Digital elevation model

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3D rendering of a DEM of Tithonium Chasma on Mars
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3D rendering of a DEM of Tithonium Chasma on Mars

A digital elevation model (DEM) is a type of Digital terrain model, recording a topographical (geomorphometric) representation of the terrain of the Earth or another surface in digital format. DEM's record altitude 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.[1]

DEMs are used often in geographic information systems, and are the most common basis for digitally-produced relief maps.

Contents

[edit] Production

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; this method is still used in mountain areas, where interferometry is not always satisfactory. Note that the contour data or any other sampled elevation datasets (by GPS or ground survey) are not DEMs, but may be considered Digital terrain models. 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;

[edit] Uses

Common uses of DEMs include:

[edit] Sources

A free DEM of the whole world called GTOPO30 (30 arcsecond resolution, approx. 1 km) is available, but its quality is variable and in some areas it is very poor. A much higher quality DEM from the Shuttle Radar Topography Mission (SRTM) is also freely available for most of the globe and represents elevation at a 3 arc-second resolution (around 90 m). It has also been developed at 1 arc-second (30 m) resolution, but this has only been declassified for United States territory. The limitation with both datasets is that they cover continental landmasses only, and SRTM does not cover the polar regions and has mountain and desert no data (void) areas. Submarine elevation (known as bathymetry) data is be generated using ship mounted depth soundings. The SRTM30Plus dataset attempts to combine GTOPO30, SRTM and bathymetric data to produce a truly global elevation model.[2]

The most usual grid (raster) is between 50 and 500 meters. In gravimetry e.g., the primary grid may be 50 m, but is switched to 100 or 500 meters in distances of about 5 or 10 kilometers.

Many national mapping agencies produce their own DEMs, often of a higher resolution and quality, but frequently these have to be purchased, and the cost is usually prohibitive to all except public authorities and large corporations.

[edit] United States

The US Geological Survey produces the National Elevation Dataset, a seamless DEM for the United States based on 7.5' topographic mapping. As of the beginnning of 2006, this replaces the earlier DEM tiled format (one DEM per USGS topographic map).[3]

[edit] DEM file formats

[edit] External links

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