Digital elevation model
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Image:Mtm-05277e 3d.png A digital elevation model (DEM) is a representation of the topography of the Earth or another surface in digital format, that is, by coordinates and numerical descriptions of altitude. DEMs are used often in geographic information systems. A DEM may or may not be accompanied by information about the ground cover. In contrast with topographical maps, the information is stored in a raster format. That is, the map will normally divide the area into rectangular pixels and store the elevation of each pixel. In that sense, digital elevation model (DEM) data are sampled arrays of surface elevations in raster form. DEMs are most commonly used to extract terrain parameters, model water flow or mass movement or for pure visualization purposes (3D draping).
Digital elevation models may be prepared in a number of ways, but they are frequently obtained by remote sensing rather than direct survey. One powerful technique for generating digital elevation models is interferometric synthetic aperture radar; two passes of a radar satellite (such as RADARSAT-1) suffice to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters. One also obtains an image of the surface cover. Older methods of generating DEMs often involve interpolating digital contour maps that may have been produced by direct survey of the land surface. Note that the contour data or any other sampled elevation datasets (by GPS or ground survey) are not DEMs. A DEM implies that elevation is available continuously at each location in the study area.
The quality of a DEM 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;
- sampling density (elevation data collection method);
- grid resolution or pixel size;
- interpolation algorithm;
- vertical resolution;
- terrain analysis algorithm;
A free DEM of the whole world called GTOPO30 (30 arcsecond resolution, approx. 1km) is available. The Shuttle Radar Topography Mission (SRTM) DEM is also freely available for most of the globe and represents elevation at a 3 arc-second resolution (around 90m). It is also available at 1 arc-second (30m) resolution for North America. The limitation with both datasets is that they cover continental landmasses only. Submarine elevation (known as bathymetry) data can be generated using satellite based sensors or ship mounted depth soundings. The SRTM30Plus dataset attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model.
Many national mapping agencies produce their own DEMs, often of a higher resolution and quality, but frequently these have to be purchased, sometimes at considerable cost.