By Dawit D. Yifru, Department of Geology, The University of Georgia, Athens, Georgia, and John A. Peck, Department of Geology, The University of Akron, Ohio

Mapping Silver Lake bathymetry was an important part of an environmental research project conducted by the Office for Terrestrial Records of Environmental Change (TREC), Department of Geology at The University of Akron, Ohio. Silver Lake is a small glacial-formed kettle lake located in west central Ohio. The purpose of the research was to reconstruct the climatic and environmental change for the past 17,000 years in west-central Ohio based on the study of lake sediments. The goal of this research was important to understand past climate change for environmental prediction and to learn more about how the earth and its environmental systems function. Sediment cores need to be collected from the deepest part of the lake because this is where maximum sediment accumulation takes place, as shallow-water sediment is resuspended and moved downslope. Therefore, understanding depth distribution of the lake was vital in this study. GIS-based mapping of lake bathymetry was chosen because several previous studies indicated that this type of mapping is a fast, accurate, and inexpensive way of identifying areas for sediment coring.

A GARMIN GPS 125 Sounder equipped with a digital transducer, in conjunction with Window's HyperTerminal computer software, was used to collect latitude-longitude and water depth data points from Silver Lake. More than 6,200 data points were gathered in one day from a boat. The GPS measured the latitude and longitude coordinates while the transducer measured water depth by transmitting sound waves at fixed frequencies and receiving echoes from the acoustic impedance contrast of the lake floor, water column boundary. These data points were collected along several transects oriented oblique/perpendicular to the shore and separated by approximately 30-50 meters. The GARMIN GPS and transducer output several data types such as latitude-longitude coordinates, water depth, altitude, bearing from origin to destination, and water temperature. However, for this project only the latitude-longitude coordinates and the respective water depths were used. Gathering data along the shoreline of the lake was difficult because the shallow lake bottom could damage the boat motor and the transducer. In addition, it was not possible to drive or walk around the entire shoreline of the lake because of the extensive fringing marsh surrounding the lake. Therefore, the shoreline of a large-scale air photo of Silver Lake was digitized on screen and converted to an ArcInfo coverage using ArcInfo software's ArcMap application. The limited shoreline GPS latitude and longitude data collected in the field was used to georeference the aerial photo before digitizing the shoreline.

A total of 6,241 data points were spatially incorporated into the ArcMap tool by adding them into a GIS database in the form of a table containing latitude-longitude coordinates and water depth. These data points were used to create a triangulated irregular network (TIN) surface model of the lake floor by using the ArcGIS 3D Analyst extension in conjunction with the ArcMap application. A contour map of Silver Lake bathymetry was created from the TIN with a one-meter contour interval using the ArcGIS Spatial Analyst extension in conjunction with ArcMap. This contouring method created a zigzagged map with several "bull's eye" local lows and highs because the data points do not have equal spacing to create smooth contour lines. Therefore, smooth contour lines were created by digitizing the contour lines and eliminating the local highs and lows. In order to see the three-dimensional view of Silver Lake bathymetry, the TIN was exported to ArcScene, which is an ArcGIS application specifically designed for viewing 3D images. For a better three-dimensional view of the lake floor, the map was vertically exaggerated five times.

The result of this study showed that GIS could be effectively used to map lake bathymetry. The integration of GIS, GPS, and fathometer measurements was successful in producing a detailed bathymetric map and three-dimensional view of the lake floor. The digital bathymetric map of Silver Lake produced in ArcInfo has many advantages over traditional lead line sounding and hand-drafted bathymetric maps. In addition to being a fast, accurate, and inexpensive method of lake bathymetric mapping, GIS also has the advantage of being able to change the contour interval of the map according to the purpose of the map. This study provided excellent resource maps and reasonable estimates of the surface area and water depth of the lake. ArcInfo calculated the area of Silver Lake as 17.8 hectares, which is substantially different from the 86 hectares calculated from a hand-drafted bathymetric map of the lake. However, the 17.8 hectares is closer to the area estimated by Camp Wilson management at Silver Lake and agrees with qualitative estimates from the United States Geological Survey topographic map.

Comparison of the 1961 hand-drafted Silver Lake bathymetric map with the new ArcInfo bathymetric map reveals an overall similarity in lake floor morphology. However, the old map has a larger contour interval than the new map; therefore, it lacks detail. The new bathymetric map is georeferenced, which helps in locating and adding points on the map. Moreover, the new map shows detailed features of the shoreline of the lake because it was digitized from the high-resolution air photograph.

For more information, contact Dawit Yifru, Department of Geology, The University of Georgia, Athens, Georgia (e-mail: dawitdy@uga.edu), or John Peck, Office for Terrestrial Records of Environmental Change, Department of Geology, The University of Akron, Akron, Ohio.