page_title: Tribrach (instrument)
section_title: Using a tribrach
hierarchical_section_title: Tribrach (instrument) / Using a tribrach
caption_attribution_description: Čeština: Optický centrovač VEB Carl Zeiss Jena Deutsch: Optisches Lot VEB Carl Zeiss Jena English: Optical plummet VEB Carl Zeiss Jena
context_page_description: A tribrach is an attachment plate used to attach a surveying instrument, for example a theodolite, total station, GNSS antenna or target to a tripod. A tribrach allows the survey instrument to be repeatedly placed in the same position over a surveying marker point with sub-millimetre precision, by loosening and re-tightening a lock to adjust the instrument base in a horizontal plane.
context_section_description: 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 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.
caption: Using an optical plummet. Here the tribrach is not central on the tripod plate as it is being aligned over the marker
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