Digital Raster Graphics
A Digital Raster Graphic (DRG) is a raster image of a published map, made by scanning a published paper map on a high-resolution scanner. The raster image is georeferenced and fit to the Universal Transverse Mercator (UTM) or a State Plane coordinate system. The colors are standardized to remove scanner limitations and artifacts.
The size of the average data set is about eight megabytes in Tagged Image File Format (TIFF) with PackBits compression. TVA DRGs are provided only in TIFF 5.0 or TIFF 6.0 files, or both. They are not available as GeoTiff files. DRGs can be easily combined with other digital cartographic products such as digital elevation models (DEM) and digital orthophoto quadrangles (DOQ).
The published maps are available on CD-ROM in blocks of 67 files that cover a one-degree block of latitude and longitude (see Status Map). DRGs are available upon request. Please contact the Map Store for prices and availability.
Customized formats, including choice of map projection, datum, and border format, can be developed upon request.
warning and limitation
This familiarity is necessary for two reasons:
Most applications that support the display of TIFF images are compatible with TVA DRGs. However, because of the large file sizes (5-20 mb) and the spatial coordinate information stored in each TIFF header, TVA cannot guarantee this compatibility.
Do not assume that any particular software package is compatible with TVA DRGs unless it has been tested with at least one of our DRG products.
The TVA Map Store will provide potential customers who wish to test any particular software package for TVA DRG compatibility with a sample DRG upon request. The sample is available for free download from the TVA corporate FTP server. Contact the TVA Map Store for details.
Multiple discs may be required in some cases to contain the data of a single cell. In cases where multiple discs are used, the data will be broken into northern and southern portions of the 1- by 1-degree cells. These portions will be stored on separate discs, designated N and S respectively.
TVA will provide a TVA DRG Project map free of charge. This small-format map shows the area of coverage for the entire project and individual DRG block numbers used for placing orders.
The TVA DRG Project map is available from the TVA Map Store. DRGs for other parts of the United States are available from the US Geological Survey (USGS).
polygon fill indicates data coverage.)
Visit the U.S. Geological Surveys Mid-Continent Mapping Center DRG page for more information on DRGs.