The differences between DEM/DTM/DSM (GIS elevation models)

The differences between DEM/DTM/DSM (GIS elevation models)

 

We have different ways to model elevation, for example, there are Digital Elevation Models (DEM), Digital Surface Models (DSM), Digital Terrain Models (DTM) and even Triangular Irregular Networks (TIN)

Today we will learn the differences between these three types of GIS elevation models.

Digital Elevation Model (DEM)

A digital elevation model is a bare-earth raster grid referenced to a vertical datum. When you filter out non-ground points such as bridges and roads, you get a smooth digital elevation model. The built (power lines, buildings and towers) and natural (trees and other types of vegetation) aren’t included in a DEM.

When you void vegetation and man made features from elevation data, you generate a DEM. A bare-earth elevation model is particularly useful in hydrology, soils and land use planning.

Example:

Soil mapping: DEMs assist in mapping soils which is a function of elevation (as well as geology, time and climate)

Hydrologic modelling: Hydrologists use DEMs to delineate watersheds, calculate flow accumulation and flow direction.

Terrain Stability: Areas prone to avalanches are high slope areas with sparse vegetation. This is useful when planning a highway or residential subdivision.

How to capture Digital Elevation Models?

LiDAR: LiDAR measures reflected light that bounces off the ground and back to the sensor to obtain elevation of the Earth’s surface.

Satellite interferometry: Synthetic aperture radar such as Shuttle Radar Topography Mission uses two radar images from antennas captured at the same time to create a DEM.

Photogrammertry: In aerial photography, photogrammetry uses photographs from at least two different vantage points. Similar to how your vision works, it’s able to obtain depth and perspective because of the separate vantage points.

Digital Surface Model (DSM)

In a LiDAR system, pulses of light travel to the ground. When the pulse of light bounces off its target and returns to the sensor, it gives the range (a variable distance) to the Earth. Hence, how this system earned its name of Light Detection and Ranging.

In the end, LiDAR delivers a massive point cloud filled of varying elevation values. But height can come from the top of buildings, tree canopy, powerlines and other features. A DSM captures the natural and built features on the Earth’s surface.

A DSM is useful in 3D modeling for telecommunications, urban planning and aviation. Because objects extrude from the Earth, this is particularly useful in these examples:

Vegetation management: Along a transmission line, DSMs can see where and how much vegetation is encroaching.

Runway approach zone encroachment: In aviation, DSMs can determine runway obstructions in the approach zone.

View Obstacle: Urban planners use DSM to check how a proposed building would affect the viewshed of residents and businesses.

Digital Terrain Model (DTM)

According to USGS LiDar base specification, a digital terrain model (DTM) actually has two definitions depending on where you live.

• In some countries, a DTM is actually synonymous with a DEM. This means that a DTM is simply an elevation surface representing the bare earth referenced to a common vertical datum.

•   In the United States and other countries, a DTM has a slight different meaning. A DTM is a vector data set composed of regularly spaced points and natural features such as ridges and breaklines. A DTM augments a DEM by including linear features of the bare-earth terrain.

DTMs are typically created through stereo photogrammetry like in the example above. For example, contour lines are in purple. The DTM points are regularly-spaced and characterize the shape of the bare-earth terrain.

In the image above, you can see how the DTM is not continuous and that it’s not a surface model. From these regularly-space and contour lines, you can interpolate a DTM into a DEM. A DTM represents distinctive terrain features much better because of its 3D breaklines and regularly spaced 3D mass points.

Quality and Accuracy of DEM/DTM

The quality of a DEM/DTM is a measure of how accurate elevation is at each pixel (absolute accuracy) and how accurately is the morphology presented (relative accuracy). Several factors play an important role for quality of DEM-derived products:

•    terrain roughness;
•   vertical resolution;
•   terrain analysis algorithm;
•  Reference 3D products include quality masks that give information on the coastline, lake,         snow, clouds, correlation.  
•    sampling density (elevation data collection method);
• grid resolution or pixel size;
• Interpolation algorithm; etc.

Common uses of DEMs

•    Extracting terrain parameters.
•   Modeling water flow or mass movement (for example, landslides)
•   Creation of relief maps.
•    Creation of physical models (including raised-relief maps).
•    Rectification of aerial photography or satellite imagery.
•    Reduction (terrain correction) of gravity measurements (gravimetry, physical geodesy).
•   Terrain analyses in geomorphology and physical geography.
•   Rendering of 3D visualizations.