Prepared in cooperation with the Iowa Department of Natural Resources
By S. Mike Linhart and Kris D. Lund
U.S. Geological Survey Scientific Investigations Map 2933
AVAILABLE ONLINE ONLY
A bar check calibration on the echo sounder was performed at the start of each day following established protocols (U.S. Army Corps of Engineers, 1994, Engineering and design: Hydrographic survey EM 1110-2-1003, chap. 9-3, p. 9-4 to 9-9). This was done to ensure that the echo sounder was calibrated correctly. The bar check involves suspending a 2-ft-diameter flat aluminum plate directly below the echo sounder. The suspension line is marked in 5-ft increments. An initial calibration is made at 5 ft by entering the speed of sound in water (based on water temperature) and then adjusting the offset of the transducer in the computer software. The offset is the draft of the transducer below the lake surface. The aluminum plate is then lowered in 5-ft increments (depending on the range of depths expected to be encountered on the day of data collection) and adjustments in the speed of sound are made until depth readings and the depth of the aluminum plate agree to within approximately 0.1 ft.
Data were collected along planned transect lines that were set-up in Hypack Max prior to data collection. Transect lines were spaced at 50-ft intervals perpendicular to the long axis of Lake Meyer. Depending on boat speed, one bathymetry data point was collected at approximately 4- to 8-ft intervals. Data points may be much closer in areas where transect or perimeter runs overlap. Perimeter bathymetry data points were collected while driving the boat around the perimeter of the lake. Target points were collected in areas of the lake where it was too shallow to collect data with the BSS+5 system. Target point depths were collected manually along with a corresponding easting and northing location. The number and location of target points were based on a judgment made in the field that was thought to be spatially representative of the area. In addition, shore point locations (easting and northing) were collected to define the edge of the lake. These locations are collected by touching the bow of the boat to the shoreline at various intervals along the shoreline and recording the position using a bow-mounted GPS antenna.
The water-surface elevation of Lake Meyer was determined by tape down (using a steel tape) from a reference point (with a known elevation) on the concrete outflow structure at the dam. The elevation of the reference point was determined by surveying from a reference mark (temporary wooden stake) nearby on the dam. The elevation (1142.388 ft NAVD88) for the reference mark (wooden stake) was determined from third-order benchmarks using an Ashtech Z-12 GPS system. The elevation of the reference point was found to be 1139.180 ft (NAVD88). The water-surface elevation was found to be 1135.76 ft (NAVD88) on July 21, 2003.
Post-processing: Bathymetry data were processed using the Hypack Max software. Post-processing involved removing obvious spikes, inputting depths for the time-tagged target points, and editing extraneous points within the shore point files. In addition, tide corrections were applied in Hypack Max to convert depths to elevations. For Lake Meyer data, it was found that some bathymetry data was lost in the deepest parts of the lake (in the old creek channel near the dam). This was due to improper gate selection in SDIDepth software (Specialty Devices, Inc., 2003, Bathymetric Survey System BSS+5 with omnistar manual: Specialty Devices, Inc., Plano, Texas, 38 p.) which controls the range in which data can be collected. SDIDepth is the software which collects seismic digital data but is also where parameters controlling transducer operation are entered. This software communicates with Hypack Max during data collection. Lost bathymetric data for these areas were recovered by exporting the depths recorded by DepthPic software (used to digitize the bottom layer of recent sediment) (Specialty Devices, Inc., 2003, Bathymetric Survey System BSS+5 with omnistar manual: Specialty Devices, Inc., Plano, Texas, 38 p.) into XYZ format. The processed bathymetry data in Hypack Max also were exported to XYZ format. A total of three separate files were exported into XYZ format: (1) the transect and perimeter bathymetric data; (2) the shore point location data; (3) and the target point data. All three XYZ files were then converted to ASCII text files for input into ESRI-ArcGIS (ver. 8.3).
NOTE: All bathymetry data processed in Hypack Max and subsequently exported to XYZ and text files were based on an incorrect water-surface elevation (1138.97 ft instead of 1135.76 ft) due to transcription error. Corrections were not made in Hypack Max or the XYZ and text files but were made in the coverage INFO files in ARC.
Generation of Bathymetry Map: The output text files (containing northing, easting, and elevation) were input into an arc macro language (AML) script called createpoint in ARC to generate the coverages mey_allpts83, mey_targpts83, and mey_edgepts83. Projections for the coverages were then defined in ARC as UTM zone 15, NAD83, and units in feet.
A shapefile (mey_bndry) was created for the lake shore by digitizing (in ArcMap) the points in the mey_edgepts83 coverage. The shapefile was then exported to the coverage (mey_bndry83). In ARC, an elevation attribute "elev" was added to mey_bndry83.aat. In ArcEdit, the "elev" attribute was calculated to a value of 1135.76 ft (NAVD88).
In ARC, the mey_allpts83 coverage was processed through the WEEDPOINT command but the points were not weeded throughout the coverage extent. After several failed attempts to use the WEEDPOINT command, the points were weeded out in the original text file (mey_allpoints.txt) using a macro in the text editor, EMACS. Of the original 5,524 data points, 1,838 data points remained after weeding. The text file (mey_tinpoints.txt) of the weeded bathymetry data was then input into createpoint in ARC to generate the coverage mey_tinpts. The mey_tinpts coverage was then projected to create the coverage mey_tinpts83 for input into a TIN model.
Using 3-D Analyst in ArcMap, a TIN model (mey_tin) was generated using the mey_tinpts83 and mey_targpts83 point coverages triangulated as mass points. Height source was "elevation". The mey_bndry83 arc was input as a hard line with a height source as "elev" (elevation value = 1135.76 ft). The mey_bndry83 coverage (polygon) was input as a hard clip to clip the TIN to the lake shoreline.
Contours were generated in ARC using the TINCONTOUR command to generate the mey_bth_orig coverage. Contour intervals were set at 2 ft with a base contour of 1108 ft. The minimum contour is 1108 ft and the maximum contour is 1134 ft. The subdivision degree was set to 4 to specify the amount of triangular subdivision to help with the smoothing. The coverage was copied to mey_bth_cont to be used as the working coverage. In ArcEdit, the mey_bth_orig coverage and the mey_tinpts83coverage were used as back coverages for comparison during adjustment of interpretive errors and smoothing. All adjustments in ArcEdit were done manually by adding, moving, or deleting vertices as needed. Contour movement was generally greatest in those contours with sharp bends. The contour interval does not reflect map accuracy.
The volume for Lake Meyer was calculated (using the TIN model mey_tin) in ArcMap/3-D Analyst above a reference plane (inverted lake-surface elevation) of -1135.76 ft. The TIN was inverted for calculation purposes only, following guidelines in ESRI-ArcGIS (ver. 8.3). ArcMap/3-D Analyst uses linear interpolation to calculate the area and volume of those of those portions of the TIN surface lying between the specified base value (lake-surface elevation) and the highest point in the TIN. Water volume was calculated to be approximately 17,831,000 cubic feet (410 acre-ft).
Glossary:
Bathymetry - Measurement of depths in water GPS - Global Positioning System NAVD88 - North American Vertical Datum of 1988 GIS - Geographic Information System NAD83 - North American Datum of 1983 UTM - Universal Transverse Mercator Easting and Northing - UTM coordinates TIN - Triangulated Irregular Network
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Generated by mp version 2.8.6 on Thu Sep 07 14:03:40 2006