page_title: Temporary adjustments of theodolites
section_title: Centering
hierarchical_section_title: Temporary adjustments of theodolites / Centering
caption_attribution_description: English: Tribrach with optical plummet for surveying instruments like for example total stations and more modern theodolites. The eyepice for the optical plummet is the black cylinder pointing to the lower left corner of the image. Svenska: Trefot med optiskt lod för geodetiska instrument som till exempel totalstationer och modernare teodoliter. Det optiska lodets okular är den svarta cylinder som pekar mot bildens nedre vänstra hörn.
context_page_description: Temporary adjustments are a set of operations which are performed on a theodolite to make it ready for taking observations. These include its initial setting up on a tripod or other stand, centering, levelling up and focusing of eyepiece.
context_section_description: Centering means bringing the vertical axis of the theodolite exactly over the station mark.
   Exact centering is done by using the shifting head of the instrument. During this, first the screw-clamping ring of the sliding head is loosened and the upper plate of the shifting head is slid over the lower one until the plumb bob is exactly over the station mark. After the exact centering, the screw clamping ring is tightened. This can be done by means of a forced centering plate or tribrach. An optical or laser plummet is normally used for the most accurate setting. The centering and levelling of the instrument is interactive and iterative; a re-levelling may change the centering, so error each is eliminated in successively until negligible.
caption: A tribrach with an optical plummet (the black cylinder pointing to the left lower corner of the image)
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