Metrology apparatus

A metrology apparatus comprising an indexed articulated joint comprising:

• first and second relatively reorientable bodies, respectively having mutually engageable engagement elements, which can be locked together in a plurality of different angular orientations about a first axis so as to provide for a plurality of angularly indexed positions of the first and second bodies;
• at least one verification sensor configured to provide a measure of the relative spatial configuration of first and second bodies when in their locked state, and
• wherein the apparatus is configured such that in the event of the first and second bodies locking together at an indexed position, the verification sensor is used to measure the relative spatial configuration of the first and second bodies, and wherein information obtained from said measure is compared to calibration information which was obtained from at least one other measure of the relative spatial configuration of the first and second bodies when the first and second bodies were locked at said indexed position at an earlier point in time, in order to establish information about the state of engagement of the first and second bodies.