A DRG is produced by scanning a USGS map on a high-resolution scanner at a minimum resolution of 250 dots per inch (dpi). The image is then compressed to reduce the size of the data set. The final result is a packbit-compressed file in the tagged image file format (TIFF), with a file size of 5-15 megabytes. The scanned image includes all map collar information. The four items used for producing a DRG of a 7.5-minute topographic map are:
A DRG may be used as a source or background layer in a geographic information system (GIS) to collect, review, and revise other digital data. A DRG can be combined with a DEM and a DOQ to produce an image with additional visual information for the extraction and revision of base cartographic information. Extent of Coverage: The standard area of coverage of a DRG is the entire area printed on a USGS standard series topographic map including the map collar, any overedge areas, and insets. DRGs are available at the following scales:
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