DSM vs DTM is one of the first distinctions that trips up pilots who move from photos into mapping deliverables. Both are elevation models. Both usually arrive as a georeferenced raster where every pixel holds a height. But they describe two different surfaces, and handing a client the wrong one can quietly break a contour map, a volume number or a drainage review.
This guide covers what each model shows, where the difference matters, what a normalized DSM adds, and which deliverable needs which surface. If you already know the definitions and want the processing workflow, our post on building a DSM and DTM on your own desktop covers how it works in SkyView.
What a DSM shows
A digital surface model (DSM) records the height of the first thing the camera saw from above. That includes the ground where it's open, but also rooftops, tree canopy, parked trucks, stockpiles, light poles and equipment. If it was visible in the imagery, it's part of the surface.
That makes a DSM the right model when the question is about everything on the site as it stands today. Stockpile tops, building roof heights, line-of-sight checks and visual context for a site plan all work from the surface model.
What a DTM shows
A digital terrain model (DTM) is the bare earth. Buildings, vegetation, vehicles and other above-ground objects are filtered out, and the gaps they leave are filled by estimating the ground underneath. The result is the shape of the land itself.
When people ask for grades, slopes or "what the dirt is doing," they almost always mean a DTM. A DSM with a row of trees across it will show a ridge that isn't there, and a contour line drawn on it will climb right over a parked excavator.
A note on DEM
You'll also see "DEM," or digital elevation model. Usage varies by vendor and agency. Some treat DEM as the umbrella term for any elevation raster, while others use it to mean bare earth. When a client asks for a DEM, ask which surface they mean before you deliver. It's a short conversation that saves a reflight.
DSM vs DTM side by side
- What it represents: the DSM shows the top of whatever is visible, and the DTM shows the ground with objects removed.
- Trees and buildings: the DSM includes them, and the DTM excludes them and estimates the ground beneath.
- Best for: the DSM suits stockpile tops, structure heights and site context, while the DTM suits contours, grading, earthwork and drainage.
- Common mistake: people use a DSM for contours on a vegetated site, or a DTM to measure something that sits above ground.
The normalized DSM: heights above ground
Subtract the DTM from the DSM and you get a normalized DSM, often written nDSM. Instead of elevation above a datum, each pixel now holds height above the local ground. A roof reads as the building's height, a tree reads as its canopy height, and open ground reads close to zero.
An nDSM is useful for vegetation clearance checks, rough structure height comparisons and spotting objects that sit above grade. It's only as good as the two surfaces that built it, so a weak DTM produces misleading object heights.
Which deliverable needs which surface
Most mapping deliverables lean clearly toward one model. Here's how the common ones break down.
- Orthomosaic: an orthomosaic is corrected using elevation data, typically the surface model, so buildings and trees sit in the right place. It's a picture, not a height product, but it depends on one.
- Contours: contours almost always come from the DTM. Contours from a DSM trace rooftops and treelines, which is rarely what a site plan needs. Our walkthrough on generating contours from a local elevation model covers the practical side.
- Stockpile volumes: the pile top comes from the surface you captured, and the base is a defined reference such as a pad, the pile toe or an earlier survey. Document which base you used, because that choice moves the number more than most pilots expect.
- Cut-fill: comparing two dates or a current surface against a design is bare-earth work. Equipment, spoil piles and vegetation left in the model show up as false cut or fill.
- Drainage and water flow: water follows the ground, not the canopy. Flow analysis needs a DTM, which is covered in our guide to drone drainage and water-flow analysis from a DTM.
Where a photo-based DTM falls short
Photogrammetry builds elevation from what the camera can see. Under dense canopy, tall crops or heavy brush, the camera may never see the ground at all. The software can remove the vegetation, but the ground it estimates underneath is an interpolation across a gap, not a measurement.
On open construction sites, pads, quarries and cleared land, a photo-based DTM can work well. On a wooded lot it can look smooth and convincing while being wrong under the trees. Be upfront with clients about which parts of the site had visible ground and which were filled in. Fly in leaf-off season when you can, and mark heavily vegetated areas as lower confidence on the deliverable.
This is one reason LiDAR comes up in terrain discussions. Laser pulses can sometimes reach the ground through gaps in vegetation where photos can't. Whether that's worth the cost depends on the site and the job, and it's a separate topic from photo-based mapping.
Practical habits that prevent bad elevation handoffs
- Name the surface in the file. Put DSM or DTM in the filename and the delivery note. An unlabeled "elevation.tif" invites misuse.
- State the coordinate system and vertical datum. A correct surface in the wrong vertical reference is still the wrong surface for the engineer who receives it.
- Say how vegetation was handled. One sentence on what was removed and where ground was estimated is enough.
- Match the surface to the question. Before processing, write down what the client will do with the data. That answer usually picks the model for you.
- Keep both when you can. Clients often come back with a second question. Having both surfaces on hand lets you answer without reprocessing.
Producing both surfaces in SkyView
SkyView Desktop builds the DSM and DTM locally on your own Windows PC, with no per-map cloud fee (that's our pricing model, not an industry rule). From there you can generate contours, run cut-fill volumes and export georeferenced rasters for the civil team or the client. The SkyView page covers the workflow, and the features page lists what each plan includes.
If you add the Survey & Earthworks add-on, you can run water flow analysis on the DTM, with Slope, percent; Drainage lines; Ponding; Catchments; Trace flow path from a click; and Outline watershed from a click. It reads the shape of the ground only. It does not model pipes, storm drains, rainfall, soil or time, and engineered drainage design stays with an engineer.
Once the surfaces are done, Pilot Ledger handles the quote, the job record and the invoice, and finished deliverables go to the client through the same account. It's the business side of the job, kept in one place.
If you want to build DSMs and DTMs on your own hardware, see Pilot Ledger plans and pricing.