DRG map

http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.html

A DRG (Digital Raster Graphic) is a USGS topographic map that has been scanned into a computer.  This is a DRG of an area around the Potomac River in Washington DC.  It is a DRG because it was a printed topographic map that was then scanned.