Digital Terrain Models

ADigital TerrainModel (DTM) approximates a part or the whole of the continuous terrain surface by a set of discrete points with unique height values over 2D points. Heights are in approximation vertical distances between terrain points and some reference surface (e.g., mean sea level, geoid and ellipsoid) or geodetic datum. Mostly arranged in terms of regular grids, the 2D points are typically given as geodetic coordinates (latitude and longitude), or planar coordinates (North and East values). DTMs usually assign a single unique height value to each 2D point, so cannot describe vertical terrain features (e.g., cliffs). DTMs are therefore “2.5D” rather than truly 3D models of the terrain (Weibel and Heller, 1991). While DTMs represent the bare ground of the terrain, a Digital Surface Model (DSM) describes heights of vegetation (e.g., trees) and of man-made features (e.g., buildings) too (Fig. 1). It is thus important to distinguish between DTM and DSM over vegetated or built areas. A closely related term is Digital Elevation Model (DEM), which is sometimes used synonymously with DTM, but often as an umbrella term to describe both DTM and DSM (Wood, 2008; Hutchinson and Gallant, 2005; Shingare and Kale, 2013). DEM is often used for elevation models from remote sensing (e.g., radar or photogrammetry). These models are rather DSM than DTM unless vegetation and building heights are removed. The concept of DTM is not only limited to Earth’s visible terrain surface. It also finds application in bathymetry (digital bathymetry models describing the geometry of the sea floor), polar geodesy (digital bedrock models to describe the rock below the ice sheets), and planetary sciences (digital elevation models of the planetary surfaces), among many other areas of application.