A method and a system for determining a terrain parameter relative to a vehicle (100) by registering a first image from the vehicle located at a first point in space, then registering a second image from the vehicle (100) when it is located at a second point in space (O.sub.2) . Receiving vehicle movement data which describes at least one of a translation (R) between the first and second points in space (O.sub.1; O.sub.2), and a rotation (A) between the first and second points in space (O.sub.1; O.sub.2). A first image velocity vector is determined, which describes a movement of the primary representation of the first terrain point (P.sub.1) in the first image to the secondary representation thereof in the second image. A second image velocity vector is determined, which describes a movement of the primary representation of the second terrain point (P.sub.2) in the first image to the secondary representation thereof in the second image. A terrain vector is then resolved from a respective relationship expressing each of the first and second image velocity vectors as a function of the translation and the rotation, the terrain vector extending between one of the first and second points in space (O.sub.1; O.sub.2) and a particular terrain point (T) in proximity of the vehicle (100).