COLLIMATION ERROR AND HOW TO CORRECT THE COLLIMATION ERROR

1. Error of Collimation:

Collimation error occurs when the collimation axis is not truly horizontal when the instrument is level. The effect is illustrated in the sketch below, where the collimation axis is tilted with respect to the horizontal by an angle a:

                                                                                                                                                       2. CORRECTION                                                                                                         This type of error can be eliminated by placing the levelling instrument at  midway between two points .due to collimation error we normally get either upward slop or downward slop