1. A method for determining a distance from a 3D point to a 3D surface from a 2D projected range image comprising: determining a projected distance from the 3D point to the 3D surface using the projected range image; determining a cliff distance from the 3D point to a nearest cliff in the 3D surface using the projected range image; and combining the projected distance and the cliff distance to determine the distance from the 3D point to the 3D surface.