Applications & special services
Terrain model and surface model: the agreed measurement
A digital terrain model represents the earth’s surface without vegetation and buildings; a digital surface model represents it with everything standing on it. Which one you need is decided by the question to be answered.
Where the heights come from
The flight delivers the points from which the terrain and surface models are built. The difference from an official dataset lies not in the technique but in the timing: your capture has a date that both contracting parties can rely on.
DTM or DSM: the difference
A digital terrain model (DTM) is a precise 3D map of the bare ground: without houses, trees or vehicles. For every point of the area the exact height is stored. The digital surface model (DSM), by contrast, shows everything standing on the ground: roof edges, tree crowns, fences.
| DTM (terrain model) | DSM (surface model) | |
|---|---|---|
| Natural ground | included | included |
| Buildings, trees, objects | filtered out | included |
| Typical use | Earthwork volumes, design, building application | Shading, tree heights, solar |
Anyone needing a volume calculation needs the terrain model. Anyone computing a daylight study or planning a solar installation needs the surface model, because that is where the neighbouring building casts the shadow.
First check: is the free state data enough?
The German states provide height data, in many cases free of charge. In North Rhine-Westphalia, for example, with a five-year flight cycle and a height accuracy in the region of ten centimetres. For an overview, a site check and preliminary design that is as a rule sufficient — and cheaper than any commission can be.
We say so actively, because it is true. Anyone who knows the free alternative and commissions work anyway has a solid reason for doing so.
When your own measurement is necessary
The decisive sentence is: an official dataset is never a basis for settlement between two contracting parties.
That is not a question of accuracy but of contract. Official data shows the state at the last flight, and when that was is not decided by your project. A volume calculation, a flood assessment or an annual report to an authority needs a measurement at a defined point in time that both sides can rely on.
Typical occasions:
- Volume calculation in earthworks and extraction: settlement on the basis of the volume actually moved
- Flood and retention assessments
- Annual reports to consenting authorities
- Securing evidence of the terrain before and after works
- Stockpile and inventory determination on a given date
How accurate is the data?
| Method and condition | Positional accuracy | Height accuracy |
|---|---|---|
| Drone with surveyed control points, open ground | 1 to 2 cm | 2 to 5 cm |
| Drone without control points | about 1 dm | worse than the position |
Reference system: position ETRS89 with UTM projection, height DHHN2016 (NHN). The height accuracy is inherently somewhat worse than the position; that is not a peculiarity of our capture but of the method. The condition “open ground” is stated deliberately: under vegetation these values do not apply.
For areas with high accuracy requirements we add terrestrial capture. The spatial reference is established through a GNSS control network and is fit for official use, so that the result fits into existing design states.
Where the drone reaches its limit
The drone captures from above. It does not see through woodland, hedges or dense vegetation. In such areas only a surface model arises at first, not a true ground DTM: where the camera cannot see the ground, no software can reconstruct it.
For vegetated areas we offer a LiDAR flight or subsequent capture on the ground. And in every delivery we state explicitly which areas are affected: filtered, interpolated or re-captured. A volume calculation hangs on exactly those places, which is why the delivery includes a statement of how the filtering was done, and not just the result.
Timing counts too: where the ground surface is the objective, we plan the flight for the leafless season where possible.
Cadastre plus drone: one model
Many tasks need both: the wider surroundings and the precise project area. We therefore combine the two — the surrounding area from official cadastral data, comprehensive and cheap but coarse and in places several years old; the project area from our own drone flight, current, high-resolution, accurate to the centimetre.
Both areas sit in the same coordinate system, with a clearly visible boundary and a statement of accuracy and capture date for each. The result is a seamless, dependable design basis rather than a patchwork of sources whose origin nobody can trace any more.
What a terrain model is needed for
Site and building. Terrain sections, drainage design, slope gradients, connection levels and the building application: an exact height basis instead of an estimate. Where building, path and terrain meet, the centimetre decides about steps and accessibility.
Civil engineering. Earthwork volumes as cut and fill, computed as the difference between two states in the same spatial reference; route design; the result usable directly in CAD and GIS.
Large areas and infrastructure. Landfill and stockpile volumes, route corridors, inventories on a given date. As a recurring time series it becomes ongoing quantity documentation: same positions, same spatial reference, comparable results.
In every case the model is not looked at but computed with. The question of format is therefore not a side issue: a picture of the model is of no use to a volume calculation.
The formats we deliver in
| Format | Suitable for | Typical recipient |
|---|---|---|
| GeoTIFF / COG | Raster analysis, GIS processing | GIS, surveying practice |
| XYZ, ASCII grid | Universally readable, simple exchange | CAD, civil engineering design |
| ESRI grid | ArcGIS environments, authority archives | GIS, authorities |
| TIFF / PNG / PDF | Visualisation, documentation, presentation | Client, design practice |
In addition contour drawings in DWG or DXF and the point cloud where it is needed. On request we deliver combined packages, for instance GeoTIFF plus XYZ plus a PDF map.
Further reading
- Drone surveying – the flight the model grows out of
- Orthophoto – the true-to-scale image from the same capture
- 3D laser scanning – terrestrial, where vegetation or accuracy demand it
- Daylight study – what the surface model is needed for
- 2D as-built plans – contours and site plans from the model
Frequently asked questions about terrain and surface models
What is the difference between a terrain model and a surface model?
The terrain model represents the earth’s surface without vegetation and buildings, the surface model with everything standing on it. Volume calculation in earthworks needs the terrain model; shading, solar and visibility questions need the surface model.
Is the free height data from the states enough?
For an overview, a site check and preliminary design, as a rule yes. They are widely available free of charge and accurate enough for those purposes.
Why commission your own measurement at all, then?
Because an official dataset is never a basis for settlement between two contracting parties. Volume calculation, flood assessment or an authority report needs a measurement at a defined point in time that both sides can rely on. That is a contractual question, not a question of accuracy.
How accurate is the model?
With surveyed control points on open ground, 1 to 2 centimetres in position and 2 to 5 centimetres in height; without control points, about a decimetre. Under dense vegetation these values do not apply; we state the affected areas explicitly in the delivery.
Can you see through trees?
No. Photogrammetry captures what the camera sees. Under woodland and dense vegetation a surface model arises at first; for the ground there we offer a LiDAR flight or subsequent capture on site, and we mark interpolated areas as such.
In which formats is it delivered?
As GeoTIFF or COG for GIS, as XYZ or ASCII grid for CAD and civil engineering, as ESRI grid for authority environments, plus visualisations as TIFF, PNG or PDF and contours in DWG or DXF. Combined packages on request.