All 70 URLs were active as of April 1998. This WWW sampling emphasizes topographic analysis, that is, surface parameterization and process modeling, rather than display via computer graphics. Terrain data are subject to several types of error, vary widely in accuracy, and must not be applied blindly. Most digital terrain data are sold by government agencies. Government DEM and bathymetry data are sold over the Internet by commercial repackagers.
Costs can vary substantially.
Increasingly, private firms are creating DEMs to order from satellite images, air photos, and radar data. Some digital terrain data are free over the Web. NGDC's TerrainBase collection, also sold on CD-ROM, is free at the above NGDC Web address. True bathymetric grids may be obtained from NGDC's TerrainBase archive. Recent DEM releases commonly appear first on the Web.