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A digital elevation data file consists of a sampled array of elevations for ground positions that are generally at regularly-spaced intervals.
The Defense Mapping Agency (DMA) produces an elevation product called Digital Terrain Elevation Data (DTED). DTED is available in two levels, DTED-1 and DTED-2. The level refers to the resolution of the elevation data. The resolution is the amount of space between each sample elevation point in the data file. DTED-1 has a resolution of roughly 100 meters, while DTED-2 has a resolution of roughly 30 meters.
Although both the DMA and the USGS distribute DTED data, each distributes it in a different tape format. The program you use to extract DTED data from tape will therefore depend upon your source of the data (DMA or USGS).