DEM

http://en.wikipedia.org/wiki/File:Dem.jpg

DEM stands for digital elevation model, is a digital model or 3D representation of a terrain’s surface, commonly for a planet, moon, or asteroid. It is in raster format, so the cartographic dats are recorded, stored and processed in a cell or pixel. DEM's are georeferenced, meaning their features are tied into a coordinate system, datum, and map projection and reference specific locations (i.e. latitude & longitude) in space. DEMs are commonly built using remote sensing techniques, however, they may also be built from land surveying. DEMs are used often in GIS, geographic information systems, and are the most common basis for digitally-produced relief maps. The image above is an example of digital elevation model of the Valestra area, Northern Apennines.