Life’s not always so simple for the GIS practitioner when it comes to elevation. This is because 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)…
Well, today we are going to set the record straight with your elevation frustration.
Let’s explore the differences between these three types of GIS elevation models.
What is a 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:
- Runway approach zone encroachment. In aviation, DSMs can determine runway obstructions in the approach zone.
- Vegetation management. Along a transmission line, DSMs can see where and how much vegetation is encroaching.
- View obstruction. Urban planners use DSM to check how a proposed building would affect the viewshed of residents and businesses.
What is a 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 (powerlines, 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
- 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.
- Soil mapping. DEMs assist in mapping soils which is a function of elevation (as well as geology, time and climate).
What is a Digital Terrain Model (DTM)?
When you refer to the USGS LiDAR Base Specification (on page 28), 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.
Elevation Data Sources
Some of the remote sensing methods for obtaining DEM surfaces are:
- Satellite interferometry with synthetic aperture radar such as Shuttle Radar Topography Mission uses two radar images from antennas at the same time
- Aerial survey photogrammetry uses photographs from at least two different locations to generate stereopairs
- LiDAR measures reflected light back to the sensor to obtain a range to the Earth’s surface.