This technique is known as 3D photogrammetry. The UAV takes images with a certain percentage of overlap, which we specify in control software. As a result, any given point on the ground has been seen from at least two different viewpoints. This technique works in the same way that your brain generates 3D information from the differences between the images from your two eyes. After the flight, specialised software generates height information from the parallax differences between overlapping images. Accuracy in the calculated 3D terrain is enhanced by using multiple Ground Control Points. These are targets placed around the site, whose positions in 3D space have been recorded at centimetre-level accuracy using special survey equipment.