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Angular Measurement
Field Use of the
Theodolite
Taking Measurements
Errors of Construction and
Adjustment
Collimation Error
Horizontal Collimation
Vertical Circle Index
Plate Level
Optical Plummet
Miscentring
Parallax in Telescope
Optical Plummet
As shown in Setting Up A Theodolite the vertical axis of the theodolite must pass through the ground (station position) mark. Theodolites are centred using a plumb bob in conjunction with the optical plummet.
The above image shows the position of the optical plummet eyepiece on a theodolite and a cross section through the base of a Kern DKM2-A theodolite
The position of the optical plummet depends upon the type of theodolite being used. In some theodolites the optical plummet is found above the footscrews in the alidade. In other theodolites it is found below the footscrews in the tribrach, as shown in the example above. The testing and adjustment of the optical plummet depends its position in the theodolite. If the optical plumment is found in the alidade, the testing and adjustment of it is easy. If found in the tribrach, it is a more difficult process.
The processes used to test and adjust the optical plummet are described in more detail in Modern Theodolites & Levels by M.A.R. Cooper.
Does the student need to know this information or only that the optical plummet exists? is this important? What is the intent in discussing it here. Does the adjustment of optical plummet ensure that miscentring does not occur??? If so the page on miscentring should follow this page and connection should be made within the notes.