A digital elevation model' (DEM) is a representation of the topography of the Earth in digital format, that is, by coordinates and numerical descriptions of altitude. DEMs are used often in geographic information systems. A DEM may or may not be accompanied by information about the ground cover. In contrast with topographical maps , the information is stored in a raster format. That is, the map will normally divide the area into rectangular pixels and store the elevation of each pixel.

Digital elevation models may be prepared in a number of ways, but they are almost always obtained by remote sensing rather than direct survey. One powerful technique for generating digital elevation maps is interferometric synthetic aperture radar; two passes of a radar satellite (such as RADARSAT-1) suffice to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters. One also obtains an image of the surface cover.

A free DEM of the whole world called GTOPO30 (30 arcsecond resolution, approx. 1km) is available (see Link below). It can be used to generate maps such as in the International gliding online contest. Several other DEMs are available, but the better they are, the more they cost.

External links

• GTOPO30 Homepage
• GTOPO30 FTP Server