1. A method of deriving a digital terrain model from a digital surface model of an area of interest, comprising: a) dividing the area of interest into a plurality of area portions; b) for each area portion calculating, from the digital surface model, a set of candidate surfaces adapted to represent a ground surface in said area of interest; c) for those area portions having at least two candidate surfaces, estimating a distance of each candidate surface from a digital surface model of the ground surface for that area portion by using a function of a set of geometrical features related to a considered candidate surface, said function being derived from a known relation between each candidate surface and the digital surface model of the ground surface, and selecting, as representative of the ground surface in each said area portion having at least two candidate surfaces, a surface among said candidate surfaces having the smallest estimated distance from said ground surface to act as a local surface model for that area portion; d) for those area portions having only one candidate surface, using that one candidate surface to act as a local surface model for that area portion; and e) merging said local digital terrain models for each portion to obtain the digital terrain model of the area of interest.