DRG

http://www.tceq.texas.gov/gis/drg.html

A DRG map, also known as a Digital Raster Graphic, is a scanned image of a USGS topographic map into a computer. These maps can include spatial data, geographical features, and any other aspect of a map. This map here is a scanned image from a topographic map that represents the Colorado River and surrounding features such as schools.