Tribrach (instrument)

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

• The tribrach is attached to the tripod and placed over the marker.
• Looking through the optical plummet pick up two of the tripod legs and position the cross-hairs over the marker. Tread all three legs into the grass, bitumen, concrete.
• Look through the optical plummet and using the thumbscrews put the cross-hairs back over the marker.
• Adjust the level of the tribrach by slackening the tripod screws and sliding the legs. Re-center the cross-hairs using the thumb screws, then fine-tune the level using the tripod legs. Once the tribrach is nearly centered over the marker, level the tribrach the last little bit using the thumbscrews.
• Slacken the tribrach from the tripod head and slide it the last 4-5mm so that it is centered over the marker and is level.

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. If there is a considerable difference between the elevation of the target and the elevation of the marker and the instrument bottom plate, such as when the target is in a deep hole, or the tripod is on a Bilby tower, the target will appear to move laterally by a large distance in the optical plummet as the tribrach level is adjusted.

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

• Gerard L'E. Turner, Nineteenth Century Scientific Instruments, Sotheby Publications, 1983, ISBN 0-85667-170-3
• Gerard L'E. Turner, Antique Scientific Instruments, Blandford Press Ltd. 1980, ISBN 0-7137-1068-3
• US 5501018, Sonny J. Porter, "Tribrach device", published March 26, 1996 

• Tribrachs and other accessories to Wild theodolites.