A digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) topographic map. DRG's include the map and all map collar information. The USGS has produced DRG's of the 1:24,000-, 1:25,000-, 1:63,360- (Alaska), 1:100,000-, and 1:250,000-scale topographic map series. Coverage includes the standard series quadrangle maps of the United States and its trusts and territories.
As with the paper topographic maps, DRG's can be used for hiking, camping, hunting, or other recreational activities. The obvious challenge for recreational users is that DRG's are digital files that require a computer for viewing. The quality of a printed DRG will vary depending on the user’s viewing and printing software and hardware. It is, therefore, not recommended for applications requiring the same clarity as a printed topographic map.
The full potential of a DRG is realized when it is combined with other digital map data. For example, when DRG's are combined with digital orthophoto quadrangles, the resulting hybrid product is excellent for collecting and revising digital map data. A DRG combined with a digital elevation model can result in a shaded-relief DRG, which provides additional visual detail for the viewing, extracting, and revising map information.
A DRG is made by scanning a printed map at a minimum of 250 dots per inch (dpi) on a high-resolution scanner. The raster image is georeferenced and put into the Universal Transverse Mercator projection. Colors are standardized to remove scanner limitations and artifacts. The image is compressed to reduce its size. The final result is a compressed TIFF file that ranges from 5 to 15 megabytes.
The DRG standard 13-color palette ensures uniform color throughout a particular map series. The red-green-blue values for a particular color, therefore, will remain consistent throughout that DRG series. Although the color values of the DRG may sometimes match those of the paper source map, small differences may be noticeable between colors on the digital image and those on the paper map. Also, the quality of the user's monitor affects the DRG colors displayed.
The horizontal positional accuracy of the DRG matches the accuracy of the source map. The 1:24,000-scale DRG at 250 dpi will have a ground sample distance of 2.4 meters (8 feet). This is adjusted on the TerraServer to display at resolutions of 2 meters and 4 meters per pixel.
DRG's can be obtained from a selected list of USGS Business Partners.
Most of the DRG's for the State of California have been produced, in cooperation with the USGS, by Teale Data Center in California. A status map, production specifications, ordering information, and current Teale prices are available at the Teale Web site, http://www.gis.ca.gov/, under "Digital Raster Graphics."
DRG's for parts of Tennessee and surrounding States have been produced by the Tennessee Valley Authority (TVA). Ordering information and current TVA prices are available through the TVA Web site, https://maps.tva.com/.
Detailed information about DRG's, including technical standards for DRG's and GeoTIFF, DRG viewing software, status and availability, and the DRG program, is available at http://mcmcweb.er.usgs.gov/drg