Manual of Instructions Bathymetric Surveys

Manual of Instructions Bathymetric Surveys
Manual of Instructions
Bathymetric Surveys
Ministry of
Natural
Resources
Manual of Instructions
Bathymetric Surveys
June, 2004
Frank Levec
Inventory Monitoring and Assessment Section
Science and Information Branch
Audie Skinner
Aquatic Research and Development Section
Applied Research and Development Branch
Cette publication spécialisée n’est disponsible qu’en anglais
2
TABLE OF CONTENTS
1
INTRODUCTION...........................................................................................4
2
PRE-SURVEY.................................................................................................5
2.1
SOFTWARE AND DATA ........................................................................................... 5
BASS Software ................................................................................................. 5
NRVIS Data ..................................................................................................... 5
2.2 EQUIPMENT .......................................................................................................... 6
2.2.1
GPS Unit .......................................................................................................... 6
2.2.2
Depth Sounder ................................................................................................. 7
2.2.3
Computer ........................................................................................................ 8
2.2.4
Power Supply ................................................................................................... 8
2.2.5
Software.......................................................................................................... 9
2.3 PLANNING YOUR SURVEY ................................................................................... 10
2.1.1
2.1.2
3
SURVEY .......................................................................................................16
3.1
3.2
3.3
3.4
3.5
3.6
SAMPLING METHODS .......................................................................................... 16
PREPARATION OF FIELD FORMS AND EQUIPMENT ............................................... 12
SAFETY AND COMMUNICATION ........................................................................... 12
SETTING UP AND TESTING THE EQUIPMENT ........................................................ 13
RUNNING THE TRANSECTS .................................................................................. 18
TROUBLESHOOTING ............................................................................................ 20
3.6.1
No GPS Signal ................................................................................................ 20
3.6.2
No Depth Signal ............................................................................................. 20
4
POST-SURVEY ............................................................................................22
4.1
4.2
4.3
DISASSEMBLING THE EQUIPMENT ....................................................................... 22
TRANSFERRING DATA TO A GIS WORKSTATION ................................................... 22
DATA ARCHIVE ................................................................................................... 23
5
ACKNOWLEDGEMENTS.............................................................................24
6
REFERENCES ..............................................................................................25
APPENDIX A - EQUIPMENT CHECKLIST FOR BATHYMETRIC SURVEYING ..................................... 26
APPENDIX B - HARDWARE OPTIONS FOR BATHYMETRIC SURVEY............................................. 27
3
1 INTRODUCTION
The purpose of a bathymetric survey is to describe the physical characteristics of the bottom of
a waterbody. Bathymetric data can be used to construct a map showing depth contours and
underwater structure and is used to calculate volume and mean and maximum depth of a
waterbody. This information is important for evaluating habitat suitability of a waterbody for
various aquatic species, assessing the sensitivity of that habitat to development (e.g. coldwater
habitat for lake trout), locating critical habitat features (e.g. spawning shoals) and selecting
sampling sites for other aquatic surveys. The information may also be used in the application
of thermal habitat models.
Contour maps were prepared for many of Ontario’s important lakes between 1968 and 1985 as
part of the Aquatic Habitat Inventory Program. To construct these maps a series of transects
were run across the lake using an echo sounder to measure water depth. Depth was
transcribed from the echo sounding tapes onto a corresponding transect map. When all the
depths had been transcribed, then points of equal depth were joined on the map to create
contours. Preparation of a contour map often required several days of processing time and
involved a certain amount of artistic interpretation. The quality of the maps depended on a
variety of factors including the number of transects that were run, the accuracy of the transect
map, the number of depths transcribed and the interpretive skills of the drawer. At best, only a
moderately accurate bathymetric map could be produced and it was inevitable that many
underwater features would be missed (Monroe and Betteridge 2000).
In more recent years, new technology became available that was used in the collection of
bathymetric data. Applying GPS (Global Positioning System) technology greatly improved the
efficiency and speed of data collection. The quality of data also improved from using GPS, and
the integration with GIS to produce maps also became much more efficient.
Since 1995, three different bathymetry collection systems integrating data from GPS and depth
sounders have been developed by Science and Technology staff within MNR. To better
facilitate support for the collection, processing and management of bathymetric data, the
Ministry has developed a new system that builds on the strengths of the earlier systems. This
new system, which has retained the name of one of the earlier systems called BASS
(Bathymetric Automated Survey System), is recommended as the general standard for Ministry
sponsored work.
This manual describes the hardware and field procedures associated with collecting bathymetric
data with the BASS system. Information on the processing of the data in a GIS (Geographic
Information Systems) is outside the scope of this manual and will be provided in the future.
4
2 PRE-SURVEY
2.1 Software and Data
2.1.1 BASS Software
The BASS software must be installed on your portable field computer prior to beginning the
field work. Check to ensure that the software is running properly. Remember to use the setup
program to install the software and be sure that you have administrative rights to the computer
if you are running Windows NT, Windows 2000 or Windows XP Professional. Also be sure to
use the Add/Remove programs option from the Windows Control Panel to properly remove the
software from the computer or when updating to a new version. It is also recommended to
familiarize yourself with the software first, before conducting a survey in the field.
The BASS software is available for download to all MNR employees. Please contact Frank Levec
at the Inventory, Monitoring and Assessment Section in Peterborough at 705-755-1634 to
obtain a copy.
2.1.2 NRVIS Data
In order to conduct a bathymetric survey effectively, it is necessary to obtain and load some
map layers or background data. This will help to give you context as you create a plan of
attack for the lake. Note that the layers gathered here would most likely be the same layers
gathered to create a hard copy map for creating an initial plan of attack for the lake. The first
of these files is an outline of the lake including all islands within the lake. This file should be
obtained from the District office in order to get the most up to date version. The District GIS
officer should be contacted with a request to obtain the lake as a polygon in shapefile format.
Other supporting layers would include, surrounding drainage features such as rivers,
surrounding contours/DTM (Digital Topographic Model) of the area. A current roads layer may
also help with planning access to a lake, especially with larger lakes where multiple access
points may be required. As a partial aid for collecting data in a grid format on larger lakes,
BASS provides the ability to overlay a grid at an interval of the user’s choice.
5
2.2 Equipment
The GPS and depth sounder equipment that you select will have a direct influence on the
quality and quantity of data that can be collected. For example, the lower end GPS units will
provide two to ten meter accuracy whereas more expensive units can obtain accuracy under
one meter. Most off the shelf sounders will not record in depths less than two feet (0.61 m).
Improved accuracy normally comes at a significantly increased cost and effort for the data
collection. Tradeoffs between economical efficiency and accuracy should be carefully
considered.
GPS units and depth sounders can be purchased as individual or combined units. Using a
separate GPS and depth sounder provides advantage of being able to use the GPS unit for
other field work in the off season. However most staff have found the combination units much
easier to wire, configure and use in the field.
The following provides general information on the minimum specifications and
recommendations for hardware associated with the collection of bathymetric data and the use
of the BASS software. These guidelines are for standard features to look for in a particular
device, and will quickly help you to eliminate many lesser makes and models. Note that
manufacturers are continually improving their equipment every year. Because of this, most
hardware devices have a duty life of about 3 years. A summary of hardware options and
recommended devices is provided in Appendix B.
2.2.1 GPS Unit
BASS is designed to accept the NMEA (National Marine Electronics Association) standard input
from GPS and depth sounder units. Almost all of the GPS and sounding units available on the
market are now NMEA compatible. The application is not vendor specific as far as the GPS unit
is concerned, but the device you purchase or plan to use must support output of the NMEA
0183 standard, particularly one or more of the following strings:
$GPGGA – Global Positioning System Fix Data (required)
$GPRMC – Recommended Minimum Navigation Information
$GPGLL – Geographic Position – Latitude/Longitude
The GPS Unit should also provide for minimum 2-5 m accuracy and have the ability to receive
real-time corrections (RTCM-104 format). With the creation of the Canada-Wide Differential
GPS system (CDGPS) (http://www.cdgps.com), we can now access free real-time corrections
with the addition of these units. In order to take advantage of the correction, the GPS unit
must be capable of receiving RTCM or RTCA corrections..
When deciding upon a GPS unit, strongly consider one which allows for 3-D data collection and
allows filtering for signal to noise ratio and satellite geometry in the field. Methods used to
obtain Real Time Corrections include:
1) Satellite subscription service
6
2)
3)
4)
5)
Wide Area Augmentation System (WAAS)
Coast Guard Beacons
Individual Base Stations.
CDGPS
Although 3-D data collection with the GPS unit is not absolutely necessary, it will provide
improved accuracy over 2-D data. 3-D positions allow for improved accuracy in the vertical
component, which in this case is being measured by the depth sounder. Three-dimensional
data collection requires signals from four satellites in order to determine position whereas 2-D
data requires only three satellites. This may restrict the ability to collect data in certain
situations (i.e. close to steep shorelines or very dense canopy or lack of visible satellites). If
data collection is not possible in these situations, settings such as the PDOP can always be
modified to allow for 2-D data
If satellite coverage is in doubt for a particular time of day, it may be necessary to conduct GPS
mission planning. This involves determining the GPS satellite constellation coverage through
the use of software or interactive websites. This software is freely available from Trimble along
with updated ephemeris data.
2.2.2 Depth Sounder
Similar to the GPS unit, your depth sounder must support output of data in the NEMA 0183
standard format, particularly one or more of the following strings:
$SDDPT – Depth (required if no $IIDBT)
$SDDBT – Depth Below Transducer (required if no $IIDPT)
$SDMTW – Water Temperature (optional – at surface).
Speed and temperature inputs are optional. However, speed can be a useful guide to maintain
speed within an effective operating range. Surface temperature can be useful for analyzing
differences in water temperature to locate springs and upwellings, plume boundaries around
industrial sites, and to show the effects of wave action on temperature, etc.
If your specific application requires shallow water collection, make sure the sounder can record
to the required minimum depth. Unfortunately, most lower-cost commercial units will not
record depths shallower than 0.7 meters.
Transducers are available in through-hull or transom-mount models. The through-hull
transducers are more stable and require only one initial setup. Through-hull transducers are not
portable, however, thus limiting data collection to a specific boat. Transom-mount transducers
are very portable but require setup each time the transducer is used. Time must be taken to
mount and test the transom-mount transducers at the proposed collection speeds to ensure
proper operation. Transom-mount transducers perform best on the starboard side of the motor
and where the bottom of the boat is relatively flat. Incorrect placement of the transducer will
result in either poor data or no data at all. A factory made adjustable transducer mount is
recommended if you plan on using a transom-mount transducer. Some transducers are more
7
aerodynamically designed than others so if possible choose a transducer that will minimize
turbulence. Finally, make sure the frequency of the transducer matches the frequency of the
depth sounder unit. All transducers should be checked for accuracy on initial set up. This is
done easily by comparing sounder depths with a graduated, weighted hand line. Variations can
be corrected after the field surveys are completed.
2.2.3 Computer
The computer must contain at least one serial port but preferably two. In the two port case,
one port is dedicated to the depth sounder and the other to the GPS unit. The assignment of
these devices to a specific port is not necessary. In the one port case, the output of one device
can be fed through the other device. Normally, the latter device is the controlling device which
can create potential problems with truncating data strings. If two serial ports are necessary an
additional I/O card must be supplied that will interface with a PCMCIA slot.
The BASS program is Window-based running on either Windows 95/98 or Windows NT version
4.0. Although the program’s memory footprint is small ( about 6 megabytes of RAM), Windows
overhead requires a minimum recommended configuration of a Pentium class (133MHz)
machine, 24 Mb of RAM and at least 600 Mb of disk space. Output data files from the BASS
program require about 100-500 Mb depending on the amount of data collected, and how often
the data is archived to a workstation.
2.2.4 Power Supply
Each of the above devices requires a power source that has to provide power for potentially 10
to 12 hours per day. Typical battery discharge rates for notebooks are two to three hours per
rechargeable battery. The other devices have somewhat reduced discharge rates. As
alternative to rechargeable batteries, it is recommended that a 12 volt marine battery be used
to ensure a constant power source for all devices over a period of a few days. Remember, if
you plan to use a 12 volt power supply to run your computer then you will require a power
inverter and transformer to convert from 12v DC to 115v AC. It is strongly recommended that
your power line to your computer is fused.
The cables necessary will be vendor specific depending on the hardware chosen. Each device
will require cables for data output as well as power supply. There are also cables required for
external interfaces to antennae (GPS) as well as the transducer (sounder). Where necessary,
these cables must be constructed and tested in the office prior to field work. Figure 2
illustrates the wiring schematic for the data connections for a serial port interface.
8
Figure 1 Data connections for serial port interface
For all installations, provisions should be made to allow for easy storage due to rain or other
adverse weather conditions. You may wish to construct a “platform” to mount and protect your
equipment while deployed during field surveys
2.2.5 Software
The BASS software installation and use is described in a separate document “BassManual2.2”
(Levec 2001) available with the software.
9
2.3 Planning Your Survey
A survey plan should be completed prior to conducting field work. This process will ensure the
most efficient use of your time in the field, help avoid missing data, and minimize the collection
of overlapping or redundant data. For smaller lakes with very few or no islands this usually
takes no time at all. For larger lakes the plan may include multiple access points if they are
available, how to divide the lake up into manageable portions, as well as how to deal with
sampling in and around islands.
The best way to make a field plan is to create a large plot of the lake and look at any problem
areas. See if access is a problem and how many islands must be taken into consideration.
Begin to break the lake up into smaller sections or basins until you have manageable sections
that can be dealt with on a daily basis. The plan should also include input from other staff that
may know the lake and can identify areas of concern such as shoals or other features that may
warrant more detailed study.
If the waterbody has a surface area greater than 500 ha it is recommended that it be
partitioned into smaller sections. Partitioned units should not be greater than 500 ha in size.
When possible, natural boundaries such as bays (basins) should be used to delineate spatial
units. Otherwise, the waterbody can be sectioned along lines that conform to the planned
survey path (Figure 3.). The objective is to define an area that can normally be surveyed in
one day (300 – 500 ha).
10
START
5085300
BLU1
Space runs ~ 50 m. apart,
plus center of bay
5084800
FINISH
5084500
START
5084300
GRN1
5083800
5083300
FINISH
5083400
START
RED1
5082800
BLU2
5082300
5082200
FINISH
BLU3
5081800
710000 710500 711000 711500 712000 712500 713000 713500 714000 714500 715000 715500 716000 716500
Figure 3. Lake Lavieille bathymetry routes
Figure 2 Example of survey route map for Lake Lavieille showing start and
end points for 3 spatial units and proposed survey routes for each unit.
2.4 Plan of Attack
Once spatial boundaries have been defined, then the next step is to decide on the best survey
path for the shoreline circuits. The objective is to avoid overlapping points and to minimize
“wasted” travel time. Survey start points and end points should be selected with this in mind.
Map the path for shoreline cruises. On the first circuit you will attempt to follow the 1 m
contour. The second circuit will be offset by approximately 25 m from the first. This will
provide a buffer from the shoreline when running the open-water transects. In some
circumstances, the entire shoreline cruise may take more than one day to complete. In this
case the cruise should be broken up into manageable chunks equal to one days’ worth of
collection per section or basin.
Based on the purpose of the survey and the size of the waterbody, the next step is to decide on
the spacing of the open-water transects. Table 1 identifies general guidelines for transect
spacing that would be adequate for producing a bathymetric map suitable for most fisheries
11
management applications. If more accurate bathymetric information is required then spacing
between the transect lines can be reduced. Keep in mind that this will increase the amount of
time required to complete the survey. For ease of navigation, it is strongly recommended that
open-water transect lines have a north/south or east/west orientation.
Figure 3 provides an example of a survey plan for a portion of Lake Lavieille. In this case most
of the lake’s shoreline had previously been surveyed as part of a Nearshore Community Index
Netting (NSCIN) project so generally only the open-lake transect lines are shown.
2.5 GPS Mission Planning
When planning a day out in the field with any GPS equipment it is always a good idea to do
some mission planning. Essentially this involves running a software program to see what the
GPS coverage will be like for that day. Usually there will be one or two periods of time (about
an hour) that will not be ideal for GPS collection. Once you know these times you can plan
your collection time around them. Software is freely available from Trimble
(http://www.trimble.com/planningsoftware.shtml). Be sure to download the latest ephemeris
file which will give the latest updates for satellite orbits.
2.6 Preparation of Field Forms and Equipment
Before going into the field the survey crew should have copies of all the necessary maps. A
hard copy of your survey route map should be printed for use in the field. You may want to
consider laminating this map to protect it from the weather. A digital copy of the same map
may be loaded on your field computer. You will also need a copy of a suitable map showing
location and access to your lake (road, trail, air, etc.). A field equipment checklist is included in
Appendix A.
2.7 Safety and Communication
Bathymetric surveys may be carried out in early spring or late fall. As such, hypothermia is a
real threat and safety is of the utmost concern. Sampling should not be attempted if there are
severe weather conditions. All safety equipment should be accessible and personal floatation
devices must be worn while on the water. Floater suits are strongly recommended for early
spring and late fall surveys. All members of the survey crew should be aware of their rights
and obligations under the Occupational Health and Safety Act. A designated person should
know where the field crew is on any given day and how to contact them.
12
2.8 Setting Up and Testing the Equipment
If you are using an externally mounted transducer, the first step will be to attach the transducer
bracket to the transom. You may find this easier to do prior to launching the boat. When
mounting the bracket choose a location on the starboard side of the motor between the ribs and
away from any other obstructions to normal water movement. Adjust the bracket so that the
bottom of the transducer is situated parallel (or in the same plane) to the water surface and
extends slightly below (~1 cm) the hull. It is crictical that the transducer be mounted at the
proper angle to ensure accurate readings. Remove the transducer and bracket before launching
the boat.
Launch the boat. Place the transducer in the water, rub the bottom surface of the transducer to
remove air bubbles and then clamp the bracket in location on the transom. Make sure that the
transducer is mounted in the correct position. Choose a convenient location to secure the depth
sounder. Depth sounders are reasonably weatherproof and can withstand rain or spray.
Clamp the GPS stand or antenna to the transom, gunnel or seat in a convenient location at the
back of the boat, near the transducer. Again, most GPS units are weatherproof.
Select a location for the portable computer that will allow you to easily view the display screen
and access the keyboard during navigation. Since this screen will be your primary navigation
aid it must be clearly visible from the driver’s seat. Remember, your computer is not
weatherproof and must be protected from rain or spray (keep this in mind when you are
choosing a location).
Place the 12 volt marine batteries in a convenient location near the middle of the boat to help
distribute weight. General setup of the hardware depends on the size and style of your boat
and the type of equipment you are using. Figure 4 provides an example of a typical hardware
setup for bathymetric data collection.
13
Laptop computer
Battery #2
GPS/Depth Sounder
Battery #1
Transducer
GPS antenna
Figure 3 A typical GPS hardware setup for bathymetric data collection
Make sure that all units are switched off before connecting the cables and power.
Connect cables as follows:
transducer cable to depth sounder
GPS antennae cable to GPS unit
Real-Time Correction antennae cable if required
serial output cable from GPS unit to COM port on computer *
serial output cable from sounder to COM port on computer *
GPS battery cable to battery*
Sounder battery cable to battery*
Computer to battery (through power inverter)
*NOTE – Only one connection is required if using a combination GPS/Sounder
When all connections have been made, check to make sure that cables will not impede movement
in the boat and that they are not lying in water in the bottom of the boat. This may cause the
system to short out and overtime can lead to corrosion of cables. Once the cables are properly
situated, switch on the depth sounder, GPS unit and computer.
Ensure the depth sounder is providing a valid depth reading and that the measurements are in the
following units: m (depth); and km/hr (speed).
14
Next, check to ensure that GPS unit is configured as follows:
 COMMUNICATION OUTPUT/COM1
 PROTOCOL NMEA 0183
 DATA CAPTURE LOGGING RATE 1 – 2 seconds
 DATUM NAD83
 COORDINATE SYSTEM - UTM
 UNITS Meters
Please note that when using the NMEA protocol, certain standards are enforced automatically. The
NMEA standard includes a baud rate of 4800, 8 stop bits and no parity. The baud rate is set to
4800 by default, the option of 9600 is included for future revisions of the NMEA protocol.
You should also ensure that data is being output to the computer. This can easily be done by
displaying the NMEA output window provided in the BASS collection software.
15
3 SURVEY
3.1 Sampling Methods
Generally, a bathymetric survey can be carried out at any time during the ice-free period.
However, aquatic vegetation can significantly interfere with accurate depth readings. In order
to minimize this error, it is strongly recommended that waterbodies with prolific aquatic
vegetation are sampled either in early spring prior to establishment of macrophytes or late fall
following macrophyte die off. Interference related to aquatic vegetation will generally not be a
problem in lakes situated on the Canadian Shield.
Weather is another factor that may influence the timing of a survey. Safety and potential water
damage (from spray) to sensitive electronic equipment are important factors that must be
considered in the daily planning of a survey. Rough water can also affect the operation of the
depth sounder. It may cause turbulence which could cause loss of signal or result in erroneous
readings due to pitching or rolling of the boat which affects the angle of the signal. It is
recommended that you not attempt to conduct a bathymetry survey if wave height (trough to
crest) exceeds 0.5 meters.
On large waterbodies it may be necessary to divide the survey area into many sections. These
should be small enough to allow for complete sampling coverage of a section in a given day.
Typically, a survey crew should be able to cover a 300 – 500 ha area in one day. However,
areas with complex shorelines, numerous islands, and widely deviating bathymetry may require
more time.
Water levels in lakes, and particularly in rivers, may fluctuate naturally depending on the
season. Although not entirely necessary, it is recommended that once a bathymetry survey is
started sampling should continue to ensure completion within the same general
timeframe/season. This will help minimize corrections that may be required to compensate for
changing water levels.
In some cases, such as on large water bodies, it will not be possible to complete the survey
within the same season. Also, river systems with hydro-electric development(s) can fluctuate
significantly even within periods as short as an hour. The water level at the time of sampling
should be compared to a local benchmark in order to ensure consistent sampling when
gathering data over extended periods of time or in areas where short-term fluctuations may
occur. This benchmark should be noted for further reference. Staff gauges should be read on
lakes with control structures to allow meaningful comparisons at later dates.
The bathymetry survey uses a systematic approach to sampling which involves a combination
of shoreline perimeter cruises and cross-lake transects (Figure 2). Within a given section, all
the mainland and island shorelines are cruised at least twice in their entirety. The first shoreline
cruise should follow a path which maintains a constant depth of around one meter. On the
second shoreline pass, a distance of approximately 25 meters from the first circuit should be
16
maintained. This data is used to define a safe navigation buffer from which open water, crosslake transects will start and end. Evenly spaced parallel transects are then run across the open
water. For navigation purposes it is recommended that these transects follow either an
east/west or north/south path. When planning the orientation of your transects, it may be
useful to consider the direction of prevailing winds to minimize spray and drift effect (i.e. avoid
crosswinds). The spacing between transects depends primarily on the surface area of the
water body. Table 1 provides recommended guidelines for transect spacing which represent a
compromise between accuracy and financial/time constraints.
Table 1 Guidelines for open-water transect spacing for bathymetry surveys
Lake Surface Area (ha)
< 200
200 to 1000
> 1000
Transect Spacing (m)
50
75
100
These are general guidelines and represent a minimum program. The GPS system allows for
versatile data collection strategies and you are not limited to driving only pre-planned routes.
For example, if you see any sudden changes on the navigation screen that might indicate a
shoal or drop-off, then you can “zigzag” over the area of interest to collect more data.
Similarly, if you wish to increase sampling coverage in some critical areas, such as the littoral
zone, you may wish to consider more closely spaced transects or a grid pattern in these areas.
17
Figure 4 – Example of bathymetry data collection pattern showing shoreline
cruises and offshore transects for Canisbay Lake, Algonquin Park
Boat speed during data collection must be considered for two reasons. First, transom-mounted
transducers may cut out at faster speeds due to greater turbulence around the transducer. By
reducing boat speed, loss of depth signals will be minimized. Second, NMEA data is generally
pulsed at intervals of around one to two seconds which results in a time lag between the times
which position and depth sentences are written to a data file. This error is in addition to any
error present in the calculation of the position. Slower speeds obviously reduce this error but
also reduce the area that can be covered in the course of a day. Although consideration must
be given to tradeoffs between operational efficiency and accuracy, an average speed of no
greater than 15 km/hr is recommended. Exceptions would be when traveling close to shore.
Speed should be reduced for safety reasons as well as adhering to a ‘no wake’ principal. Speed
should also be reduced when making turns at the end of a transect. This will help to reduce
cavitation and improper sounder operation.
3.2 Running the Transects
At the start of your survey it is recommended that you consider establishing a benchmark so
that water level can be monitored over the course of the survey and at future dates. This is
especially important if the survey will span weeks or months or if you expect water levels will
18
fluctuate (e.g. rivers with hydro development). When establishing a benchmark, use an
existing, permanent feature such as the bottom of a bridge, top of a dam or a prominent mark
on a rock face. Record a thorough description of the benchmark, including geographic
location, and the distance between the water surface and bench mark. If possible, for river
surveys, try to tie in to existing survey monuments.
Use the survey route map that you prepared in the office to determine the start location for
your survey. Although you are not limited to driving pre-planned routes you should attempt to
adhere as closely as possible to your plan. Generally, your survey will begin with the shoreline
cruise. Two complete circuits of the mainland and island shorelines will be completed to define
a safe navigable buffer. On large, wide lakes it may be more efficient to postpone the island
shoreline cruise until you have begun the open-water transects. When navigating the shoreline
it is essential that you always be on the lookout for rocky shoals or deadheads. Usually the
driver will be monitoring depth on the computer screen while a spotter in the front of the boat
is watching the waters ahead. Polarized sunglasses are recommended to help reduce glare and
make spotting underwater hazards easier. For safety reasons, it is strongly recommended that
you travel at slow speeds (e.g. ~ 5 km/hr) when cruising the shoreline.
During the survey you should frequently check to ensure data is being collected and received
by the computer. Output can be checked by periodically checking the size of the collection file.
Note that the software actually outputs three files for backup; a NMEA log file, a text file, and a
shapefile.
The open-water transects are started once the shoreline cruise is completed for the lake or a
particular section of the lake. Begin the cross-lake transects using the survey route map.
Running accurate transects will depend on you using the computer screen for navigation. You
will also use the sounder or computer to constantly monitor depth. If you observe sudden
changes in depth, you may want to loop back and run a zigzag pattern or series of closely
spaced transects over the area of interest. You can also mark this location using the attribute
tools and revisit the site at another time. The objective is to more accurately define the spatial
extent and depth of potential shoals or holes. When deviating from your planned route it is OK
to cross existing transects (preferably at right angle) but you should avoid direct overlaps for
extended periods.
Although it is recommended that you generally not exceed 15 km/hr when running your openwater transects there may be times when you will want to deviate from this guideline. If you
are traversing a broad expanse of open water and depth is constant or changing very slowly
you can increase boat speed without significantly affecting the accuracy of the data. If you
choose to increase speed you will need to closely monitor depth for change and also to make
sure that the sounder continues to operate normally. A speed of 25 km/hr is the recommended
maximum that should not be exceeded. As depth begins to change more rapidly, then speed
should be reduced accordingly.
At the end of each day you must back up your data from the hard drive to floppy disk. It is
strongly recommended that you do two backups to floppy disk. Make sure that all disks are
clearly labeled with lake name, sector name and date and stored in a safe place. A more
common storage media now being used is a compact flash card with a PCMCIA adapter. This
19
setup will allow you to use one of the slots on the laptop. The advantage to this media is that
it can hold larger amounts of data and is easily transferable back to your office PC.
3.3 Troubleshooting
Equipment malfunction can be due to cabling problems such as loose connections, wrong or
faulty cables. Check the cables before leaving the office to ensure the system is operational
when you arrive in the field. Check cable connections and inline fuses if problems are
encountered in the field.
As mentioned previously, the devices must output NMEA data. The standard for NMEA data
transfer is 4800 baud, no parity, eight data bits and 1 stop bit. Ensure your devices are set to
this standard otherwise the BASS software will not be able to communicate with the devices.
3.3.1 No GPS Signal
A ‘No GPS Signal’ error usually occurs because the GPS unit is not currently determining a
position or the output interval of the NMEA data from the device has exceeded five seconds.
The output frequency of the GPS unit should not exceed two seconds. If required, reset this
value on the GPS unit to ensure continuous data collection. In either case, check to see that
the GPS unit is actually collecting a position.
You can also open the NMEA window in BASS to see that a data stream is coming through to
the computer. If there is no activity in the NMEA window, sometimes the COM port needs to be
reset. Restart the computer without any device connected to the COM port. Plug in the
GPS/Sounder into the COM port and turn it on. In windows, open up the program called
HyperTerminal found under Programs -> Accessories -> Communications. Run this program
and set up a connection with the COM port. Enable the port by clicking on the small telephone
icon. You should see activity in the window. If there is no activity in HyperTerminal, check the
settings on the GPS for proper NMEA output or a faulty cable. If there is activity in
HyperTerminal, close the program, start BASS, and open communications with the GPS.
If the satellite geometry is not currently suitable, or the filters for signal to noise ratio and
satellite geometry are set incorrectly, the GPS may not determine a position. Obstruction of the
unit’s antenna by any objects including topography (cliffs), shoreline vegetation or even human
bodies may interfere with the reception of signals from the satellites.
Very seldom during the course of a day should there be satellite reception problems under open
sky conditions. If this occurs, waiting fifteen minutes may improve any problems relating to the
satellite constellation. This can also be avoided by doing some mission planning the day
before.
3.3.2 No Depth Signal
20
A ‘No Depth Signal’ error usually occurs because the water may be too shallow or there may be
an excessive amount of turbulence around the transducer. The excessive turbulence
(cavitation) may be caused by fast speeds or improper mounting of the transducer. If a
turbulence problem is suspected try repositioning or adjusting the mounting bracket. Also note
that because cavitation increases on turns, it is recommended that boat speed is reduced
before entering and during a turn.
Note that most tranducers also have a limited shallow water range, usually around one meter.
Accuracy at less than a meter will be limited. Also in severe weedy conditions the transducer
may register 0, failing to receive a signal off the bottom. These values can eventually be
filtered out when pre-processing the data in a GIS.
3.3.3 Erratic Laptop Behavior
With the increased use of Panasonic Toughbook computers, many users have found these
laptops, particularly the mouse/pointer acts very erratically. This problem may be due to the
mapping of the COM port to a mouse at sometime, which conflicts with the touchpad mouse.
Sometimes a quick reboot will solve the problem. Otherwise you may have to go into the
device settings and remove the mouse driver. This should reset the mouse the next time the
computer is restarted. Also the COM port mapping may be affected when a Microsft’s
ActiveSync program is used for downloading data from the laptop. ActiveSync will search all
ports (COM, Infrared, etc...) and mark them as “in-use”. Again the COM port may have to be
reset in order to function properly.
21
4 Post-Survey
4.1 Disassembling the Equipment
When the transect work is completed and the data has been backed up then the equipment can
be disconnected and stowed in travel cases. Before disconnecting cables make sure that all
units are switched off. It is strongly recommended that you use a rugged travel case that is
padded and weatherproof for transporting electronic equipment. When a survey is completed
all equipment should be cleaned, dried and checked to make sure it is in working condition.
Recharge batteries. If necessary repair or replace damaged or malfunctioning components.
This is also a good time to download the data from the laptop to some sort of other media.
Choices could include a disk or CD, but the easiest choice now available is a USB jump drive.
These devices can store upwards of 10 gigabytes of data and are very reliable. Be sure to
make a copy of each days work a dn transfer it to a workstation in the office for surther GIS
work or archiving in NRVIS.
4.2 Transferring data to a GIS workstation
The corporate standard for Geographic Information Systems within the MNR is ESRI’s ARCGis.
Although many of the GIS processes can be duplicated on GIS software from other vendors, no
attempt has been made to adapt any of the operations for software other than ARCGis.
Once the data has been collected, there is more in-depth GIS analysis that can take place. One
of the most useful forms for continuous data such as bathymetry is the creation of a grid or
raster surface. Most GIS packages provide routines for the interpolation of irregularly spaced
point data. This surface layer can then be used to derive lake volumes, determine “home
ranges” based on depth, use them as a predictor of preferred habitat, and be used in
combination with other data to create other models.
Since the vendor of record for GIS at the MNR is ESRI, there is an easy transition to using
products such as ArcMap, along with its extensions. Spatial Analyst is used for handling raster
data and 3D Analyst is used for three dimensional display and analysis. Since existing digital
data within the MNR is stored as Arc/Info coverage or Arcview shapefiles, the integration of
bathymetric data with other data will be seamless. In addition, the resulting bathymetric
information can easily be input into the Natural Resource and Values Information System
(NRVIS).
22
4.3 Data Archive and NRVIS
In the longer term it is anticipated that NRVIS will be the repository for the bathymetry layer.
This will ensure broad access to this information and that it is maintained and up to date.
Planning for bathymetry requirements in NRVIS began in the spring of 2002. The data
management model was completed in 2004 and the document has been posted on ODMS for
review.
The data model for bathymetric data allows for data to be stored in one of two layers.
Bathymetry – Point will contain all raw observations collected using the BASS or similar
systems. Note that this is not a readily usable layer, but must be modeled in a GIS to create an
elevation model or contour line layer. The class definition in NRVIS is points, indicating the
measurement of water depth at various places in a body of water.
The layer Bathymetry – Line will contain derived contour bathymetry maps from various
sources. These could include old lake survey maps which have been scanned and
georeferenced, derived contour maps from GPS point data, as well as other contour maps from
outside the MNR. The class definition in NRVIS is a continuous line formed of vertices
indicating the same measurement of water depth at various places in a body of water. It is
used for a description of the terrain relief below the surface of the water.
These two models were decided upon due to the different nature of the datasets. Obviously
point features are different than line features, and must be stored accordingly. Point data
derived from GPS work is considered much more accurate than those derived from old transect
methods. Confusion may arise if contour maps, derived from BASS point data, are then
uploaded to the bathymetry line layer. Again the bathymetry line layer was originally intended
to store old historical data. Whether this layer becomes a working layer in NRVIS will be
determined by the users.
23
5 ACKNOWLEDGEMENTS
The methods described in this report are based largely on three existing bathymetry collection
systems. They are BASS (Bathymetric Automated Survey System) developed by Andy Todd
and Scott Christilaw at the North Bay Science and Technology Unit, the SLAPS system
(Shoreline and Profundal Surveyor) developed by Rob Kushneriuk at the Centre for Northern
Forest Ecosystem Research and the Real-time GPS Lake Bathymetry System developed by Brian
Monroe and Greg Betteridge at the Algonquin Fisheries Assessment Unit. The contributions of
these individuals to the field methodologies contained in this manual and to the development of
the new BASS application were critical.
24
6 REFERENCES
Monroe, B. and G. Betteridge, 2000. The Algonquin Fisheries Assessment Unit Real-time GPS
Lake Bathymetry System. Algonquin Fisheries Assessment Unit, Algonquin Park.
Christilaw, S. 1996. Bathymetric Automated Survey System Data Collection User Manual.
South Central Science and Technology Unit, North Bay.
Levec, F. 1996. B.A.S.S. Bathymetric Automated Survey System GIS Technical Guide. South
Central Science and Technology Unit, North Bay.
Levec, F. 2001. B.A.S.S. 2.0 User Manual. Inventory, Monitoring and Assessment,
Peterborough.
25
Appendix A - Equipment checklist for Bathymetric
Surveying
Log book / working paper map
Motor repair kit
spark plugs and spark plug wrench
large screwdriver
pliers
cotter/shear pins
wire
manual pull cord
spare gas line
Duct tape
Whistle
Fuel for the day
Paddles or oars
Bailing bucket or bilge pump
Waterproof flashlight
Buoyant heaving line
Anchor line (for the boat) - 30.5 m (100 ft) long
Required number of approved personal flotation devices
Rain suits, rubber boots (or waders)
GPS Unit (and antenna)
Depth Sounder (plus transducer and portable mount)
Portable computer (with BASS software, OBM outline, survey planning map)
Blank diskettes or other removable storage media (Compact Flash)
Appropriate cables
Power
2 GPS extendable battery cables
1 extendable GPS serial cable
1 sounder NMEA input cable
DC-AC power inverter
6 spare 5 amp fuses
Two 12 volt marine batteries
permanent transparency markers
HB pencils (ten or more)
Waterproof tarp large enough to protect electronic equipment
Copy of Bathymetry Survey Field Manual
Copy of BASS User Manual
Data Jump drive or other download media
26
Appendix B - Hardware Options for Bathymetric
Survey
Minimum requirements
Required
NMEA output
Ability to accept RTCM / RTCA differential corrections
WAAS capable receiver
Configurable GPS options like PDOP, elevation mask, and SNR
Long battery life
Waterproof
8-channel receiver, 12-channel preferred
Optional
Post-processing correction capability (Trimble)
External high-gain antennae
Additional waypoint / map storage memory
NMEA - National Marine Electronics Association
RTCM - Radio Technical Commission for Maritime Services
RTCA - Radio Technical Commission for Aeronautics
WAAS - Wide Area Augmentation System
PDOP – Precise Dilution of Precision
SNR – Signal to Noise Ratio
Recommended Devices (see manufacturers website for new models)
GPS
Garmin GPSMap76
Trimble Pathfinder ProXT/XH
Magellan Mobile Mapper
Sounder
Garmin 400c
Lowrance X510C
Combination GPS/ Sounder
Garmin GPSMap 420s/450s (new 2007)
Garmin GPSMap 238 (discontinued)
Lowrance LMS-480M / LMS-520C
27
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement