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Digital terrain data models & difference between DEM, DSM and DTM

Digital terrain data models & difference between DEM, DSM and DTM

SEAMCAT needs digital terrain elevation data in PMPs for calculation of transmission loss over site-specific paths. There are several propagation models implemented in SEAMCAT which consider digital terrain, such as PMP for ITU-R P.452 (vers. 16, 17, 18), ITU-R P.1546 ver. 6, and ITU-R P.2001 ver. 4.

For inclusion of digital terrain data in SEAMCAT, the user needs to download terrain data from outside sources, define the data format and the path to the folder where the data is downloaded. SEAMCAT use the embedded terrain data readers to extract terrain height profiles and provide them to PMPs.

Digital terrain elevation data is generally denoted as DEM (digital elevation model).

DEM: This is a general term for a digital representation of elevations (or height) of a topographic surface in form of a geo-rectified point-based or area-based grid, covering the Earth or other solid celestial bodies. Currently most common DEMs use rectangular grids and raster image file storage formats. At the global scale, elevations are measured from space using near-infrared, radar and visible sensors, then geo-rectified and stored in a gridded storage structure.

DEM is a rather broad category of terrain data and could be further subdivided in different sub-categories. For radiocommunication application domain purposes and SEAMCAT consideration it is specifically interesting to distinguish between:

  • digital surface models (DSM), which measure the upper surface of trees, buildings and other man-made features; and

  • digital terrain models (DTMs) which measure the elevation of the ground surface, or the ‘bare-Earth’

 

This distinction is nicely illustrated in Fig.1 where A represents illustration of artefacts on the Earth surface for hypothetical terrain profile, and B where difference between DTM and DSM models is shown.

 

image-20250626-134028.png
Fig.1: Illustration of different representation of terrain artefacts in A) by digital surface model (DSM) and digital terrain model (DTM) in B) [1]

This difference is of particular interest for PMPs which consider additional losses (clutter loss or building entry loss) coming from biosphere (e.g., trees, forest, foliage, etc.) or anthroposphere (e.g., man-made objects such as buildings, road infrastructure, industrial objects, etc.). For example, PMP ITU-R P.452-18, that was recently implemented in SEAMCAT, has a distributed clutter loss model (instead of terminal clutter loss model) and requires the information on both the bare terrain profile and the clutter height profile along the path (for biosphere and anthroposphere) (see STG(24)028R1, STG(24)036, STG(24)037).

Freely available DEM models mostly fall under the DSM category. DSMs exist at the global scale, or near-global scale, at 3 arc second (∼90 m), and more recently 1 arc second (∼30 m) grid spacing. They are usually available for free download under specific license terms and registration. Such models are for example Shuttle Radar Topography Mission (SRTM), ASTER GDEM (v3), Copernicus GLO-90, Copernicus GLO-30 (COPDEM30), etc.