Abstract: A system enables indirect determination of a position vector (VR) of a point position (P). The system uses two fixed trackers (10 and 10?) whose absolute positions are known. A movable measuring device (20) provides a rigid rod (25) supporting a pair of reflectors (30 and 30?) which are mounted at fixed positions. A reference point (R) is mounted at a further fixed position on the rod (25) and is on a straight line (L) through the reflectors (30 and 30?). Light beams from the trackers (10 and 10?) acquire the reflectors (30 and 30?) so that when the reference point (R) is positioned at the point position (P), the position vector (VR) of point position (P) is determinate by vector addition.