DSM vs DTM: What's the Difference?

A drone flight can produce two different elevation models, and both get filed under the same label — "DEM". The difference fits in one sentence: a DSM tells you what you would land on, a DTM tells you where the ground is. Picking the wrong one shifts volume and elevation results by metres.

DSM: everything on the surface (buildings, trees)

A Digital Surface Model records the first surface a ray hits. Fly over a building and you get the roof; fly over a tree and you get the canopy. DSM is the natural output of photogrammetric dense matching — a camera cannot see under the ground, so it models what it can see.

Use a DSM for:

• Building heights, envelope and skyline checks
• Powerline / vegetation encroachment clearance
• The top surface of material in a stockpile
• Line-of-sight and shadow studies

DTM: bare ground

A Digital Terrain Model represents bare earth after vegetation and structures have been stripped out. A DTM is a derived product: ground classification is run over the DSM or the point cloud, non-ground points are discarded, and the remainder is interpolated into a surface. That means DTM quality follows classification quality directly — under dense canopy where no pulse reached the ground, the DTM is interpolation, not measurement.

Use a DTM for:

• Cut-fill and platform level calculations
• Slope, drainage and catchment analysis
• Road and channel longitudinal profiles
• Zoning and land development studies

Which analysis needs which

What you want to measure	Correct model	Why
Stockpile volume (top of material)	DSM	The real top surface lives in the DSM
Cut-fill (earth movement)	DTM	Vegetation and temporary objects must not enter the volume
Building / envelope height	DSM − DTM	The difference is height above ground
Longitudinal profile / slope	DTM	Tree canopy destroys the profile
Vegetation-to-powerline clearance	DSM	The stripped objects are exactly what you care about

Rule of thumb: measuring moving earth → DTM; measuring a standing object → DSM.

Uploading DSM/DTM and reading elevations in CartaX

Elevation models are attached alongside your orthophoto — they are not a separate visual layer, they are the ortho's elevation source. A DEM cannot be uploaded on its own — a colour orthophoto must accompany it: you hand over the orthophoto and the DSM/DTM files in the same upload selection, and the system automatically treats the single-band GeoTIFF as an elevation model and attaches it to the orthophoto. To add a DEM to an existing orthophoto later, the upload is redone. CartaX Studio photogrammetry produces the DSM and DTM together with the orthophoto and attaches them automatically. There is no separate per-file size cap for DEMs; only your storage quota applies (see all limits).

If more than one model is attached to a layer, you choose which one is read with the layer panel's "Measurement elevation" selector: DSM ("surface (including buildings/trees)"), DTM ("bare ground") and, on 3D mesh layers, additionally Surface ("model surface"). A model that was not uploaded stays greyed out and says "DSM has not been uploaded for this layer"; with no DEM at all you see the notice "No DEM attached — elevation is read directly from the clicked surface". While measuring, the on-screen status line states which model is being read — "DSM attached — elevation is read from the surface (incl. buildings/trees)" or "DTM attached — elevation is read from bare ground". That line is deliberately visible: at the same coordinate the two models can differ by metres, and no report should be written without knowing which surface produced the number.

With a model attached, the following become available:

1. Spot height — the elevation of any point you click
2. Line / profile — an elevation graph and slope along a section
3. Line of sight — in 2D view the visibility check between two points is also read from the DEM (in 3D the terrain surface is used)
4. Cut-fill — real cubic metres in a two-epoch comparison

In an orthophoto A/B comparison, cut-fill needs the model present in both epochs; if only one has it, the tool stays disabled and says "Cut-fill is disabled: both epochs need a DSM/DTM." In a 3D mesh A/B comparison, however, the volume is computed directly from the model surface even without a DEM; if a DEM is attached, the more accurate raster path is preferred.

Summary

Question	Answer
What is a DSM?	The first surface hit — roofs and canopy included
What is a DTM?	Bare ground, vegetation and structures removed
Which one is the raw output?	DSM; the DTM is derived from it
Which for cut-fill?	DTM — it isolates earth movement
CartaX limit?	DSM/DTM GeoTIFF; no per-file cap, only storage quota — uploaded together with the orthophoto
Which model is being read?	"Measurement elevation" selector in the layer panel; named explicitly in the measurement status line