Mapping river and lake bathymetry has long been used as a means to define channel and lake bed characteristics. Repetitive bathymetric mapping can help determine sedimentation rates, scour and deposition of bed material, and the effectiveness of dredging. In the past, bathymetry mapping involved establishing permanent benchmarks along the banks of the water body of interest. Transects were established from benchmarks on opposite banks and a boat equipped with a sounding line and weight or fathometer crossed the transects, recording water depths along the way. The process was time consuming and costly. The combination of a global positioning system (GPS) with an acoustic Doppler current profiler (ADCP) provides a fast, accurate method of mapping bathymetric surfaces. The GPS measures the longitude- and latitude-coordinate location of the point where the ADCP measures water depth. The two data sets are merged, and a longitude and latitude coordinate is determined for each water depth recorded by the ADCP. The combination of the two technologies also provides an easy method for locating and measuring the same coordinates in the future without the need to establish permanent benchmarked reference sites.
The Indiana District of the U.S. Geological Survey (USGS) used the GPS-ADCP combination to map channel cross sections at large, unwadeable river sites (Shoals, Hazleton, Elnora, and Centerton, (fig. 1) in the White River Basin, Ind., and to map lake-bathymetric surfaces on Geist and Morse Reservoirs in Indiana (fig. 1) . Channel cross sections were measured for an aquatic-community analysis of streams in the White River Basin as part of the USGS National Water Quality Assessment (NAWQA)Program. NAWQA is a nationwide program established to describe the status and trends in the quality of a large, representative part of the Nation's surface and ground-water resources (Hirsch and others, 1988). One aspect of the Program is to collect information about biological communities in streams. The stability of bed materials is important in the establishment and continuation of aquatic communities. Channel cross-section profiles measured over time help determine the characteristics of the channel bed. By knowing the GPS coordinates for the channel cross-section profiles, it is possible to return to the same location in the future to repeat mapping procedures and to chart changes in the river channel.
Figure 1. --Location of river-bathymetry mapping sites
in the White River
Basin, Indiana, and lake-bathymetry mapping sites on Geist and Morse
Reservoirs, Indiana.
(Available:
on-line GIF)
The GPS-ADCP combination was adapted for mapping
the bathymetric surfaces for Geist and Morse reservoirs.
Lake bathymetry was mapped as part of a cooperative
project with the Indiana Department of Natural
Resources (IDNR) and the USGS to determine sedimentation
rates in the two water-supply reservoirs (fig. 1) .
Because the cost of mapping the reservoirs by establishing
permanent benchmarks was prohibitive for the
IDNR and because the results of the survey were needed
in a short time, the GPS-ADCP method provided an ideal
solution to the problem. The lake surveys were dependent
on the GPS longitude and latitude coordinates to
transform the ADCP depth data to real-world coordinates.
The data then could be contoured, plotted on
existing maps, and compared to earlier bathymetric surveys.
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DESCRIPTION OF EQUIPMENT
The equipment required for a bathymetric survey
include a mobile GPS receiver and a stationary (base
station) GPS receiver, an ADCP, a laptop computer to
collect the ADCP data, and a boat equipped with the
hardware to mount the ADCP. If real-time coordinate
data are needed, the mobile GPS unit and the stationary
unit should be equipped with two-way communication
devices; GPS coordinates then can be differentially
corrected (described in the following section) as the
data are collected.
The GPS Mobile Receiver and Base Station
The GPS mobile receiver and base-station receiver are needed to provide real-world coordinates for the water-depth data provided by the ADCP. Both receivers are needed to obtain the level of accuracy (2-5 meters) required to complete the surveys.
The GPS calculates the receivers' position on Earth by determining the distance from the receiver to 3 or more of the 25 GPS satellites currently orbiting the Earth. The position obtained by a stand-alone GPS receiver is determined by satellite trilateration. Satellite trilateration is the process of calculating the intersection of three or more spheres, the centers of which are the positions of the observable GPS satellites (Trimble Navigation, 1994). Accuracy can be affected by the Department of Defense, which has the ability to degrade GPS accuracy at any time with Selective Availability (SA). The resulting absolute positional accuracy on the ground can be anywhere between 25 and 100 meters (Cloyd and others, 1995).
The application of differential techniques provides the solution to obtaining higher GPS accuracy. Differential techniques use two receivers, a stationary base-station receiver at a known location and in close proximity (within 500 kilometers) of a mobile receiver. Both receivers operate simultaneously. The reason for the two receivers is that the receivers are influenced almost equally by SA positional errors and by atmosphere error. If one receiver is at a known location, it is possible to correct the positional data collected by the mobile receiver by applying the amount of difference between the known location and the calculated location of the base-station receiver to the data collected by the mobile receiver.
Data can be corrected differentially as the data are collected or by post-processing. If the data are differentially corrected as they are collected, real- time communication between the base station and the mobile unit is required. Real-time data were not needed for the initial river and lake surveys conducted by the Indiana District of the USGS; therefore, the differential corrections were applied after the data were collected.
Data collected for the river and lake surveys were
differentially corrected with base-station files from
the USGS, Indiana District first-order base-station
receiver located at the Indianapolis office. A first-
order station means that the location of the station is
known with an accuracy of 1:100,000, or 10 centimeters
over 10 kilometers. The station is centrally located
and provides sufficiently accurate base-station data
for most GPS data-collection efforts in the State.
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The Acoustic Doppler Current Profiler
The ADCP measures water velocity and depth for discreet vertical columns of water. ADCP's transmit sound and listen to echoes returning from sound scatterers in the water (such as small particles or plankton) and from the bed material (RD Instruments, 1989). The ADCP transmits groups of sound pulses into the water column. The groups of pulses include water-profiling pulses and bottom-tracking pulses. A group of pulses containing an operator-set number of water-profiling pulses interspersed with an operator-set number of bottom-tracking pulses in an "ensemble"; a single ensemble may be compared to a single vertical sounding from a conventional discharge measurement (Oberg, 1994). The ADCP collects data continuously and may collect hundreds or thousands (for large lakes) of ensembles for a given transect. The ADCP measures water depth with an accuracy of plus or minus 10 centimeters (for this particular instrument configuration and application) (RD Instruments written commun., 1995).
The ADCP has the capability of bottom tracking. Bottom tracking is the ADCP's internal x-y coordinate location system. As the boat moves, the ADCP tracks and records its x-y location in northing and easting from the origin (the point at which the ADCP began to collect data). The ADCP loses its ability to bottom track if the instrument is operated in water less than 1 meter or greater than 90 meters (maximum depth dependent on transducer frequency). The ADCP will not track the bottom if weeds are present. These limitations make it necessary to use GPS coordinates to supplement the ADCP data. The accuracy of the ADCP bottom tracking is plus or minus 9 centimeters per second (for this instrument configuration and application) (RD Instruments written commun., 1995).
The ADCP requires a laptop computer to run the
ADCP software and to store the ADCP data. The software
has the capability of merging GPS point data with each
depth ensemble collected. This option is practical only
if GPS data are differentially corrected as they are
collected. In the application described here the data
were merged when differential corrections were made
rather than during data collection.
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METHODS
Water depths were measured along transects for the
river and lake surveys. The ADCP and GPS were mounted on
a boat, and the boat was driven along several transects
in both surveys. GPS data were collected at selected
sites at the same time that water depths were measured.
Mapping River Bathymetry
The objective of the river survey was to obtain river-channel profiles for selected reaches of the White River and East Fork White River. Three sites with a single reach surveyed and one site with three reaches surveyed required the type of channel-profile data provided by the GPS-ADCP instruments (fig. 1) . Reach lengths ranged from 765 meters to 989 meters. Six cross sections were measured at each of the six reaches. Channel cross sections were measured with the two instruments mounted on a boat at each cross section along a river reach; the boat was driven perpendicular to the channel from bank to bank along the cross section. GPS latitude and longitude coordinates were collected at the starting and ending points of each cross section. At some reaches, the water depth near the bank was less than one meter; when this situation occurred, water depths were measured with a standard wading rod with depth graduations. The GPS receiver was used to record longitude and latitude for the depths made with the wading rod.
GPS data were collected at each starting and ending point until at least 120 position locations were collected for each point. After the 120 points were differentially corrected, they were plotted and analyzed to determine if multipath errors were present. Multipath errors occur when the GPS receives satellite signals reflected by some other object (a tree or cliff) before the signal reaches the receiver. After multipath errors were eliminated, the remaining points were averaged and a single longitude and latitude coordinate was obtained for each starting and ending point of the cross section.
The bottom-tracking feature of the ADCP was used to obtain a temporary x-y coordinate location for each depth measured. Depth- and bottom-tracking data were stored in separate computer files for each cross section. A starting and ending x-y coordinate (in northing and easting from the origin) from the bottom-tracked data were recorded.
A Geographic Information System utilizing the ARC/ INFO software was used to convert the ADCP data to coordinates that could be plotted on a map. ARC/INFO point coverages were generated from the longitude and latitude data collected by the GPS. The coverages were converted to the Albers Equal Area projection because the base maps that will be used to illustrate the channel bathymetry data are in that projection. The ARC/INFO TRANSFORM command was used to convert the ADCP x-y coordinates to the Albers Equal Area projection. To perform the conversion, at least two points (the starting and ending points) in the ADCP coverage had to correspond to two points in the GPS coverage. Depth data were stored for the corresponding point locations in the ARC/INFO data base. Figure 2 shows the channel profiles for the first, third, and sixth cross sections measured at one large, unwadeable river site--the East Fork of the White River at Shoals in the White River Basin.
Figure 2. --Channel profiles derived from GPS and ADCP data
collected for
the first, third, and sixth transects at the East Fork White
River at Shoals, Indiana.
(Available:
on-line GIF)
The ADCP provides virtually all the data required
to map channel cross sections. The GPS makes it possible,
however, to convert the ADCP x-y data into coordinates
that can be plotted on existing maps. With the use
of real-time differentially corrected GPS, or GPS with
SA removed, it will be possible to return to the same
site in the future and map the same cross sections with
a high degree of accuracy. Such repetitive mapping is
needed to meet some of the data-collection requirements
of the NAWQA program.
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Mapping Lake Bathymetry
The objective of the lake survey was to obtain enough depth data to generate a depth-contour map of the bottom surface for each reservoir and to compute the volumes of the reservoirs. The surface areas of Geist and Morse Reservoirs are about 728 and 556 hectares, respectively (Ruddy and Hitt, 1990). Because the reservoirs are large, it was necessary to map small sections at a time. Bathymetric and GPS data were collected with the two instruments mounted on the boat; the boat was driven in a zigzag pattern along the length of the reservoir section being mapped Figure 3. Data also were collected along transects running the length of the section; when all the data were collected for a given section, a grid pattern of data points of at least 300-meter intervals was collected for that section. The distance between grid intervals varied in shallow areas, inlets, and headwater areas. Soundings were made with a wading rod in areas where the water depth was less than one meter. The GPS receiver was used to obtain longitude and latitude coordinates for the wading measurements.
Figure 3. --Representation of the zigzag pattern used to
collect
bathymetric data with ADCP and GPS instruments on a section of
Geist Reservoir, Indiana.
(Available:
on-line GIF)
ADCP bottom tracking was used to obtain temporary x-y coordinates of the transects for each section. Latitude and longitude were collected with the GPS unit for at least three registration points in each section. Corresponding x-y coordinates from the ADCP bottom- tracked data also were recorded; the registration points obtained with the GPS unit then could be used to convert the ADCP coordinates to real-world coordinates. The GPS receiver was used to collect at least 120 coordinates for each registration point. After the coordinates were differenced, they were analyzed for multipath errors. Multipath errors were deleted, and the remaining coordinates were averaged; a single longitude and latitude coordinate was obtained for each registration point. The GPS unit also was used to collect latitudes and longitudes for shoreline points around the lake.
Water-surface elevations were recorded during the bathymetric surveys for each reservoir. Reference marks were established on bridges at several locations on each reservoir. Water-surface elevations were computed by measuring the distance between reference marks and the water surface with a steel tape. Standard vertical control surveys using an automatic level were made to establish a mean-sea-level datum for each reference mark; the water- surface elevations then could be computed in mean sea level. Surveys for each reservoir were made during the spring when water levels were high.
ARC/INFO software was used to convert the ADCP
data to coordinates that could be plotted on a map. ARC/
INFO point coverages were generated from the longitude
and latitude data collected by the GPS. The coverages
initially were converted to the Universal Transverse
Mercator (UTM) projection. The ARC/INFO TRANSFORM command
was used to convert the ADCP x-y data into UTM
coordinates. The shorelines of each reservoir were digitized
from large-scale air photos. The shoreline-point
data collected in the field were used to verify and
adjust the digitized shoreline data. After the conversions
were made for each of the sections mapped, the
shoreline data and the coordinate data with their associated
water depths were combined into a single ARC/
INFO GRID coverage. ARC/INFO contouring software was
used on the GRID coverage to generate contoured surfaces
for the lakes. An inspection of the final contours
were made to ensure that the contours agreed with
the point data.
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SUMMARY
A global positioning system (GPS) and an acoustic Doppler current profiler (ADCP) were used to map river and lake bathymetry in Indiana. Water depths at selected locations were measured with the two instruments mounted on a boat. The GPS measured the longitude- and latitude-coordinate location of the ADCP with an accuracy range between 2 to 5 meters. The ADCP measures the depth of water along transects. The combination of the two systems provides a fast, accurate method of mapping lake and river bathymetric surfaces. The combination of the two technologies also provides an easy method for locating and measuring the same transect in the future without the need to establish permanent benchmarked reference sites. Mapping river and lake bathymetry over a period of time makes it possible to determine changes in the character of the lake or river.
The use of brand names is for identification purposes
only and does not imply endorsement by the U.S.
Geological Survey.
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REFERENCES
Cloyd, P., Gray, E.R., and van Gelder, B.H.W, 1995. Status of the Proposed Federal/Cooperative Base Network (FBN/CBN) for the State of Indiana: School of Civil Engineering, Purdue University, West Lafayette, Ind., 15 p.
Hirsch, R.M., Alley, W.M., and Wilber, W.G.,1988. Concepts for a National Water-Quality Assessment Program: U.S. Geological Survey Circular 1021, 42 p.
RD Instruments, 1989. Acoustic Doppler Current Profilers Principles of Operation--A Practical Primer: RD Instruments, San Diego, Calif., 36 p.
Ruddy, B.C. and Hitt, K.J., 1990. Summary of Selected Characteristics of Large Reservoirs in the United States and Puerto Rico, 1988: U.S. Geological Survey Open-File Report 90-163, 295 p.
Trimble Navigation, 1994. GPS Mapping Systems, General Reference: Trimble Navigation Limited, Surveying and Mapping Division GeoExplorer Software and Manuals PN 24177-00, Trimble Navigation, Sunnyvale, Calif.
1.
Nancy T. Baker
U.S. Geological Survey, WRD
5957 Lakeside Boulevard
Indianapolis, IN 46278
Email: ntbaker@usgs.gov
2.
Scott E. Morlock
U.S. Geological Survey, WRD
5957 Lakeside Boulevard
Indianapolis, IN 46278
Email: smorlock@usgs.gov
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