DGR is an acronym for Digital Raster Graphic. Pretty much, a Digital Raster Graphic is a scanned image of a USGS map which includes all different types of information. The DRG maps are of extreme high quality with maps which are scanned with a minimum of 250 dots per inch.
http://www.ghostdepot.com/rg/maps/drg%20map%201899%20red.jpg
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