1. A method of datuming an analogue measurement probe in position determining apparatus, the probe having an output indicative of a distance of a surface of an object from the probe along a measurement axis of the probe when the distance is within a measuring range of the probe, the apparatus having means for moving the probe relative to the object and position determining means for taking a reading of the position of the probe with respect to at least one measurement axis of the apparatus, the method comprising: (a) providing a test object having a datum point; (b) moving the probe into a plurality of surface-sensing positions with respect to the test object and using the position determining means to take a first set and a second set of said readings, the readings within each set being in respect of different surface positions of the test object and being sufficient to make a determination from that set of readings of the position of said datum point of the test object, each reading of the first set being taken when the probe output indicates that the probe is substantially at a first point within its measuring range and each reading of the second set being taken when the probe output indicates that the probe is substantially at a second point within its measuring range; (c) making a first determination of the position of said datum point of the test object from the first set of readings and a second determination thereof from the second set of readings; and (d) deriving from the first and second determinations of the position of said datum point a correction factor for use in correcting the probe output for any angular deviation between the measurement axis of the probe and the measurement axis of the apparatus.