A Total Station is modern, automated and much more complicated combination of theodolite integrated with an electronic distance meter (EDM), microprocessor with an internal data storage or external data collector.
The total station is designed for measuring of slant distances, horizontal and vertical angles (earlier theodolite was used for this purpose) and elevations in topographic and geodetic works as well as for solution of application geodetic tasks.
Like every optical and electro-mechanical instrument total station does have some source of error that need to be understand and instrument must be calibrated before the instrument move to field.
Following are different types of errors in a total station:
1. Horizontal Collimation or Line of Sight Error
Horizontal collimation or line of sight error is when the line of sight is not perpendicular to the tilting axis of the instrument. This is an axial error.
Line of sight error effects the horizontal angle readings and increases with steep sightings. The error can be overcome or eliminated by observing on two faces.
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.
The catch is here if the deviation of line of sight from actual line of sight exceeds more than a desired value, the instrument must be send to service centre or manufacturer for manual calibration.
2. Tilting Axis Error or Tilt Error
Tilting axis or tilt error is the error when the axis to the total station is not perpendicular to the vertical axis or plumb line. The error effect on horizontal readings when the instrument is tilted (steep sightings) but have no effect on sightings taken when the instrument is horizontal.
Like horizontal collimation error the tilting error can be eliminated by two face measurement. Another method is to apply the measured tilting error at the time of calibration process for all readings.
If the tilt error is more than the specified error for instrument, must be send to calibration lab.
3. Vertical Collimation Error or Vertical Index Error
If the horizontal base line of angle from 0° to 180° in the vertical circle does not coincide with the vertical axis of instrument. This zero point error is present in all vertical circle readings and like the horizontal collimation error, it is eliminated by taking FL and FR readings or by determining i.
4. Compensator Index Error
This error is caused by not leveling the total station correctly and carefully. This error can’t be eliminated by taking two face (face left and face right) readings unlike the horizontal collimation error.
If the instrument is fitted with a compensator it will measure residual tilts of the instrument and will apply corrections to the horizontal and vertical angles for these.
However all compensators will have a longitudinal error l and traverse error t known as zero point errors. These are averaged using face left and face right readings but for single face readings must be determined by the calibration function of the total station.
Reference: Total Station Lecture Series by Trinity College Dublin