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:
Digital Elevation Models (DEM), Digital Surface Models (DSM), Digital Terrain Models (DTM), even Triangular Irregular Networks (TIN)…
Well, today we are going to set the record straight with your elevation frustration. We 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. They return and are detected by the sensor giving the range (a variable distance) to the Earth. Hence, how they earned their name of Light Detection and Ranging.
In the end, LiDAR delivers a massive point cloud filled of varying elevation values. But these elevation values can come from the top of buildings, tree canopy, powerlines and other types of features. A DSM captures the natural and built features on the Earth’s surface.
A DSM is especially useful in 3D modelling and is relevant in telecommunications, urban planning, aviation and forestry. This is because objects extrude from the Earth, which is particularly useful in these examples:
- Runway approach zone encroachment – In aviation, runway obstructions in the approach zone can be examined with a DSM to ensure no collisions.
- Vegetation management – Along a transmission line, overlaying a DSM to 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 other residents and businesses.
What is a Digital Elevation Model (DEM)?
A digital elevation model is a regularly-spaced bare-earth raster grid referenced to a common vertical datum. When you filter out non-ground points such as bridges and roads, you are left with a smooth digital elevation model. The built (powerlines, buildings and towers) and natural (trees and other types of vegetation) aren’t extruding in a DEM.
When you void vegetation and man made features from elevation data, you obtain a DEM. A smooth, bare-earth elevation model is particularly useful in fields of study such as hydrology, soils and land use planning/safety. Here are examples how a DEM can be used in GIS:
- Hydrologic modelling – A DEM is used to delineate watersheds, calculate flow accumulation and find out flow direction.
- Terrain stability – Areas prone to avalanches are high slope areas with sparse vegetation, which 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 this 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. Contour lines have been converted into points and are shown in purple. The DTM points are regularly-spaced that 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.