SOURCES OF ERROR FOR TOTAL STATIONS
1 CALIBRATION OF TOTAL STATIONS
To maintain the high level of accuracy offered by modern total stations, there is now much more emphasis on monitoring instrumental errors, and with this in mind, some construction sites require all instruments to be checked on a regular basis using procedures outlined in the quality manuals.
Some instrumental errors are eliminated by observing on two faces of the total station and averaging, but because one face measurements are the preferred method on site, it is important to determine the magnitude of instrumental errors and correct for them.
For total stations, instrumental errors are measured and corrected using electronic calibration procedures that are carried out at any time and can be applied to the instrument on site. These are preferred to the mechanical adjustments that used to be done in labs by technician.
Since calibration parameters can change because of mechanical shock, temperature changes and rough handling of what is a high-precision instrument, an electronic calibration should be carried our on a total station as follows:
Before using the instrument for the first time
After long storage periods
After rough or long transportation
After long periods of work
Following big changes in temperature
Regularly for precision surveys
Before each calibration, it is essential to allow the total station enough to reach the ambient temperature.
2 HORIZONTAL COLLIMATION (OR LINE OF SIGHT ERROR)
This axial error is caused when the line of sight is not perpendicular to the tilting axis. It affects all horizontal circle readings and increases with steep sightings, but this is eliminated by observing on two faces. For single face measurements,
an on-board calibration function is used to determine c, the deviation between the actual line of sight and a line perpendicular to the tilting axis. A correction is then applied automatically for this to all horizontal circle readings.
3 TILTING AXIS ERROR
This axial errors occur when the titling axis of the total station is not perpendicular to its vertical axis. This has no effect on sightings taken when the telescope is horizontal, but introduces errors into horizontal circle readings when the
telescope is tilted, especially for steep sightings. As with horizontal collimation error, this error is eliminated by two face measurements, or the tilting axis error a is measured in a calibration procedure and a correction applied for this to all horizontal circle readings – as before if a is too big, the instrument should be returned to the manufacture.
4 COMPENSATOR INDEX ERROR
Errors caused by not levellinga theodolite or total station carefully cannot be eliminated by taking face left and face right readings. If the total station 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.
A vertical collimation error exists on a total station if the 0o to 180o line in the vertical circle does not coincide with its vertical axis. 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
For all of the above total station errors (horizontal and vertical collimation, tilting axis and compensator) the total station is calibrated using an in built function. Here the function is activated and a measurement to a target is taken as shown below.
Following the first measurement the total station and the telescope are each rotated through 180o and the reading is repeated.
Any difference between the measured horizontal and vertical angles is then quantified as an instrumental error and applied to all subsequent readings automatically. The total station is thus calibrated and the procedure is the same for all of the above error type.
Fig 3.9 Compensator Index Error