http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.html
A DRG map stands for a digital raster graphic which is essentially a scanned in digital version of the standard 7.5 minute USGS topographic maps. Many GIS type softwares use these maps as they are in digital form and can have layers added or removed from them. The above DRG represents an area of western Washington D.C. on the Potomac River.
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