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.

Formats
DRGs are offered in two standard formats:

• UTM: Universal Transverse Mercator grid coordinate system, NAD27 datum, map contains collar information
• TNSP: Tennessee State Plane coordinate system, NAD83 datum, borderless.

Customized formats, including choice of map projection, datum, and border format, can be developed upon request.

Product warning and limitation
The data contained on the TVA DRG CD-ROMs is technical in nature; it requires its users to be familiar with CD-ROM devices, file management techniques, procurement and installation of TIFF viewing software, and manipulation of DOS ASCII text files.

This familiarity is necessary for two reasons:

• TVA provides these CD-ROMs as a “data only” product; no software of any kind is included. Users must procure and install their own software for use with DRG images.
• TVA makes no attempt to create an extensive user manual for the DRGs because of the wide variety of individual uses for the product and the vast array of software packages that will process such data.

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.

Distribution media
DRG images of quadrangles at scales of 1:24,000 and 1:100,000 are stored on CD-ROM as 1- by 1-degree cells. The associated 1:250,000-scale quadrangle comprising the cell is also included on each disc.

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.

Area of coverage
DRGs are available for 3,129 maps covering the Tennessee Valley and the TVA power service area (see Status Map). Included are the entire state of Tennessee, the northern two-thirds of Mississippi, the northern half of Alabama, north Georgia, western North Carolina, southwestern Virginia, most of Kentucky, and small portions of Arkansas, Illinois, Missouri, and West Virginia.

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).

Digital Raster Graphics Status Map

(Blue polygon fill indicates data coverage.)

Visit the U.S. Geological Survey’s Mid-Continent Mapping Center DRG page for more information on DRGs.