USGS distributes 3″ terrain data free of charge. Can I use such data?
Basically, no, although such data can be reformatted and used in Don Plouff’s terrain correction program for distant terrain, OUTERTC. The 3″ data distributed by USGS were derived from 1:250,000-scale maps, and are simply not accurate enough to be used to correct gravity data for near-in terrain. At distances greater than about 3-4 km, such data could be used. Note that the data format used for the 3″ terrain data is completely different from the format used for the 30-m DEMs distributed by USGS.
