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New Contour Map Available for Dredge Area on Clear Lake
CLEAR LAKE - A map showing the contours and depths of the newly dredged west end of Clear Lake, known locally as Little Lake, has been completed and is now available on the DNR's website. Little Lake was mapped soon after the $9 million dredging project was completed this summer, and then data was added to create a depth map usable by the public. To view and download the map, go to http://www.iowadnr.gov/fish/fishing/lakes/cle17.html. The maps are created using data collected by a single beam sonar and Global Positioning System (GPS) clamped to the side of a boat and driven over the planned grid. The more variable the lake bottom, the greater the number of lines in the grid. The sonar device is an 8-inch diameter brass transducer that is positioned 9 to 10 inches under the water and has a survey-grade GPS antenna placed above it. The sounder works kind of like a fish finder. It shoots a sound pulse at the bottom of the lake, which bounces off the lake bottom and back to the boat where that data is collected in an onboard laptop computer. Recording the data is only the first step. Each sounding must be checked to make sure it is correct and false soundings are eliminated. A 600-acre lake can have between 200,000 and 300,000 soundings. The next step is to create a bottom surface, then add contour lines. Finally, a map must be created. The rest of Clear Lake is tentatively planned to be mapped in the spring. The old map of Clear Lake will remain until next summer.
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