Tribrach (instrument)

In surveying science, a tribrach means an instrument attachment plate containing three thumbscrews (see theodolite). The device consists of two triangular metal plates, which are connected at their corners by thumbscrews. By turning these, it is possible to level the top plate, when the bottom plate has been mounted atop a tripod.

Both metal plates making up the tribrach have a large circular hole in the center, through which goes the attachment screw of the theodolite. When tightened, this screw firmly attaches the theodolite, placed atop the tribrach, to a flange pressing against the bottom surface of the tribrach. In this way, it is possible to mount the instrument firmly, but freely (within some inches) choose the horizontal position of attachment.

A tribrach is used to iteratively and simultaneously realize the dual requirements placed on a theodolite mounted for measurement over a benchmark: that it be "centered" and "levelled". Usually the tribrach also contains a forced centering mechanism, allowing the theodolite to be replaced by a target, optical plummet or other instrument to the same position with sub-mm precision, by just loosening and re-tightening a locking screw.