Digital Raster Graphics (DRGs)
Digital Raster Graphics (DRGs) are scanned images of U.S. Geological Survey (USGS) topographic maps. The scanned image includes all standard map collar information. The image inside the map neatline is georeferenced to the surface of the earth, making it possible to use DRG as a map layer or theme in many GIS software packages.
For some GIS applications, 7.5 minute quads may be the most detailed, accurate base map available. Many features have been manually table digitized from these paper maps into GIS systems in the past. DRGs make it possible to utilize quads in a GIS without digitizing.
The DRG can be used to collect, review, and revise other digital data, especially Digital Line Graphs (DLGs).
When the DRG is combined with other digital products, such as digital orthophoto quarter quadrangles (DOQQs), or digital elevation models (DEMs), the resulting image provides additional visual information for the extraction and revision of base cartographic information.
Hard copy maps produced from DRGs look very much like the original quad sheets, except they can be at different scales, overlap quad boundaries, and include any other type of GIS data.
For information and standards concerning DRGs, please visit the following USGS page which describes Digital Raster Graphics in further detail: http://topomaps.usgs.gov/drg/ .
The DRGs for the entire state of Texas are now available at the Texas Natural Resource Information System (TNRIS) via their new Data Distribution System. DRGs can be located by county, quad, or one-degree block.
TNRIS also provides DRGs in CD format. Each CD contains the 1:24,000, the 1:100,000 and the 1:250,000 digital raster graphics for an entire one-degree block area. Cost for the CDs is $28 plus shipping. For more information visit the TNRIS web site at www.tnris.org or call 512-463-8337.