Digital Raster Graphics are raster images of standard quadrangle format USGS topographic maps. A DRG is a raster image of a scanned USGS topographic or planimetric map including the collar information, georeferenced to the UTM grid. A DRG is useful as a source or background layer in a GIS as a means to perform quality assurance on other digital products, and as a source for the collection and revision of vector data.
Figure 2. USGS Digital Raster Graphic (DRG)
Within the 1:24,000 and 1:25,000 series are included Provisional (P) maps as well as Orthophotomaps. DRG's retain the geometric and visual qualities of the source maps in all respects with two exceptions; DRG's are referenced to the UTM grid, and the colors of the DRG may differ slightly from the original printed map. They serve a variety of purposes, from use as backgrounds in a geographic information system (GIS) to source for collecting or revising USGS base cartographic data. The DRG shows potential as a tool for validating and assessing other non-USGS digital data. DRG's can be combined with DOQ's, DEM's, and DLG's, or combinations of each to create several "hybrid" products, each with its own unique applications. DRG prototypes have been utilized as "browse images" on USGS data management systems, and may serve similar functions in production systems and sales databases. While a DRG can be made from any map, the DRG program is oriented primarily toward the production of digital raster graphics from 1:24,000 and 1:25,000 USGS topographic maps to be used in the validation and revision of digital line graph data. DRG's adhere to the USGS NMPTI Standards for Digital Raster Graphics.
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