Elevation data in the form of a digital elevation model (DEM) is an essential component of many analyses derived from EO. DEMs are used to represent every kind of surface, including terrain surface, vegetation canopy surface, sea surface, sea-ice surface, glacier surface etc. This description focuses on DEMs for representing terrain. A digital terrain model (DTM) describes the bare ground of the terrain, a digital surface models (DSM) described heights of vegetation (e.g. trees) and of man-made structures (e.g. buildings) reaching above the terrain. DEM is often used as an umbrella term for DTM and DSM. EO-derived DEMs are usually DSMs and require removal of vegetation and buildings in order to represent the terrain (DTM). DEMs are multi-purpose products used in various applications. They are available for global scale (SRTM, WorldDEMTM), regional scale (ArcticDEM, Copernicus EU-DEM v1.1) or for national levels and local regions. Various techniques exist to generate DEMs from SAR data, stereographic optical EO (as well as airborne and drone) data and from airborne laser scanning.