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Angular Measurement
Field Use of the
Theodolite
Taking Measurements
Errors of Construction and
Adjustment
Horizontal Collimation
Testing For Collimation
Error
Vertical Circle Index
Plate Level
Optical Plummet
Miscentring
Parallax in Telescope
Miscentring
Miscentring is the error caused by the theodolite not being correctly plumbed over the reference mark. It is caused by a combination of operator carelessness and either the optical plummet or plumb bob. It effects horizontal angles.
The diagram above shows a plan view with a theodolite set up at Station A. However, because the theodolite has not been centred properly over the station position, it is actually vertically above Point D. Angle BAC is the "correct" angle q, that is the angle observed if we were set up above the mark at Station A. Due to miscentring the angle actually read is BDC, the measured angle q'.
The following formula allows the error to be corrected:
Then for example, if e = 2mm and L = 30m, the error in the horizontal angle is approximately:
| ( 2 x 0.002 ) | x (206265) seconds = 27" |
| 30 |
These are guide figures only.
Miscentring is important in short lines.