A definition of a DRG is a scanned image of a U.S. Geological Survey (USGS) standard series topographic map. DRGS are most useful as backdrops where other digital data is superimposed. The san of a DRG is 250 pixels per inch resolution. The picture above is a DRG of Bushkill, Pennsylvania.
https://www.e-education.psu.edu/natureofgeoinfo/c6_p6.html
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