Abstract: A laser beam (22) is used as a spacial reference and its position is measured relative to a target device (200A, 200B, 200C) mounted on a structure (210) being monitored (FIG. 6). The target device has two edges or slots which are moved to intersect the beam such that the two intersections occur at right angles to one another at the beam for maximum accuracy. The edges may be fixed and are in such relation that one coordinate of beam position is a monotonic function of the displacement of the beam between positions at which one and the other edge intersect the beam. The other coordinate is obtained by the relative displacements for intersection of the beam from a reference position. In one embodiment (FIG. 2) the target (30) is moved linearly and the edges (40,42) are straight and mutually orthogonal. In another embodiment (FIG. 3) the target (100) is mounted for rotational displacement with arcuate intersecting edges (104, 106).