Earth , moon , or asteroid — created from a terrain's elevation data. A "global DEM" refers to a discrete global grid.
DEMs are used often in geographic information systems, and are the most common basis for digitally produced relief maps. While a digital surface model DSM may be useful for landscape modeling, city modeling and visualization applications, a digital terrain model DTM is often required for flood or drainage modeling, land-use studies,  geological applications, and other applications,  and in planetary science.
In most cases the term digital surface model represents the earth's surface and includes all objects on it. In contrast to a DSM, the digital terrain model DTM represents the bare ground surface without any objects like plants and buildings see the figure on the right. A DEM can be represented as a raster a grid of squares, also known as a heightmap when representing elevation or as a vector-based triangular irregular network TIN.
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DEMs are commonly built using data collected using remote sensing techniques, but they may also be built from land surveying. The digital elevation model itself consists of a matrix of numbers, but the data from a DEM is often rendered in visual form to make it understandable to humans.
This visualization may be in the form of a contoured topographic map , or could use shading and false color assignment or "pseudo-color" to render elevations as colors for example, using green for the lowest elevations, shading to red, with white for the highest elevation. Visualizations are sometime also done as oblique views, reconstructing a synthetic visual image of the terrain as it would appear looking down at an angle.
In these oblique visualizations, elevations are sometimes scaled using " vertical exaggeration " in order to make subtle elevation differences more noticeable. Mappers may prepare digital elevation models in a number of ways, but they frequently use remote sensing rather than direct survey data.
Digital elevation model
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 contour line 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.
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One powerful technique for generating digital elevation models is interferometric synthetic aperture radar where two passes of a radar satellite such as RADARSAT-1 or TerraSAR-X or Cosmo SkyMed , or a single pass if the satellite is equipped with two antennas like the SRTM instrumentation , collect sufficient data to generate a digital elevation map tens of kilometers on a side with a resolution of around ten meters.
The SPOT 1 satellite provided the first usable elevation data for a sizeable portion of the planet's landmass, using two-pass stereoscopic correlation. A tool of increasing value in planetary science has been use of orbital altimetry used to make digital elevation map of planets.
A primary tool for this is laser altimetry. 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:.
SRTM does not cover the polar regions and has mountain and desert no data void areas. SRTM data, being derived from radar, represents the elevation of the first-reflected surface—quite often tree tops. So, the data are not necessarily representative of the ground surface, but the top of whatever is first encountered by the radar. Submarine elevation known as bathymetry data is generated using ship-mounted depth soundings.
When land topography and bathymetry is combined, a truly global relief model is obtained. Other than SRTM30plus, Earth provides information on ice-sheet heights and bedrock that is, topography below the ice over Antarctica and Greenland.
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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.
DEMs are often a product of national lidar dataset programs. OpenTopography  is a web based community resource for access to high-resolution, Earth science-oriented, topography data, and related tools and resources that houses large volumes of DEM data. OpenTopography  is a U. From Wikipedia, the free encyclopedia. Balenovic, H.
Marjanovic, D. Vuletic, etc. Quality assessment of high density digital surface model over different land cover classes. UK: Environment Agency. Archived from the original PDF on Archived from the original on Encyclopedia of Geodesy : 1—6.
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Kirkman, Peter M. West Sussex. IAHS Publ. Retrieved 11 March K; Chrysoulakis, N.
International Journal of Remote Sensing. Archived from the original PDF on July 21, Retrieved June 22, Cavanaugh, et al. Journal of Soil and Water Conservation. Journal of Geophysical Research: Earth Surface. Pernar, A. Estimation of the mean tree height of forest stands by photogrammetric measurement using digital aerial images of high spatial resolution.
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