http://www.tceq.state.tx.us/assets/public/gis/images/drg.gif
A DRG is a digital image created by USGS that are typically scanned at 250 dpi and saved as a TIFF (a file format for storing images). The image usually includes the original border information, commonly referred to as the "map collar". DRG's are regularly used in GIS applications.
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