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 100-ft intervals perpendicular to the long axis of Lake Macbride. Depending on boat speed, one bathymetry data point was collected at approximately 5- to 10-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 shoreline 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 Macbride 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 verified by leveling from a known benchmark (716.219 ft, NAVD88) located north of the dam about 200 ft. The elevation of the reference point was found to be 714.96 ft (NAVD88). From the tape down, the water-surface elevation was found to be 711.83 ft (NAVD88) on July 3, 2003. Tape downs were made every day during data collection.
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 sounding depths to elevations. The processed bathymetry data in Hypack Max were exported to XYZ format. The perimeter and transect bathymetry data were weeded to reduce the density of points in one direction (along data-collection transects) to minimize the spatial pull of contours toward the densified lines. Of the original 137,477 data points, 14,134 data points remained. A total of four separate files were exported into XYZ format: (1) transect bathymetry data; (2) perimeter bathymetric data; (3) shore point location data; (4) and the target point data. All four XYZ files were then converted to ASCII text files for input into ESRI-ArcGIS (ver. 8.3). The perimeter and transect data were combined into one data set (alllines_perim_sort.txt). The file allpts_noweed.txt contains all data points (not weeded) exported from Hypack Max (not including edge and target points).
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 (macb_edgepts, macb_tinpts30, and macb_targpts). The projections for the coverages were then defined in ARC as UTM zone 15, NAD83, and units in feet. Some shore points were moved, deleted, or added to the coverage macb_edgepts in ArcEdit as some data points were either missing or shifted due to loss of differential GPS (primarily in the south arm) during data collection in the field. Data points were added or moved by digitizing along the shoreline of digital orthophoto quads (DOQ's) and 1:24,000 USGS topographic quads (brought into ArcEdit as background images). The resulting shore point coverage (macb_edgeedit) contains a total of 1,303 shore points defining the lake boundary. Some points in the macb_targpts coverage were deleted, either because of positional errors, or because the locations were in areas of the lake where bathymetry data were already collected. The final target point coverage is macb_targedit. The coverage macb_allpts contains the unweeded data points.
A polygon coverage (macb_bndry) was then created for the lake shore by digitizing (in ArcEdit) the points in the macb_edgeedit coverage. Iterative edits resulted in the final coverage macb_bndry3. Another polygon coverage (macb_islands) was created by digitizing in ARC the corresponding points in the macb_edgeedit coverage. This coverage was used to define the location and boundary of islands in Lake Macbride. The coverages were defined in ARC as UTM zone 15, NAD83, with units in feet. Copies of these coverages (macb_bndelev and macb_islelev) were created in ARC and an elevation attribute "elev" was added to the macb_bndelev.aat and the macb_islelev.aat. In ArcEdit, the "elev" attribute of both these coverages were calculated to a value of 711.8 ft (NAVD88).
Using 3-D Analyst in ArcMap, a TIN (macb_tin4) model was generated using the macb_tinpts30 and macb_targedit point coverages triangulated as mass points. Height source was "elevation". The macb_bndelev and macb_islelev arcs were input as hard lines with height source as "elev" (elevation value = 711.8 ft). The macb_bndry3 coverage (polygon) was input as a hard clip to clip the TIN to the lake shoreline. The islands coverage was input as a hard erase to clip out the islands.
NOTE: Prior to generation of the TIN (macb_tin4), a datum conversion (-0.51 ft, adjustment of 1912 to NAVD88) was applied to the elevation or elev items in the info tables of all point and polygon coverages. The datum conversion was found after a static GPS survey on 10/27/04 near the USGS benchmark near the Lake Macbride dam. The original XYZ and resulting text file data exported from Hypack Max all retain elevation values in relation to the adjustment of 1912 datum as applied in the tide files during processing in Hypack Max.
Preliminary contours were generated from the TIN model (macb_tin4) in ARC using the TINCONTOUR command to generate the macb_bth_orig coverage. The subdivision degree was set to 4 to specify the amount of triangular subdivision to help with smoothing. Contour intervals were set at 4 ft with a base contour of 670 ft. The minimum contour is 670 ft and the maximum contour is 710 ft. The coverage macb_bth_orig was then copied to macb_bth_cont. In Arcedit, the macb_bth_orig and macb_tinpts30 coverages were used as back coverages for comparison during adjustment of interpretive errors and smoothing. Most adjustments and smoothing were performed manually by adding, moving, or deleting vertices as needed. For a few selected contours (arcs), smoothing was performed using ArcEdit commands. The 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. The command UNSPLIT NONE was then issued. For smaller contour arcs, (i.e. those representing small enclosed areas), final grain size was usually set between 5 and 10. Contour movement was generally greatest in those contours with sharp bends. The contour interval does not reflect map accuracy.
Using the TIN model (macb_tin4), the volume for Lake Macbride was calculated in ArcMap/3-D Analyst above an inverted reference plane (lake-surface elevation) of -711.8 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 590,501,000 cubic feet (13,600 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 Tue Jul 25 09:32:41 2006