Digital Raster Graphics (DRGs) are scanned images of U.S. Geological Survey (USGS) topographic maps. The scanned image includes all standard map collar information.
When the DRG is combined with other digital products, such as digital orthophoto quarter quadrangles (DOQQs), or digital elevation models (DEMs), the resulting image provides additional visual information for the extraction and revision of base cartographic information.
http://www.tceq.state.tx.us/gis/drg.html
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