1. A method of determining the 3-D position and orientation of an object relative to a scanning device, given a surface model of the object, range profiles that have been acquired by scanning the object, and the object's approximate location defined by the 3-D position and orientation relative to the scanning device, the method comprising the steps of: a) generating a set of at least 2 synthetic 2-D profiles of the object from a model of the object by simulating how the scanning device would have scanned the object if the object were located at the approximate 3-D location; b) searching through the acquired range profiles to select a subset of these profiles whose 2-D waveforms most closely match the 2-D waveforms of the set of synthetic profiles; c) determining the amount of 2-D translational and rotational offset between the 2-D waveforms of the synthetic profiles and the 2-D waveforms of the acquired range profiles that produced this most close match; d) calculating a correction of the approximate three dimensional position and orientation of the model representing the object from these 2-D offsets between the synthetic profiles and the acquired range profiles that produced the best match in step (b) to obtain a closer approximate 3-D position and orientation of the object; and, e) repeating steps (a) to (d) until the correction is below a predetermined threshold.