In today’s world where everyone is looking for ways of improving efficiencies, Digital Elevation Models have many applications including Mining exploration, flood modelling, city planning, river and aquifer mapping, vegetation(density, height) mapping and many others in area such as road, rail, pipeline planning.
So you may be asking “What exactly is a Digital Elevation Model?”
Well, a Digital Elevation Model is a way of digitally representing the elevation at a given geographic coordinate. For every point in the DEM there are 3 values; the normal X and Y values represent the coordinates, and the Z value represents the relative height. Using these three points, we can accurately plot terrain into a visual format that makes it easier to use, while retaining the underlying data and allowing the user to extract accurate height data. The images below show 3D render of a DEM over the Coorong and part of Hindmash Island in South Australia beside a satellite image of the same area.
Coorong 50cm Z accuracy IFSAR DEM
So “Are all Digital Elevation Models the same?”
The short answer is no. There are 2 forms of DEM, DTM and DSM. DSM stands for Digital Surface Model and includes the vegetation and building elevation with the ground elevation. DTM stands for Digital Terrain Model. In a DTM the vegetation and building data have been artificially removed from the DEM to provide an elevation model of the underlying terrain.
How Digital Elevation Models are acquired?
DEM can be generated using different techniques and sensors:
- Radar Interferometry (InSAR): a SAR instrument sends microwave radiation and then record the strength and time delay of the returning signal to produce images of the ground. Elevation information can be extracted from the time delay difference between 2 SAR images over the same area. The Shuttle Radar Topography Mission (SRTM) is the most famous example of a global Digital elevation model based on space borne InSAR. InSAR DEM can also be produced using Aerial IFSAR radar imagery to achieve better resolution and accuracy than space borne sensors. Intermap technologies provides this kind of digital elevation model worldwide.
- Aerial and Satellite photogrammetry: Two or more images are used to extract the height of any pixels on image stereo pairs utilising acquisition parameters such as focal length, principal points, platform location etc. (example ALOS PRISM DEM, SPOT DEM, EROS DEM)
- Direct coordinate acquisition: It can be achieved with surveying techniques and GPS measurement where 3 dimensional object positions are accurately determined and associated with positions on the surface the earth.
- LiDAR: LiDAR is a remote sensing mapping technique which uses a laser scanner to measure the distance between the sensor and the surfaces. Mounted on an aircraft of helicopter, millions of x,y,z positions are acquired and form a surface map. Ground LiDAR are used to measure features along profiles.
What can DEMs be used for?
When it comes to DEMs the uses are unlimited. They are essential for things such as urban planning, construction of pipelines and other infrastructure, hydrological modelling from flood prevention to tsunami predictions – remotely sensed elevation maps are invaluable to accurately map the terrain over large areas where the same data using conventional surveying methods would be prohibitively expensive. Accurate DEMs are used in flood mapping, physical modelisation, where accuracy of the terrain mapping determines the accuracy of the resulting flood map. DEMs also have applications for precision agriculture and many other scientific and commercial areas.
Flood level using Adelaide IFSAR DEM
Interesting Related Links:
NEXTMap USA
