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

Horizontal Collimation Error

The effect of collimation error is similar to that in a level, the line of sight produced by the cross hairs is not parallel with the optical axis of the telescope.

Unlike a level where only horizontal angles are read, in a theodolite this will cause systematic errors in the horizontal as well as vertical directions. It is caused by the cross hair not being on the mechanical centre of the telescope, that is not on the horizontal and vertical axes of the instrument. The image above shows a cross section through a theodolite telescope. The reticule is shown in red, this is the glass disc upon which the cross-hairs are etched. There are procedures for detecting this error but it is eliminated during the observation process by observing both face left and face right directions.

There are two possible ways of looking at each target:

Face Left is when the vertical circle is situated on the left hand side of the observer's face.

Face Right is when the vertical circle is situated on the right hand side of the observer's face.

Collimation error will cause the line of sight to fall to one side of the telescope axis on one face, and it will cause the error to fall on the other side on the other face. The mean of these two observations will mathematically cancel the effect of the error.

In effect, most of the common errors of adjustment and construction can be eliminated by observing angles on both face left and face right.

Therefore all angles are always observed on both faces!