A digital raster graphic (DRG) is a scanned image of a U.S.
Geological Survey (USGS) topographic map. The scanned image
includes all map collar information. The image inside the map
neatline is georeferenced to the surface of the Earth. Between
1995 and 1998, the USGS produced DRGs of the 1:20,000- (Puerto
Rico) 1:24,000-,1:25,000-, 1:30,000- (Caribbean Islands),
1:63,360- (Alaska), 1:100,000-, and 1:250,000-scale topographic
map series. Most USGS 7.5-minute DRGs produced between 1995 and
1998 have the following specifications: The source material for
a DRG is a USGS topographic paper map. USGS DRGs are in TIFF 6.0
format, with GeoTIFF 0.2 or 1.0 extensions to define
georeferencing. The map is scanned at a minimum resolution of
250 dots per inch (dpi). If scanned at a finer resolution, the
image is resampled to 250 dpi. The digital image is
georeferenced to the true ground coordinates of the 2.5-minute
grid ticks and projected to the Universal Transverse Mercator
(UTM) for projection consistency with USGS dig