DRG

http://images.google.com/imgres?imgurl=http://lter.kbs.msu.edu/maps/GIS/DRG_d.jpg&imgrefurl=http://lter.kbs.msu.edu/maps/gis_data.php&usg=__VaxqDKfw1nQxdpExshqwnZSwVTw=&h=602&w=640&sz=609&hl=en&start=7&um=1&itbs=1&tbnid=EXfqRc2BKG8f3M:&tbnh=129&tbnw=137&prev=/images%3Fq%3D%2522DRG%2522%26um%3D1%26hl%3Den%26sa%3DN%26rls%3Dcom.microsoft:*%26tbs%3Disch:1

DRG, which stands for digital raster graphic and is a scanned image of a U.S. Geological Survey topographic map. In these maps, raster is how the map is represented as you can notice the grid-like squares.