The files describing the 3D terrain shape, through a differently dense grid of plano-altimetric points, are generated with different tools and methodologies:

• interpolation of contour lines from vector maps.
• semi unsupervised sampling from digital stereopair images. (This technique is optimized if applied to images directly acquired in raster format, using off the shelf aerial digital cameras)
• direct measure of every single point of the grid, using “active” airborne sensors, capable of recording the terrain reflections of instrument’s irradiated energy. Depending on the waive length selected and on their specific engineering features, two different type of tools are used: LIDAR (LIght Detection and Ranging) emitting Laser-type energy, effective from medium to low altitudes, and IFSAR (InterFerometric Synthetic Aperture Radar), capable of surveying extensive areas from high altitude.

“Active” sensors provide the most accurate mean of representing the terrain in 3 dimensions, because every single point of the grid is directly measured and it’s hight value is not interpolated from the value of surrounding points.