
qualified contractors. It is recommended the Contract Administrator provide a clear definition of the
feature(s) to be surveyed, which point features are to be considered “High-Significance” and which are to
be considered “Standard-Significance”, boundaries of the features, guidelines for interpretation of special
features - if necessary, it is recommended a specimen layout for interpretative purposes. It is
recommended the Contract Administrator also provide a clear definition of the deliverables, services, work
quality, payment schedule, and other relevant contract issues. There should be no doubt or confusion as
to the nature and quantity of work expected.
2. It is recommended the Contract Administrator advise the Contractor of the Audit process (i.e. the method
and frequency of data/field inspections and surveys that will be used in determining achievement of end
specifications in compliance with the conditions of the contract).
3. It is recommended the Contract Administrator conduct a field inspection with the Contractor, advising
them of specific details to include or exclude in the contract work so that there is no doubt as to the
nature and quantity of work expected in the contract.
4. If physical reference markers are required to be established, it is required that the interval and type of
markers be stated in the contract, and be established according to existing Agency guidelines or
requirements (e.g. the Forest Practices Code guidebooks for forest road engineering and boundary
marking).
5. It is recommended all projects include sufficient map ties such as creek junctions, road intersections or
other features to enable accurate geo-positioning and to provide reliability checks. It is recommended the
Agency representative specifies the number of tie points required, and if possible, specify where and what
these tie points should be.
6. An official land survey may only be legally defined by a licensed land surveyor. None qualified
individuals attempting to present a survey as official can result in legal action being taken against
the Contractor or the Agency if damages occur on adjacent lands.
7. The required survey accuracies (i.e. target accuracies at 95%) for
(Class =
Network Horizontal Accuracy =
____
m
Interpretative Horizontal Accuracy =
____
(Class =
m
Network Orthometric Height
____
(Class =
Accuracy =
m
Interpretative Vertical Accuracy =
____
(Class =
m
the project are:
__ meter )
__ meter )
________ )
_________)
For clarification, the definition of meeting the above accuracy class is that for GPS point features, at least 95%
of the individual position fixes are within the above-specified accuracies (horizontal linear measure) of the true
position of the point according to the National Spatial Standards for Data Accuracy. See Section B: Accuracy
Standards.III.B “Determining the NSSDA”.
Similarly, for GPS traverses done in dynamic linear mode, at least 95% of the individual GPS position fixes are
within the specified accuracies (horizontal measurements perpendicular to this line) from the true position of
this line.
VI. FIELDWORK
1. The field GPS receiver is to be set to position or record observations with a minimum of four (4) satellites
without constraining/fixing the height solution (sometimes known as “3D” positioning mode).
80
2. The minimum satellite elevation angle/mask for the field GPS receiver is 15 degrees above the horizon.
3. It is required that the DOP not exceed the following values:
DOP Figure
Geometrical DOP (GDOP)
Positional DOP (PDOP)
Horizontal DOP (HDOP)
Vertical DOP (VDOP)
Maximum DOP Value
--6.0
-----
Not all DOP values are required to be completed.
VDOP limits need be followed only in surveys where accurate elevations are required
4. During Static (point-mode) surveys, occupations will adhere to the minimum values below, or the values
used during the Validation survey, which ever is higher.
Point Significance
StandardSignificance Point
High-Significance
Point
Minimum
Occupation Time
(sec)
30 seconds
Minimum Number of Fixes
250 seconds
50 fixes
30 fixes
5. It is required that position fixes for linear features mapped statically (i.e. static or point-to-point
traverses) be no more than ______ meters apart, with the traverse points defined as Standard Significance
Points.
6. It is required that position fixes for linear features mapped dynamically (i.e. dynamic traverse) be no more
than _______ meters apart.
7. It is required that dynamic traverses begin and end on a physically marked static High-Significance point
(commonly referred to as the Point of Commencement (PoC), and the Point of Termination (PoT)).
8. All significant deflections required to delineate linear features at the required accuracy are to be mapped.
This includes significant vertical breaks if elevations are required.
9. Times of GPS Events (i.e., interpolated points) on dynamic traverses should be accurate to at least
________ seconds.
10. It is required that for point offsets, the following specifications be observed:
™ The Field Operator is to record the following information: slope distance; vertical angle; and magnetic
or true azimuth from the GPS antenna to the feature.
™ Magnetic Declination is to be applied to all compass observations before computing offset coordinates.
™ The maximum distances for point offsets are ____ meters, and ____ meters if offset observations are
measured forward and backwards.
™ Bearings are to be accurate to at least ____ degrees, and distances to at least ____ meters.
11. It is required that for linear offsets, the following specifications be observed:
81
™ The Field Operator is to record the following information: horizontal distance and the direction (left or
right) perpendicular to the direction of travel.
™ The maximum linear offset (i.e. horizontal distance) allowable is _____ meters.
™ Linear offset distances are to be checked and adjusted periodically.
12. It is required that supplemental traverses meet these following rules:
™ The supplemental traverse is to begin and end on physically marked High-Significance GPS static points
(PoC and PoT).
™ The distance traversed is to be less than _______ meters.
™ The supplemental traverse is to close between the GPS PoC and PoT by ___ meters+1:___00_ of the
linear distance traversed.
™ The supplemental traverse is to be balanced between the GPS PoC and PoT by an acceptable method
(i.e., compass rule adjustment).
13. Physical reference markers are to be established every _____ meters along linear features (enter N/A if
not applicable). These markers must adhere to contracting Agency standards, or be accepted before the
work commences.
14. It is required that static point features be collected at all physical reference markers. These static point
features are to be collected as STANDARD / HIGH (circle one) Significance points.
15. It is required that the GPS receiver’s default Signal to Noise Ratio (SNR) mask (6) for high accuracy be
used. This CAN/ CANNOT (circle one) be relaxed during traversing of linear features.
VII.
GPS REFERENCE STATIONS
1. If the Contractor chooses to establish or use a previously established reference station and not a CORS Base
station then it must be monumented (physically marked) to allow the contracting Agency or other
Contractors to re-occupy the same location. Physical reference marks are to be left and the station
referenced using adjacent features (i.e. road intersections, sign posts, bearing trees, etc.) to assist in the
future location, and in determining that it has remained undisturbed. Suitable markers include iron bars
driven into the soil, spikes in asphalt or concrete, or other markers that the Contractor and Agency
determine will remain stable during and, for a reasonable time, after project completion.
2. It is required that the separation distance between the GPS Reference Station and field receivers be less
than _____ kilometers, or the separation distance used during Validation, whichever is less.
3. The minimum elevation angle/mask of the GPS Reference Station should be 10 degrees.
4. If real-time corrections are used, it is required that the Contractor validate the GPS Reference Station
according to accepted industry procedures.
5. If real-time corrections are used, it is required that the RTCM-Age of the rover GPS system not exceed
_____ seconds. See Table IV-1: Suggested Maximum RTCM Correction Age Settings for information on
correction ages appropriate for various accuracies.
VIII.
PROCESSING AND QUALITY CONTROL
1. All GPS positions are to be corrected by standard differential GPS methods (pseudorange or navigation
corrections). If navigation corrections are used, the same set of GPS satellites are to be used at the GPS
Reference Station as at the field receiver for all corrected positions.
2. If the GPS receiver and/or post-mission software provides the option for dynamic filtering, the filters are to
be set to reflect the speed of the operator or vehicle, and the software versions and filter settings are to
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be noted in the project returns. If filtering is applied to GPS Reference Station data, this is also to be
noted.
3. It is recommended the Contractor implement a Quality Control (QC), or reliability assessment, program in
order to show compliance to specified standards (i.e. positional accuracy, content accuracy, completeness,
data format adherence, and data integrity assurance).
4. It is recommended the Contractor be prepared to entirely re-survey those areas that do not meet the
compliance standard at their own cost.
IX.
PROJECT DELIVERABLES
1. It is recommended the Contractor submit a project report that includes the following information, as a
minimum.
™ A brief description of the Contract particulars, including the contracting Agency that commissioned the
work, the Contract Administrator, a project name (if available), and a project identifier (e.g. provincial
government’s ARCS/ORCS number, etc.).
™ A brief description of the project work (i.e. purpose, target accuracy, location, etc.).
™ A key map showing the project area and a description of any GPS Base Stations used.
™ A schedule of events showing key dates/milestones (i.e. contract award; field data acquisition; problems
encountered; data processing; delivery of results; etc.).
™ A listing of all personnel (Contractor and Subcontractors) involved in this project detailing their particular
duties and background (i.e. their educational background; formal GPS training details (courses with dates);
their experience on similar projects, etc.) - this could be a copy of what was provided with the prequalification package.
™ A list of all hardware and software used on the project; including but not limited to:
o GPS hardware (i.e. receiver model, antenna, data logger, firmware versions, etc.);
o GPS software (i.e. name, version number, settings, etc.)
o Mapping software (i.e. name, version number, settings, etc.)
o Utility software (i.e. name, version number, settings, etc.)
™ Detail regarding the GPS Reference Station used (i.e. private, local and/or government, validation status,
etc.).
™ A summary of the project including planning, field data collection methods and parameters (i.e. GPS
receiver settings/defaults), data processing methods and parameters (i.e. post-processing
settings/defaults), any project problems, anomalies, deviations, etc.
™ An explanation of deliverables (digital and hard copy) including data formats, naming conventions,
compression utilities used, media, etc.).
™ A copy of all field-notes (digital or hard copy).
™ A list of all features that have been mapped or surveyed.
2. It is recommended the Contractor submit the following digital deliverables in the indicated format and
datum (see APPENDIX D - DIGITAL MAPPING and GIS INTEGRATION for details).
Deliverables
Format
GPS Reference
Station Data
Raw Field GPS Data
Original Corrected
GPS Data
Final Interpreted
GPS Data
Proprietary or
RINEX
Proprietary
Proprietary or
ESRI Format
ESRI Format
Datu
m
WGS84
Notes
WGS84
NAD83
Unedited
Unedited
NAD83
Edited
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Merged if possible
As noted in the table above, two digital and/or hard copy data sets should be submitted. One dataset must
show all the GPS data collected after it has been corrected; before there has been any “cleaning” (i.e.
filtering, pruning, averaging, etc.). The second dataset must show the resulting GPS data that has been
“cleaned” (and is eventually used in the final survey plans/plots). The provision of these products will allow
the Contract Administrator to do a visual Quality Assurance check on the GPS data.
3. The Final Interpreted GPS data is to be provided in a digital format to be specified by the contracting
Agency, and a hard copy map/plan may also be required. Map hard copies are to conform to Agency
cartographic standards.
The following map submission is provided as a suggested minimum:
o Map Surround, which includes the following project information: Project Title; Project
Number/Identifier (e.g. provincial government’s ARCS & ORCS identifier); contracting Agency name;
Contractor name; and date of survey.
o Plan datum (e.g. NAD83) and the Map Projection (e.g. State Plane Long Island).
o Plan scale (e.g. 1:20,000) with BCGS map identifier.
o Plan orientation, (e.g. north arrow annotating True North, Magnetic North and Grid North).
o Geographic (e.g. latitude/longitude) and/or Mapping Projection (e.g. VCS) graticule as requested.
o Source of any non-project information (i.e. TRIM backdrop, Forest Cover data, etc.).
4. Final data (i.e. Original Corrected GPS data and Final Interpreted GPS Data) is to be reduced and presented
referenced to the NAD83 datum. If the Contract Agency requires data to be provided on the NAD27 datum,
then it is required it be a copy of the data. If the Agency requires any other local datum, the methods
used to transform the data are to be explicitly described in the project report and approved by the Agency.
5. If orthometric elevations, i.e., Mean Sea Level, are required for submission, vertical data is to be
referenced to the NAVD88 using the standard geoid model for the United States - with local geoid modeling
if required (i.e. for high vertical accuracy projects).
6. The data files created by this project are the property of the contracting Agency and access to all files
created in the completion of the works is required to be made available to the Contract Administrator or
designate. It is recommended the Agency forward a copy of none sensitive data to the Vermont Center for
Geographic Information for distribution to the GIS user community. In addition, the Agency should be
responsible for storage or destruction of the data files in accordance with government standards.
7. It is recommended the data provided be catalogued with the following information for archiving purposes:
™ General project information; such as: the contracting Agency; the Contract Administrator; a project
name; and a project identifier (e.g. Agencies internal project number, etc.).
™ Type, model and version number of hardware used to collect and store data.
™ GPS Reference Station used to correct field data (include coordinates and validation information).
™ Details of post-processing conversions used.
™ Software used in calculations and conversions and version number.
™ Any non-standard data handling method, technique or principle used.
8. Digital returns are to be submitted on the storage media and format as required by the Agency.
X.
TECHNOLOGICAL/ PERSONNEL CHANGE
1. If there are any significant changes in the Contractor’s GPS system components (i.e., hardware, firmware,
software, methodology, etc.) or personnel during the period of the contract, the Contractor should consult
with the Contract Administrator. A decision will be made as to whether the Contractor GPS System
Validation; the personnel qualification, and/or the GPS Reference Station Validation survey are required to
be repeated.
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APPENDIX J - SAMPLE GPS CONTRATOR REPORT
1.) Company/ Agency Information
Company/ Agency:
Contact:
2.) Field Operator Information
Field Operator Name:
Company/Agency:
Formal Credentials:
Experience:
3.) Data Processor Information
Data Processor’s Name:
Company/Agency:
Formal Credentials:
Experience:
4.) Field GPS Receiver Information
FIELD GPS RECEIVER – GENERAL INFORMATION
GPS Manufacturer/ Model:
GPS Mobile Software:
FIELD GPS RECEIVER – DATA COLLECTION SETTINGS
Data Rate Used:
Data Format:
Satellite Elevation Mask: _______ Degrees
PDOP Mask: ______
Minimum Number of SVs: ______
SNR Mask: ______
5.) GPS Base Station Used
GPS Base Station Used: ________
6.) Deliverables
GPS CONTRACTOR REPORT
DATA
1.) Uncorrected Data
2.) .SSF File
3.) Corrected Data (Shapefile or Geodatabase in NAD83 State Plane New York Long Island FIPS 3104 feet )
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APPENDIX K - FIELD EQUIPMENT LIST
GPS Equipment
__ GPS Datalogger
__ Integrated GPS Antenna
__ TSCI Data Cable
__ Dual Battery Cable
__ Data Power Cable
__ NMEA/RTCM Cable
__ Real-Time GeoBeacon
__ Antenna Cable
__ Vehicle Magnet
__ Backpack
__ GPS Carrying Case
__ Range Pole
Safety Equipment
__ Safety Vest and Hats
__ Rotating Safety Light
__ Rear Safety Sign
__ Road Flare
__ Sunglasses
__ Cellular Phone
__ Tire Chains
__ First Aid Kit
Miscellaneous Items
__ Pencils (standard and colored)
__ Pens
__ Compass
__ Field book
__ Highlighters
__ Tape Measure
__ Road Maps
__ Topographic Maps
__ Paper Clips
__ Clipboard
__ Calculator
__ Extra Clothing
__ Food
__ Water
__ Bug Spray
__ Sun block
__ Tripod
__ Property Access Papers
__ Purpose of Project letter
__ 2-Way Radios
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APPENDIX L - EVALUATING GPS PROFESSIONALS
ITEMS TO BE CONSIDERED IN EVALUATING GPS PROFESSIONALS
1. Responsiveness to the specifications and the contractor's proposed plan of performance. The plan of
performance should include a schedule for accomplishing the work, including the time required for each
phase.
2. Experience. Request a client list. Review one or two of the most recent projects, by examining the work
and discussing the client's satisfaction with the mapping contractor's work.
3. Equipment and production facilities. Request a written statement of how maps are prepared. Ask for a
listing and description of equipment to be used on the project.
4. Personnel. Ask for a listing of full-time employees of the firm available to work on the specified project
and brief resumes of key mapping personnel. The caliber of workforce can be an important factor in a firm's
ability to produce acceptable products.
5. Financial status. Request a current financial statement. Check the statement and the contractor's credit
rating.
6. Bonding. Bonding should be required for the bid price and 100 percent performance.
7. Support programs. Technical assistance and questions regarding the delivered data should be provided.
8.) Cost. Cost should be measured in relation to the service to be provided.
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