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).

Abstract: A laser beam from a source 20 is used to provide a reference in space adjacent a portion of a structure movements of which are monitored. The beam is received by a receiver 30. A number of targets 300 may be mounted to move with parts of the structure extending alongside the beam path. Target constructions (FIGS. 6-8) are described which allow for normal passage of the beam past the target but which enable the position of the beam relative to a given target to be measured by intercepting the beam. In a variation (FIG. 13) for civil engineering use, a single target is selectively mountable to one of a number of supports. Another variation (FIG. 11) uses graduated mesh targets on which the beam impinges on each mesh but is also transmitted through the mesh. Dynamic movement of a part of structure such as a bridge is measured with a single target attached to the part in question and comprising a barred reflective assembly (FIG. 3), a fibre optic array (FIG. 4) or a position sensing photo-diode (FIG. 5).