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DRG Frequently Asked Questions
What are DRGs?
DRG stands for Digital Raster Graphic. It is a file format used by the US Geological Survey to distribute their topographic map series in a format usable by computers. Radford University makes available the USGS 7.5' topographic map series covering Virginia, North Carolina, West Virginia, Maryland and the District of Columbia. Each computer file contains one topographic sheet.
What file format are DRGs stored as?
Each DRG is composed of two files, one ending in a TIFF file extension and one ending in a TFW file extension. The TIFF file is the actual graphics file that may be read using any software package that reads TIFF files (Adobe PhotoShop, for example). The TFW file contains geographic coordinates that reference the map sheet to its proper location on the face of the earth. Software like ESRI's ArcView, ERDAS' Imagine, as well as many others can thus accurately use a DRG with other existing spatial data.
What projection is used with DRGs?
DRGs come from the USGS in Universal Trans Mercator (UTM), North American Datum '27 (NAD 27). Be aware that the DRGs thus may not be in the same projection as that posted on its map collar!
What's the resolution of a DRG?
DRG's are scanned by the USGS at a resolution of 500 dots per inch (DPI). They are then resampled at 250 dots per inch. The DRGs provided here are the 250 DPI product.
How may I view DRGs on my computer?
You may use any software that reads TIFF files to view DRGs. We have included a link to Enironmental Systems Research Institute (ESRI), who provide a free viewer called ArcExplorer. ArcExplorer may be used on computers that use Windows 95/98 and Windows NT. While we cannot endorse any product, ArcExplorer does a good job of providing an easy to use way to work with DRGs. It will even uncompress the DRGs downloaded from this site and immediately make them viewable.
Why is Radford University making them available?
Radford University makes these files available as a service to the community. We make available the surrounding states (RU is located in Virginia) because analysis of spatial issues doesn't stop at the state line. Watershed analysis, air and water pollution, animal migration, as well as many other topics benefit by analyzing neighboring areas.