DRG - Digital Raster Graphics - Scanned images of US Geological Survey Topographic maps. Maps are scanned at a minimum resolution of 250 dpi. Maps are georeferenced to the Earths surface to fit the Universal Transverse Mercator Projection.
http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.html
http://egsc.usgs.gov/isb/pubs/factsheets/fs08801.graphics/drg.jpg
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