Digital Elevation Models is a generic term to indicate the discrete representation of the surface of the Earth using points generally placed on a regular grid. For each point its position is known in a chosen reference frame and represented through a chosen coordinate system (horizontal coordinates: geographic (latitude, longitude) or cartographic (East, North); elevation (heights or depths): orthometric with respect to a chosen geoid model or ellipsoidal.
DEMs can be obtained, among others, by conventional ground topographic surveys, global navigation satellite system positioning, mapping techniques (photogrammetry), laser altimetry, satellite radar altimetry, synthetic aperture radar techniques (interferometry and radargrammetry), echo sounding, airborne optical sensing of bathymetry, ice penetrating radio-echo sounding, or by digitising of topography contours.
At present, DEMs are mostly inferred from remote sensing techniques; therefore, they natively include all objects located on the Earth’s surface, like vegetation, ice covers, buildings, infrastructures and generally all man-made objects. These DEMs are referred as Digital Surface Models (DSMs); in contrast to DSMs, Digital Terrain Models (DTMs) represent the bare topographical surface of the ground, and they are derived from DSMs through proper filtering.