Tribrach (instrument)

The optical plummet has either a bullseye or cross-hair sight for positioning the instrument over a survey marker or ground control point.

The older tribrachs required a plumb line to allow them to be positioned vertically over a point and took longer to set up.

The process is iterative between horizontal position and top plate level, because they are interactive.

The following general process is followed: If the target over which the instrument is to be placed is roughly at the same elevation as the tripod feet, the preceding technique is applicable.

Tribrachs are precision instruments and should be checked regularly to ensure they do not become a source of error.

A tribrach with an optical plummet (the black cylinder pointing to the left lower corner of the image).
View through an optical plummet of a prism adapter.
The head of a surveyor's tripod with the screw for mounting the tribrach.
Using an optical plummet. Here the tribrach is not centered on the tripod plate as it is being aligned over the marker
Laser reflector target on tribrach and marker column