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Tribrach (instrument)
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 with sub-millimetre precision, by just loosening and re-tightening a locking handle or lever.
== Components of a tribrach ==
The device consists of two triangular metal plates connected at their corners by leveling thumbscrews, a bubble level, a locking mechanism and often an optical plummet. The centre of the bottom plate of the tribrach has a 5/8-11 UNC thread bolt hole which is used to attach the tribrach to the tripod. The top plate of the tribrach has three small holes or slots spaced at 120 degrees above leveling thumbscrews, and a locking mechanism which enables a survey instrument to be placed on the tribrach and locked in place.
Some modern total stations have a laser plummet which allows the instrument to be levelled and positioned vertically over a survey monument or ground control point. Other tribrachs have an optical plummet with either a bullseye or cross hair sight for positioning the instrument. The oldest tribrachs required a plumb line to allow them to be positioned vertically over a point and were slower to set up.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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