digital raster graphic (DRG) is a scanned image of a U.S.
Geological Survey topographic map. Geo-referencing information
is added to DRGs to associate the maps with their true position
on the ground. The best quality images are available in 250 to
500 dpi scans, depending upon when they were produced. DRGs are
available free for many states (see Links for sources). They are
also available for download from TerraServer (including automatic
retrieval with USAPhotoMaps) and other sources, but DRGs served
as web images are generally lower resolution.