DRG

http://www.tceq.texas.gov/gis/drg.html
DRG maps, or Digital Raster Graphic maps, are digital images obtained from scanning and georectifying a topographic map from a U.S. Geologic Survey (USGS). They are useful tools for many geographic information systems, as they use raster format technology, so the images are already coded in pixels. This DRG of the Colorado River is standard size of a USGS topo map: 7.5 x 7.5 minute boundaries.