The data collected from each device did not directly give the position of the tip of the probe. Therefore, in order to compare the measurements of the OPTOTRAK and the FARO arm, the position of the actual tip of the probe had to be calculated from these data, in each case. The offset between the tip of the probe and the point to which raw data refer to was determined, for both devices, by a least square fit procedure (described below). This approach has been adopted because it is believed to be more robust than a direct mechanical measurement. Furthermore, the point for which the FARO arm raw data are given (situated in the probe holder) is not accessible physically, making the latter type of measurement impossible.