From the list of network points, a surface model is built consisting of triangles glued together. In the case of non-triangular networks, a step is added to select the points that should be joined into triangles.

In the case of infographics, the triangles are dressed with a layer, thus restoring the general appearance of the terrain, a satellite image or a map.

In the case of a low-flying trajectory, we establish the shortest and least exposed path, checking that each point of the trajectory is above the surface defined by the MDT network.

Increasingly, geographic information systems (GIS) incorporate the third dimension in the form of MDT, even though the costs related to the acquisition of altitude information are relatively high. This allows these GIS to be used for applications such as calculating the implementation of transportation infrastructure (underground ducts, land roads, overhead power lines, GSM antennas, etc.). In this case, and depending on the MDT resolution, the information related to the coverage of the land by buildings or plants is integrated, to add their height to the altitude of the land on which they are located. Digital terrain models are also applied in the field of earth sciences, to perform quantitative analyzes of morphology, which can indicate to the researcher the presence of a tectonic, climatic or lithological signal.

Some mapping organizations (mainly American) make important databases freely available to the public, which can be accessed via the Internet. Let's name some of the main ones: NASA (DEM ASTER, SRTM-1, SRTM-3, SRTM30, MOLA MEGDR), the National Imagery and Mapping Agency (NIMA) (SRTMs) and the USGS (DEM SDTS, 1 degree, 7.5 minutes, NED, GTOPO30).

The amount of free data remains limited, since the cartographic organizations that have it are generally financed through its sale (this is the case, in particular, of the IGN "National Geographic Institute (France)" in France, which publishes the MDT BD Alti). However, the situation is improving, since public administrations are becoming aware of the importance of this data in numerous civil spheres that cannot afford to buy it at a gold price. For example:.

Three main characteristics allow you to get a quick idea of ​​a digital terrain model and judge its suitability for a particular need:.

Characteristics of some formats available over the Internet (see the External links section to find out where to download them from):.

* Payment formats (for MDT BD Alti, orders from 400 euros, according to the IGN 2003 catalogue).

** Total coverage of the French metropolitan coastline in 2013 with a vertical precision of 20 cm.

Several free or open source GIS have appeared. This is the case of GRASS (Geographic Resources Analysis Support System), for example, which can be linked to free databases (MySQL, PostgreSQL, etc.), and use an open source statistics program such as R (programming language). Similarly, we have also witnessed the emergence of 3D visualization tools adapted to Linux, such as TruFlite.

There are also other collaborative tools, such as JMap Imagerie, that allow you to interact with matrix-type images, specifically with digital terrain models (DTM) or satellite images with several bands. It allows the calculation of the covered area, the slope, the volume of cut and fill, the section view, the search for pixels, as well as visibility studies. It is part of the GDAL (Geospatial Data Abstraction Library) library.

From the list of network points, a surface model is built consisting of triangles glued together. In the case of non-triangular networks, a step is added to select the points that should be joined into triangles.

In the case of infographics, the triangles are dressed with a layer, thus restoring the general appearance of the terrain, a satellite image or a map.

In the case of a low-flying trajectory, we establish the shortest and least exposed path, checking that each point of the trajectory is above the surface defined by the MDT network.

Increasingly, geographic information systems (GIS) incorporate the third dimension in the form of MDT, even though the costs related to the acquisition of altitude information are relatively high. This allows these GIS to be used for applications such as calculating the implementation of transportation infrastructure (underground ducts, land roads, overhead power lines, GSM antennas, etc.). In this case, and depending on the MDT resolution, the information related to the coverage of the land by buildings or plants is integrated, to add their height to the altitude of the land on which they are located. Digital terrain models are also applied in the field of earth sciences, to perform quantitative analyzes of morphology, which can indicate to the researcher the presence of a tectonic, climatic or lithological signal.

Some mapping organizations (mainly American) make important databases freely available to the public, which can be accessed via the Internet. Let's name some of the main ones: NASA (DEM ASTER, SRTM-1, SRTM-3, SRTM30, MOLA MEGDR), the National Imagery and Mapping Agency (NIMA) (SRTMs) and the USGS (DEM SDTS, 1 degree, 7.5 minutes, NED, GTOPO30).

The amount of free data remains limited, since the cartographic organizations that have it are generally financed through its sale (this is the case, in particular, of the IGN "National Geographic Institute (France)" in France, which publishes the MDT BD Alti). However, the situation is improving, since public administrations are becoming aware of the importance of this data in numerous civil spheres that cannot afford to buy it at a gold price. For example:.

Three main characteristics allow you to get a quick idea of ​​a digital terrain model and judge its suitability for a particular need:.

Characteristics of some formats available over the Internet (see the External links section to find out where to download them from):.

* Payment formats (for MDT BD Alti, orders from 400 euros, according to the IGN 2003 catalogue).

** Total coverage of the French metropolitan coastline in 2013 with a vertical precision of 20 cm.

Several free or open source GIS have appeared. This is the case of GRASS (Geographic Resources Analysis Support System), for example, which can be linked to free databases (MySQL, PostgreSQL, etc.), and use an open source statistics program such as R (programming language). Similarly, we have also witnessed the emergence of 3D visualization tools adapted to Linux, such as TruFlite.

There are also other collaborative tools, such as JMap Imagerie, that allow you to interact with matrix-type images, specifically with digital terrain models (DTM) or satellite images with several bands. It allows the calculation of the covered area, the slope, the volume of cut and fill, the section view, the search for pixels, as well as visibility studies. It is part of the GDAL (Geospatial Data Abstraction Library) library.