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Digital terrain modeling

A digital terrain model, also known as a digital elevation model, is a digitally created representation of the topography of the ground and terrain. Although maps representing topographic information have been produced for hundreds of years, it is only recently that elevation data has been collected in such a digital form as to enable digital modeling.

The digital elevation model can be used to model water flow or other motion, for example to run simulated avalanches or landslides, or for land use studies, transportation system planning, and geological applications. Other uses include the creation of physical relief maps, flight simulator programs, or other visualization and modeling applications. Digital terrain models are also incorporated into geographic information systems.

There are many ways to obtain the information displayed on a digital terrain map. Often these data are obtained using remote sensing equipment rather than direct survey methods. Radar satellites are often used to model large areas of terrain. Although these satellites often only have a resolution of about ten meters, they can obtain information over an area tens of kilometers wide in a single pass. There are also other methods. A pair of images acquired at different angles taken from an aircraft or satellite can be used to infer terrain. The first digital terrain models using this method were created in 1986 for a large part of the planet using data from the SPOT 1 satellite.

In many cases, digital terrain models are generated from contour maps, often those that have been produced by direct surveying of the earth’s surface. Contour data is obtained by various survey methods, including lidar, Doppler radar, theodolite, or total station survey equipment. By using GPS, elevation data can be related to a specific location. This information can then be converted into a digital contour map or terrain model, which converts the raw data into a model that allows the viewer to virtually “see” the landscape.

Unlike contour maps, DTMs provide continuous elevation information. Contour maps, on the other hand, connect points that have the same elevation, but generally do not provide elevation data for intermediate points. A digital terrain model also differs from a contour map in its visual appearance. A digital terrain map appears in 3D; in many cases, “flights” or similar programs allow the user to manipulate the map to see all areas and angles of terrain.

A digital terrain model typically includes only the Earth’s surface, excluding vegetation, as well as buildings or other man-made features. This is sometimes called the bare earth model. A digital surface model, on the other hand, displays such features in addition to the natural terrain. The problem with some survey methods used to create these models, such as radar, is that they reflect the highest point of elevation at a given location, be it a treetop, building, or bare ground.

A low-resolution digital elevation model of the earth, known as GTPPO30, is available. A much higher quality DEM is available on the ASTER instrument on the Terra satellite. The US Geological Survey also provides the National Elevation Data Set, continuous elevation data for the contiguous United States. Free digital elevation models for Mars can also be found. However, for a specific application, specifically created DEMs, DTMs, or digital surface models may be required. These models are very accurate and are often requested by public agencies or large corporations.

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