DRG

DRG maps are digital raster graphics. They are scanned images of a U.S. Geological Survey standard series topographic map. The image inside the map neat line is geo-referenced to the surface of the earth and fit to the Universal Transverse Mercator projection. The map is scanned at a minimum resolution of 250 dots per inch. The map to the right is scanned from a USGS topographic map from an area near the Colorodo River.


http://www.tceq.state.tx.us/assets/public/gis/images/drg.gif