Digital Terrain Modeling

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A digital terrain model, also known as a digital elevation model, is a digitally-created representation of ground topography and terrain.
Although maps depicting topographical information have been produced for hundreds of years, it is only recently that such elevation data has been collected in such a precise digital form as to allow the creation of digital models.
The digital elevation model can be used to model water flow or other movements, 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 raised-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 shown in a digital terrain map.
Often this data is obtained using remote sensing equipment rather than direct surveying methods.
Radar satellites are often used for models of large areas of terrain.
Though these satellites often only have a resolution of about ten meters, they can obtain information on an area tens of miles wide in a single pass.
There are other methods, too.
A pair of images acquired with different angles taken from an airplane or satellite can be used to infer the terrain.
The first digital terrain models using this method were created in 1986 for a large portion 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 land surface.
Contour line data is obtained by various surveying methods, including LIDAR, Doppler radar, Theodolite or total station surveying equipment.
Using GPS, elevation data can be related to a specific location.
This information can then be turned into a digital contour map or terrain model, which turns the raw data into a model enabling the viewer to "visualize" the landscape in a virtual manner.
Unlike contour maps, DTMs provide continuous elevation information.
Contour maps, on the other hand, connect points that are equal in elevation, but do not generally 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 3D; in many cases, "fly throughs" or similar programs allow the user to manipulate the map to view all areas and angles of the terrain.
A digital terrain model usually includes only the Earth's surface, excluding vegetation as well as buildings or other man-made features.
This is sometimes referred to as a bare-earth model.
A Digital Surface Model, on the other hand, shows such features in addition to the natural terrain.
The problem with some surveying methods used to create these models, such as radar, is that they reflect the highest elevation point on a given location, whether this is the top of a tree or building or bare ground.
A free low-resolution digital elevation model of the earth, known as GTPPO30, is available.
A much higher quality DEM is available from the ASTER instrument of the Terra satellite.
The US Geological Survey also provides the National Elevation Dataset, seamless elevation data for the contiguous United States.
Free digital elevation models can also be found for Mars.
However, for a specific application, purpose-created DEM, DTM, or Digital Surface Models may be required.
These models are very accurate, and are usually requested by public agencies or large corporations.
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