Recently, someone in the MalaGIS group asked again: which of DEM, DSM, and DTM actually represents digital elevation? And what are the relationships among them? In practice, these three abbreviations are very common, but it is indeed very easy to mix up their differences. I looked up some materials and decided to record the findings here.

Concepts of DEM, DSM, and DTM

All three abbreviations describe the height of a certain point, but each emphasizes a different object. DSM stands for Digital Surface Model. It records the first surface that the sensor “touches”: treetops, roofs, utility poles. Where there is an obstruction, it records the top of that obstruction; only where there is no obstruction does it reach the ground. DTM stands for Digital Terrain Model. It emphasizes the bare-earth undulations after removing vegetation and man-made features. DEM stands for Digital Elevation Model. In many textbooks and vendor documents, DEM is treated as a broader term that encompasses both DSM and DTM, serving as a general term.

Common Ways to Distinguish Them

In everyday projects, I suggest first distinguishing them by “whether features are present”: DSM includes surface objects, while DTM does not. Take a look at this classic illustration:

In the diagram, the line that follows the treetops and roofs is the DSM; the line that follows the true ground surface is the DTM.

In practice, the initial height data obtained from remote sensing and photogrammetry tends to be closer to a DSM. Only after ground-point classification and filtering of LiDAR point clouds can a near-bare-earth DTM be derived. In field surveying contexts, irregularly distributed bare-earth points are often called a DTM, and once they are regularized into a raster, the result is typically referred to as a DEM. Subtracting a DEM (or DTM) from a DSM yields a Canopy Height Model (CHM), which some also call an nDSM (normalized DSM).

Application Scenarios

For analyses that rely on true terrain, such as inundation analysis, watershed delineation, and slope/aspect calculations, it is preferable to use a DTM. Using a DSM for these purposes can easily introduce distortions in valleys, roads, and urban areas. For urban skyline studies, viewshed analysis, UAV mapping quality checks, and building height extraction, a DSM is the preferred choice. When the relative height of buildings or trees is needed, subtract the DTM from the DSM to obtain the relative height.

Summary

In fact, the answer lies in the names themselves. The key to DSM is the “S” (Surface), which emphasizes the surface—including treetops, roofs, and utility poles. The key to DTM is the “T” (Terrain), which emphasizes the original bare terrain. DEM, on the other hand, is a general term used to represent ground-surface elevation.