All the imageries that are collected, be it the satellite imageries or UAV data, all are 2D imagery. But for relevant study, it is at times required to understand the images in a more realistic approach. Hence, to understand better and make the images real, elevation is required. Elevation can be used for a lot more than just rectifying 2D imagery. Let us now have a detailed look at the digital elevation model.
What do you understand by DEM?
DEM or digital elevation model is basically the model of the earth’s terrain in a digital format. DEM is made up of gridded elevation values that when mapped out helps to generate the shape of a location on the earth surface or any other outer planetary terrain.
DEMs are used in many fields, few of which are hydrological and geological analysis, hazard monitoring, exploration for natural resources and agricultural management.
There are two main factors that influence a digital elevation model namely Posting and Accuracy. Generally posting is used for the name of the value of the DEM. It describes the distance between the measures of elevation on the ground. The tighter the posting the more detailed terrain can be derived. Accuracy refers to how closely the heights in the model match the actual ground heights that you are measuring. Accuracy is independent of posting, but the tighter the posting the better the accuracy. It is just like that the imagery terrains come in different flavors depending on the need. There are various factors that influence the quality of DEM. Few of them are listed below:
- Terrain roughness
- Vertical and grid resolution or pixel size
- Interpolation and terrain analysis algorithm
- Sampling density
Different types of DEMs
The common types of DEMs include DSM, DTM, Point Cloud and 3D. Details are as below:
DSM or the Digital surface model: This shows the elevation of the ground as well as all the things on the ground like trees building, towers and all. This can be helpful in setting up mobile signal towers which would give you a better understanding of how these structures on the ground are going to impact the flow of the signal.
DTM or Digital terrain model: This model shows the bare earth surface without consideration of any trees or structure on the surface. DTM type of model becomes helpful more when you consider the level of water that can flood the area, or the amount of dirt required for a construction.
Contours: Contours represent terrain with lines of the same elevation value. When you look at all the lines together you will see the 3D image of the terrain on a 2D map. Contours, though not very popular now, is still used for the study of mountainous terrains and it largely depends on land survey data.
Point cloud: These are basically elevation DSM or Digital terrain model but in a point cloud format. Which means that its points are randomly placed like a cloud. And hence, point cloud model.
3D: This 3D is an imagery of the future. It is a transformation of the 2D images into a 3D representation. You do not just get imageries from above. You see the surface from every angle which gives you a lifelike view of the world.
Moreover, the DEM can be represented as a raster, which is a grid of squares known as the heightmap, or as a vector-based triangular irregular network known as TIN. TIN is often referred to as the Primary DEM while the raster DEM is referred to as the secondary DEM. There are various methods used to obtain data for elevation study. few of which includes as below:
- Stereo Photogrammetry from aerial surveys
- Structure from motion
- Interferometry from radar data
- Real time Kinematic GPS
- Topographic maps
- Doppler radar
- Focus Variation
- Surveying and mapping drones
- Range imaging.
Many of the mappers prepare the DEM in many ways frequently making use of remote sensing data rather than land survey data. DEMs are used for DIS and are the basis for digitally produced relief maps. One very powerful technique used is the Interferometric Synthetic Aperture Radar in which two passes of a radar satellite collect data to generate a digital elevation map with a resolution of around ten meters.
The first usable elevation data for a sizeable portion of the earth was provided by the SPOT1 satellite in 1986. Further data using the same technique was provided in 1991 by the European Remote Sensing Satellite, ERS, in 2000 by Shuttle Radar Topography Mission, SRTM, using the single pass SAR and the Advanced Spaceborne Thermal Emission and Reflection Radiometer, ASTER 2000 instrumentation on Terra satellite using double pass SARs.
Older methods involved the generation of DEMs by interpolation digital contour maps that were produced by the direct land survey. Although not a very common method now it is still used in mountain areas.
DEM is used widely for the study of the terrain and various other related surveys. Some of the common uses are:
- DEM is commonly used in GIS
- For archaeological study
- Advanced driver assistance systems
- Intelligent transportation system
- Precision farming and forestry
- Surface analysis
- Base mapping
- Flight simulation
- Satellite navigation
- The Line of sight analysis
- Relief maps
- Civil engineering
- Land survey for geomorphology
- Gravimetry and physical geodesy
- Rectification of 2D imagery from UAV or satellite
- Deriving terrain parameters for geomorphology
- Extracting water flow information for hydrology or mass movement like landslides
- Modeling Soil wetness with cartographic DTW-index
- Creating relief maps
- Rendering 3D visualization
- 3D flight planning and TERCOM
- Creation of raised maps for relief or strategic land survey.
Details on current DEM sources usage:
- GTOPO30 provides a free DEM of the whole world and is approximately 1Km along the. But the quality of the representation is variable and some areas are very poor.
- The Advanced Spaceborne Thermal Emission and Reflection Radiometer of the Terra Satellite provide a higher quality DEM which is also available free. It covers 99% of the globe and represents elevation at 30-meter resolution.
- Bathymetry or Submarine elevation data is obtained from ship-mounted depth soundings. Combining topography and Bathymetry yields the truly Global Relief Model and also, SRTM30Plus combines GTOPO30, SRTM and Bathymetric data to yield a truly Global Elevation model
- EARTH2014 global topography and relief model provide information on ice sheets heights and bedrock over Antarctica and Greenland with grids at 1 arc-min resolution.
- The Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) combines SRTM data and that outside the coverage of SRTM to provide information at 7.5 arcs Second resolution
- The Radar Satellite RADARSAT-2 has been used to provide DEMs for military and commercial purposes.
- TerraSAR-X and RanDEM-X provide DEM in the form of a uniform global coverage with a 12m resolution.
- ALOS provides a free global DSM with 1arc second resolution and a commercial DSM/DTM with 5meter resolution.
Hence, we now can conclude that DEM is not just limited to the study of the earth’s surface any longer. With the increasing advancement in the space sciences DEMs are used for the study of the terrains of other planetary objects like that of the Mars and the moon. NASA’s Mars Digital Terrain Model and the Mission Experiment Gridded data record from the Mars Orbiter Laser Altimeter instrument of Mars Global Surveyor provides with crucial information on the terrain formation of the planet. Digital Elevation Models have now become popular to derive land information for various purposes. It not only provides the data with greater accuracy and quality but also has a quick turn-around time. DEMs allow extensive geographic coverage without the need to be near the location which is really benefitted and also now a day, multiple solutions are available to meet the requirements and budget of the project.