A DRG map or a digital raster graphic map is a map that has been scanned through the United states geological services catalog and shows topography of an area by using a 250 dot mechanism. This one shows the Potonoc River as it flows through Washington and the District of Colombia. They are useful just as any other maps to convey size, shape, and topographic data within the area.
http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.graphics/drg.jpg
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