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 perpendicular to the long axis of Easter Lake. 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 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 Easter Lake was determined by tape down (using a steel tape) from a reference point (with a known elevation) on the north rim of a manhole located on top of the concrete emergency spillway west of the main spillway. The elevation of the reference point, according to the Polk County Engineers Office (oral commun., 2003), was 816.85 ft above NGVD29. The water-surface elevation was found to be 813.0 ft above NGVD29 on July 30, 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 and perimeter 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 (eas_allpts, eas_edgepts, and eas_targpts). Projections for the coverages were then defined in ARC as UTM zone 15, NAD83, and units in feet.
Polygon coverages (eas_bndry and eas_island) were created to define the lake shore by first connecting (digitizing in ArcEdit) the points in the eas_edgepts coverage. The polygon coverages were used to generate the line coverages (eas_bndelev and eas_islelev). An elevation field (elev) was then added to the attribute table of the line coverages (eas_bndelev.aat and eas_islelev.aat). In ArcEdit, the elev field was set to a value of 813.0 ft (lake-surface elevation). The projection for the coverages were defined in ARC as UTM zone 15, NAD83, and units in feet. A digital orthophoto quad (DOQ) (Iowa Department of Natural Resources, 2002-2003, Iowa 2002 Digital Orthophoto Update Project, information available on World Wide Web at URL <http://www.igsb.uiowa.edu/nrgislibx/>) was later used as an aid in further adjusting the coverage of the shoreline.
In ARC, the eas_allpts coverage was processed through the WEEDPOINT command with a weed-tolerance of 30 ft to create the output coverage eas_tinpts30. Of the original 14,934 data points, 3,038 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 eas_tinpts30 coverage was then input into a TIN model.
The point coverage eas_ptsinterp was created using an arc macro language (AML) script. The script interpolated additional data points between bathymetry data points collected in the field. Data points were interpolated along creek channels and within lake coves to aid in generating contours.
To generate contours and calculate lake volume, three-dimensional surfaces (TIN models) were created using 3-D Analyst in ArcMap. The TIN eas_tin30 was used to generate the preliminary contours. The TIN model (eas_tin30) was generated using the eas_tinpts30, eas_targpts, and eas_ptsinterp point coverages triangulated as mass points. Height source was "elevation". The eas_bndelev and eas_islelev line coverages were input as hard lines with height source as "elev" (elevation value = 813.0 ft). The eas_bndry polygon coverage was input as a hard clip to clip the TIN to the lake shoreline. The eas_island polygon coverage was input as a hard erase to remove the island from the TIN. Another TIN model (eas_tin) was later generated to further adjust contours and to calculate lake volume after the boundary coverages (eas_bndry and eas_bndelev) were adjusted using the DOQ.
Preliminary contours were generated in ARC using the TINCONTOUR command to generate the eas_bth_orig coverage. The subdivision degree was set to 2 to specify the amount of triangular subdivision to help with smoothing. Contour intervals were set at 2 ft with a base contour of 793 ft. The minimum contour is 793 ft and the maximum contour is 811 ft. The coverage eas_bth_orig was then copied to eas_bth_cont. The eas_bth_cont coverage was the edit coverage in ArcEdit during smoothing. The eas_bth_orig coverage was used as a back coverage and for comparison during 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 coverage, eas_bth_orig2, was later generated from the TIN eas_tin to check and further adjust the outer most contours in the eas_bth_cont coverage after the boundary coverages were adjusted. The contour interval does not reflect map accuracy.
The volume for Lake Easter was calculated using the TIN model eas_tin in ArcMap/3-D Analyst above a reference plane (inverted lake-surface elevation) of -813.0 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 for Easter Lake was calculated to be approximately 61,026,000 cubic feet (1,400 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 13 09:08:09 2006