- Specialist module: the Numerical Terrain Model
The specialised definition is too detailed, so I will use a maximal simplification. A Numerical Terrain Model is a file in which elevation data is stored.
Most often these are raster files (such as normal camera photos) in GeoTiff format, where each pixel (instead of a value describing the colour) has elevation information associated with it (e.g. 245 m.).
What is the source of these files? They are most often created from laser scanning performed from aircraft or satellites. In Poland, NMT data are publicly available free of charge. They can be downloaded via Geoportal.gov.pl.