Question

Define the horizontal collimation?

Answer 1

Horizontal collimation is also known as line of sight error. It occurs when line of sight is not perpendicular to the tilting axis of the instrument.

This is an axial error which occurs in total station. Total station is a device used in surveying by replacing total station.

Horizontal collimation effects the horizontal angle readings and increases with steep sightings.

• How to test the horizontal collimation error-

To test for horizontal collimation error, point to a target in face one then point back to the same target in face two; the difference in horizontal circle readings should be 180 degrees.

• How to eliminate this type of error-

This type of error is eliminated by observing on two faces.

Horizontal collimation error can always be corrected for meaning the face one and face two pointings of the instrument as under

1. Most electronic theodolites have a method to provide a field adjustment for horizontal collimation error. Again, the manual for each instrument provides detailed instructions on the use of this correction.
2. In some instruments, the correction stored for horizontal collimation error can affect only measurements on one side of the circle at a time. Therefore when the telescope is passed through zenith (other side of the circle is being read), the horizontal circle reading will change by twice the collimation error. These instruments are functioning exactly as designed when this happens.
3. When prolonging a line with an electronic theodolite, the instrument operator should either turn a 180 degree angle or plunge the telescope and turn the horizontal tangents so that the horizontal circle reading is the same as it was before plunging the telescope.

• For single face measurements, an on-board calibration function is used to determine the deviation (c) of actual line of sight and deviated line of sight. The on-board software then apply a correction for each measured horizontal angles reading automatically. It means if the deviation of line of sight from actual line of sight exceeds more than a desired value, the instrument must be send back to service centre or manufacturer for manual calibration.