Introduction
If you work with aerial surveying or GIS analysis, you've encountered the acronyms DEM, DSM, and DTM. Often these terms are used interchangeably, which can lead to confusion about which model is right for your project.
The differences are subtle but important. Choosing the wrong elevation model can lead to incorrect calculations and flawed decisions. In this guide, we'll clarify what each model represents and when to use it.
What Is a Digital Elevation Model (DEM)?
A Digital Elevation Model (DEM) is an umbrella term for any gridded digital representation of terrain elevation. It's a 3D map of the landscape stored as a grid of height values. The most important thing to understand: DEM is a generic category that encompasses both Digital Surface Models and Digital Terrain Models.
What Is a Digital Surface Model (DSM)?
A Digital Surface Model (DSM) represents the elevation of the actual surface you see from above—buildings, trees, power lines, vehicles, and bare ground all at their true heights. It's a complete picture of everything on the landscape.
Think of it as a "first bounce" elevation model. When a drone or sensor measures the landscape, a DSM captures the first surface it encounters.
DSM Use Cases:
- Urban planning and development—understanding how structures fit into the landscape
- Vegetation analysis and forestry—measuring tree canopy heights
- Aviation safety and obstacle assessment
- 3D visualisation and site visualisation for marketing
What Is a Digital Terrain Model (DTM)?
A Digital Terrain Model (DTM) represents only the bare ground—the actual topography of the earth. All artificial structures and vegetation are removed through processing, leaving just the underlying landscape.
This is achieved by applying classification algorithms and filters to remove features like buildings, trees, and other objects that sit above the bare ground.
DTM Use Cases:
- Civil engineering and infrastructure design—slope calculations and cut-and-fill volumes
- Drainage and flood modelling—water flow analysis
- Mining and quarrying—ore body assessment and pit design
- Environmental planning—understanding true ground topology
DEM vs DSM vs DTM: Quick Comparison
| Aspect | DEM | DSM | DTM |
|---|---|---|---|
| Definition | Generic elevation grid | Surface including all features | Bare ground only |
| Buildings | Depends on type | Included at true height | Removed |
| Vegetation | Depends on type | Included at canopy height | Removed |
| Processing | Minimal | Minimal | Extensive filtering |
| Best For | General reference | Urban, planning, visibility | Engineering, hydrology, mining |
How Are DEMs Created from Drone Data?
Modern DEMs are created using photogrammetry—a technique that reconstructs 3D information from overlapping aerial photographs. Here's the process:
- A drone captures hundreds of overlapping images in a grid pattern over the site.
- Specialised software identifies common features ("tie points") between images and creates a point cloud—millions of X, Y, Z coordinate points representing the landscape.
- This point cloud is gridded into a regular mesh, with elevation values assigned to each cell.
- The result is a DSM. If a bare-ground model (DTM) is needed, additional processing removes non-ground features using classification algorithms.
Choosing Between Models: A Practical Guide
Ask yourself: Do I need to understand what's on the ground, or what's underneath everything?
- Need DSM? You're doing urban planning, 3D visualisation, forestry assessment, or anything where the actual surface matters.
- Need DTM? You're doing engineering design, drainage modelling, flood risk assessment, or mining operations where bare ground is essential.
Accuracy and Resolution
Both DSMs and DTMs can achieve centimetre-level accuracy with proper ground control and processing. Resolution (grid cell size) typically ranges from 5 cm to 1 metre, depending on the application and drone altitude.
Conclusion
Understanding the difference between DSM and DTM is crucial for getting the right data for your project. DSM captures the complete landscape as it appears; DTM strips away structures and vegetation to show bare ground. Both are valuable—you just need to know which one answers your question.
Not sure which model you need? Contact Aerial Imaging Australia—we'll help determine the right approach for your specific requirements.