Digital Elevation Models: DSM vs DTM and When Each Matters

A Digital Elevation Model (DEM) is a raster dataset where each pixel represents the elevation of the ground (or surface) at that location. DEMs are the foundation of terrain analysis, drainage modeling, cut/fill calculations, and engineering design.

But not all DEMs are the same. The distinction between a Digital Surface Model (DSM) and a Digital Terrain Model (DTM) is critical — and choosing the wrong one for your application can produce seriously misleading results.

A DSM represents the elevation of the first surface the sensor encounters — which includes everything on the ground: buildings, trees, vehicles, structures, and the terrain itself.

When a drone captures imagery over a forested area, the DSM shows the top of the tree canopy, not the ground beneath it. When it captures imagery over a building, the DSM shows the roof, not the ground level.

DSM is appropriate for:

• Documenting current site conditions including structures
• Calculating volumes of stockpiles and earthworks
• 3D visualization and modeling
• Solar analysis (understanding shadows from buildings and trees)
• Urban planning and building height analysis

A DTM represents the bare earth surface — the ground elevation with all above-ground objects removed. Buildings, trees, vehicles, and other features are filtered out, leaving only the terrain.

Producing a DTM from photogrammetry requires additional processing to classify and remove above-ground points. This is straightforward on open sites but challenging in areas with dense vegetation, where photogrammetry may not capture ground points at all.

LiDAR is often preferred for DTM production in vegetated areas because its ability to penetrate vegetation canopy provides ground returns even under dense cover.

DTM is appropriate for:

• Drainage analysis and hydrological modeling
• Flood plain mapping
• Cut/fill calculations for grading design
• Infrastructure corridor planning
• Erosion and sediment modeling
• Any application where you need to understand the terrain, not the surface

Consider a 100-acre site with a mix of open fields, a woodlot, and several farm buildings. You need to design a drainage system for the site.

If you use a DSM, your drainage model will show water flowing over the tops of the trees and pooling on the roofs of the buildings. The drainage analysis will be meaningless.

If you use a DTM, your drainage model shows water flowing across the actual ground surface — through the fields, around the buildings, and through the woodlot. The analysis is accurate and useful.

For drainage design, you need a DTM. Using a DSM will produce wrong answers.

Contour lines are derived from DEMs — they connect points of equal elevation to show the shape of the terrain. Like DEMs, contours can be derived from either a DSM or a DTM.

For engineering and construction applications, contours are almost always derived from a DTM. Contours derived from a DSM will show the tops of trees and buildings as terrain features, which is not useful for site planning.

Contour interval (the elevation difference between adjacent contour lines) is specified based on the application:

• 1-foot contours — detailed grading design, drainage analysis
• 2-foot contours — general site planning, earthwork design
• 5-foot contours — large-area terrain overview, regional planning

The accuracy required for a DEM depends on the application:

Achieving higher accuracy requires RTK GPS, ground control points, and careful processing methodology.

Every DEM has a vertical datum — a reference surface from which elevations are measured. Common vertical datums include:

• NAVD88 — the standard vertical datum for the contiguous United States, used for most engineering and surveying work
• Ellipsoidal height — elevation above the WGS84 ellipsoid, used by GPS receivers
• Local datum — an arbitrary datum used for site-specific work

For most professional applications, deliverables should be in NAVD88. GPS receivers measure ellipsoidal heights, which must be converted to NAVD88 using a geoid model (GEOID18 or similar).

The choice between DSM and DTM is not a technical detail — it determines whether your elevation data is useful for your application. For terrain analysis, drainage design, and engineering applications, you need a DTM. For surface documentation and volumetric analysis, a DSM is appropriate.

At Blackridge Geospatial, we discuss deliverable requirements with every client before the survey to ensure the right product is produced. Contact us to discuss your elevation data requirements.