DTM vs DSM: what's the difference (and which do you need)?
A DTM (Digital Terrain Model) represents the bare ground with buildings and vegetation stripped away. A DSM (Digital Surface Model) represents the top of everything on the ground — roofs, tree canopy, structures. For site topography in BIM you almost always want the DTM.
Both come from the same kind of source data, usually airborne LiDAR, but they answer different questions. The DTM asks "where is the ground?"; the DSM asks "what is the highest thing at this point?" Choosing the wrong one is one of the most common reasons a site model comes in looking lumpy or several metres too high.
Side by side
| DTM | DSM | |
|---|---|---|
| Represents | Bare earth | Top surface of everything |
| Includes buildings/trees | No | Yes |
| Best for | Site grading, terrain under a building, context models | Existing building heights, canopy, volume studies |
| Height at a tree | Ground level | Top of the canopy |
Which do you need?
For nearly all architectural site work — placing a building, modelling cut and fill, showing context — you want the DTM, because you need the real ground surface. Reach for a DSM only when the objects on the site are the point: assessing the height of neighbouring buildings, tree screening, or rights-to-light studies.
How Topo-grapher handles it
Each country's source is labelled as a DTM or DSM. When you add building footprints on a DSM-based source, their base is set to the surface height minus the building height, so they sit correctly on the ground rather than floating on top of the surface model.
Generate a terrain model ->Frequently asked questions
What is the difference between a DTM and a DSM?
A DTM represents the bare ground with buildings and vegetation removed. A DSM represents the top of everything on the ground, including building roofs and tree canopy. The DSM is higher wherever objects sit on the terrain.
Should I use a DTM or DSM for site topography in BIM?
Use a DTM. For site context, grading, and a terrain surface under your building, you want the bare ground. A DSM is useful when you specifically need the heights of existing buildings or tree cover.
Does Topo-grapher use a DTM or a DSM?
It uses the best terrain source per country and labels whether that source is a DTM or DSM. When buildings are added on a DSM source, their base is set to the surface height minus building height so they sit correctly on the ground.
Further reading
- USGS 3D Elevation Program — the US federal DEM programme; explains DTM vs DSM terminology in a national context.
- Open Geospatial Consortium (OGC) — open standards body for geospatial data formats including elevation models.