7.5-min DEM Data Source
The 7.5-min USGS DEMs have been converted from the SDTS format to the native DEM format using the corrected SDTS2DEM conversion software produced by Dr. Gregg Townsend of the University of Arizona.
The 7.5-min USGS DEMs are the corrected files produced by the USGS to correct positional errors that existed in previous DEMs produced prior to 2001.
Digital Elevation Model (DEM)
A Digital Elevation Model (DEM), consists of a sampled array of elevations for ground positions that are normally at regularly spaced intervals. The 1-Degree DEM (3- by 3-arc-second data spacing) provides coverage in 1- by 1-degree blocks for all of the contiguous United States, Hawaii, and limited portions of Alaska. The basic elevation model is produced by or for the Defense Mapping Agency (DMA), but is distributed by the USGS, EROS Data Center, in the DEM data record format. In reformatting the product the USGS does not change the basic elevation information. 1-degree DEM's are also referred to as "3-arc second" or "1:250,000 scale" DEM data.
The 7.5-minute Digital Elevation Model (DEM) data files are digital representations of cartographic information in a raster form. DEMs consist of a sampled array of elevations for a number of ground positions at regularly spaced intervals. These digital cartographic/geographic data files are produced by the U.S. Geological Survey (USGS) as part of the National Mapping Program. DEM data for 7.5-minute units correspond to the USGS 7.5-minute topographic quadrangle map series for all of the United States and its territories except Alaska. Each 7.5-minute DEM is based on 30- by 30-meter data spacing with the Universal Transverse Mercator UTM projection. Each 7.5- by 7.5-minute block provides the same coverage as the standard USGS 7.5-minute map series.
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