Photogrammetry, GIS and Remote Sensing

# Terrain Modeling

• In terminology, the  Digital Elevation Model,  known as DEM  is actually a three-dimensional structure, acquired by photogrammetric techniques,  LiDAR,  RADAR,  topographic and other techniques and methods. (Li et al., 2005). So The DEM can be a structure with regular XY spaces, expressed in linear or geographic units or a point cloud-like structure. The Z-value of each grid or point represents the height of the terrain (or surface) relative to a vertical datum.
• The Digital Surface Model (DSM)  is a three-dimensional graphic representation that includes all the elevation elements (all objects on that surface). When we use the elevation term of a geographic location, we refer to the height referenced to a reference system (geodetic system with a vertical datum). The reference system is actually a coordinate system and a set of reference points, used to accurately determine any location on Earth.
• The Digital Terrain Model (DTM) represents the real terrain surface,  without objects, such as buildings, vegetation, bridges, and so on. . According to USGS specifications, in some countries, DTM is synonymous with DEM, while in the United States, DTM is actually a vector data structure composed of dots and 3D lines, called lines and “breaklines.” Practically, the DTM is the improved version of DEM, through the existence of slope slopes drawn in 3D.

Therefore, we can say that DEM is a general term used for both DSM and DTM. We are talking about DSM or DTM, we mean a DEM that has undergone automatic / semi-automatic and manual interventions, to improve quality / precision.

In some situations, we can talk about DSM meaning a selective surface model, and its end use. Such a situation is in the case of true-ortho generation, where the DSM model contains ( as an over-ground element) only the anthropic elements (without vegetation), in the idea of ​​generating a nadiral orthophoto, for all elements strongly affected by perspective. The true-orho technique is applied only in the case of a special flight,  by using a vertical perspective camera with 80%-80%  image coverage , both lateral (along strips) and longitudinal (between strips), or by using an oblique pentacam camera (4 oblique sensors in addition to the nadir sensor), such as Leica RCD30, Trimble AOS, Vexcel UltraCam Osprey or IGI Penta-DigiCam.

The DTM can be obtained  from LiDAR,  after specific classification for terrain extraction,  or from aerial/satellite imagery, from stereo-correlation and manual enhancement processes, in 3D environment, based on aerial/spatial triangulation process. The DTM quality is specific to each project and it depends of the input data specific quality (GSD size). It’s direct use consist as a base for orthorectification and further analyses and processing in GIS environment.