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 viewable planned transect lines that were set-up in Hypack Max using the internal line editor prior to data collection. Transect lines were spaced at 50-ft intervals parallel to the long axis of Don Williams Lake. Depending on boat speed, one bathymetry data point was collected at approximately 4- to 10-ft intervals. Data points may be much closer in areas where transect or perimeter runs overlap. 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 were 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 Don Williams Lake 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 known benchmark (1130.258 ft, NGVD29) southwest of the dam about 1.5 miles. The elevation for the reference point (chiseled arrow) was determined from the third-order benchmark using a Topcon (GTS-220) total station system. The elevation of the reference point was found to be 1071.908 ft above NGVD29. The water-surface elevation was found to be 1065.33 ft above NGVD29 on July 28, 2003.
Post-processing: Bathymetry data were processed back in the office 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. The processed bathymetry data in Hypack Max were exported to XYZ format. A total of three separate files were exported into XYZ format: (1) transect bathymetric data; (2) 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).
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 three corresponding coverages (dw_allpts83, dw_edgepts83, and dw_targpts83). Projections for the coverages were then defined in ARC as UTM zone 15, NAD83, and units in feet.
A shapefile (lakebnd) was then created for the lake shore by digitizing (in ArcMap) the points in the dw_edgepts83 coverage. The shapefile was then exported to the polygon coverage (dw_bndry) and defined in ARC as UTM zone 15, NAD83, with units in feet (dw_bndry83). From the polygon coverage dw_bndry83, a line coverage (dw_bndelev83) was created in ARC. In ARC, an elevation attribute (elev) was added to dw_bndry83.aat. In ArcEdit, the elev attribute was set to a value of 1065.3 ft (NGVD29).
In ARC, the dw_allpts83 coverage was processed through the WEEDPOINT command with a weed-tolerance of 30 to create the output coverage dw_tinpts83. Of the original 11,162 data points, 2,349 data points remained after weeding. The weeding was performed to reduce the density of points in one direction (along data-collection transects) to minimize the spatial pull of contours toward the densified lines. The dw_tinpts83 coverage was then input into a TIN model.
Using 3-D Analyst in ArcMap, an initial TIN model (dw_tin) was generated using the dw_tinpts83 and dw_targpts83 point coverages triangulated as mass points. Height source was "elevation". The dw_bndelev83 arc was input as a hard line with a height source as "elev" (elevation value = 1065.3 ft). The dw_bnd83 coverage (polygon) was input as a hard clip to clip the TIN to the lake shoreline.
Preliminary contours were generated using Spatial Analyst in ArcMap using the dw_tin TIN model. Contour intervals were set at 2 ft with a base contour of 1024 ft. The minimum contour is 1024 ft and the maximum contour is 1064 ft. A shape file (bathycont2.shp) of the contours was generated. In ArcCatalog, bathycont2.shp was exported to the coverage dw_bth_orig. The coverage dw_bth_orig was then copied to the coverage dw_bth_work. The dw_bth_work coverage was the edit coverage in ArcEdit where the contours were smoothed and adjusted for interpretive errors. The coverage dw_bth_orig was used as a back coverage and for comparison during editing in ArcEdit. The dw_tinpts83 and dw_targpts coverages were also used as a back coverages for comparing data point elevations and contour values during editing in ArcEdit. For selected contours (arcs), grain size (using the GRAIN command) was set to 0.01. The SPLINE command was then issued. Then the grain size was set to 30 and splined again. For smaller contour arcs (that is, those representing small enclosed areas), final grain size was usually set between 5 and 10. Most interpretive adjustments and smoothing were done manually by adding, moving, or deleting vertices as needed. Contour movement was generally greatest in those contours with sharp bends using the GRAIN and SPLINE commands. A final copy (dw_bth_final) of dw_bth_work was generated for additional smoothing and editing. To assist in improving interpolation of the TIN model, an additional point coverage (dw_ptsinterp) of computer generated interpolated bathymetry points was created using an Arc Macro Language (AML) script that executes a linear interpolation between two known (elevation) data points (existing data points in the dw_tinpts83 coverage). The dw_ptsinterp coverage was used as an additional elevation source in generating the final TIN model (dw_tin2). The dw_tin2 TIN model was then used to further adjust contours to connect creek channels, run contours further into the coves, and to maintain consistency along steeply sloping contour intervals. Final edits were made to the contour coverage (dw_bth_final) using the interpolated data points. The contour interval does not reflect map accuracy.
The volume for Don Williams Lake was calculated using the TIN model, dw_tin2, in ArcMap/3-D Analyst. The volume was calculated above a reference plane (lake-surface elevation) of -1065.3 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 portions of the TIN surface lying between the specified base value (lake-surface elevation) and the highest point in the TIN. Water volume for Don Williams Lake was calculated to be 100,485,000 cubic feet (2,310 acre-ft).
Glossary:
Bathymetry - Measurement of depths in water GPS - Global Positioning System NGVD29 - National Geodetic Vertical Datum of 1929 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 Wed Sep 06 13:23:37 2006