A tribrach is an adjustable mounting plate for a surveying theodolite or sighting target. For accurate surveying measurements, the tribrach must be adjusted to be perfectly level and absolutely plumb above a survey reference point. A tribrach uses an optical plumb sight for vertical orientation. Rough field conditions can jar the optical plumb sight out of adjustment, leading to inaccurate measurements by survey instruments mounted on the tribrach. Readjustment is relatively easy to do.
Set up the surveyor’s tripod and attach the tribrach to the tripod head. Lay a sheet of white paper on the floor below the middle of the tripod and secure it to the floor with the masking tape. Use the marker to draw three alignment marks on the tripod head, each 120 degrees of arc apart. Line up the tribrach’s optical plumb with the first mark on the tripod head and adjust the tribrach until it is level.
Sight through the optical plumb and have an assistant mark the spot on the paper where the sight’s cross hairs indicate plumb. Rotate the tribrach 120 degrees to line the optical plumb up with the next mark on the tripod head. Level the tribrach again and take another plumb sight, marking the plumb spot on the paper. Rotate the tribrach another 120 degrees and repeat the sighting process, again marking the plumb spot on the paper. If the optical plumb is out of adjustment, the three marks will form an equilateral triangle. Put a mark at the center of that triangle.
Adjust the plumb. Most tribrachs have two small adjusting screws on the optical plumb arm under the instrument. One screw adjusts the cross-hair holder left to right, the other to and fro. Turning the screws simultaneously in opposite directions produces diagonal movement. Use the tiny screwdrivers to move the cross-hair holder until the hairs center on the mark at the center of the triangle. Repeat the three-sight test. If the adjustment is correct, all three sights should hit on the same plumb spot.