TERMS USED IN DIRECT METHOD OF LEVELLING

In direct method of levelling following terms are used:

(i) Plane of Collimation: with respect to the datum selected it is the reduced level of plane of sight. It is also called as 'height of instrument'. It should not be confused from the ground with the height of telescope where the instrument is set.

(ii) Back Sight (BS): with an intension of determining the plane of collimation it is the sight taken on a level staff held on the point of known elevation. Always in a place it is the first reading after the instrument is set. It is also called as plus sight, as this reading is to be added to RL of the point (Benchmark or change point) to get plane of collimation.

(iii) Intermediate Sight (IS): Sights taken on staff after back sight (first sight) and before the last sight (fore sight) are called as intermediate sights. Intension of taking these readings is to determine the reduced levels of the points where staff is held. As the IS reading, these sights are called a as 'minus sights' is to be subtracted from plane of collimation to get RL of the point where staff is held.

(iv) Fore Sight (FS): before shifting it or just before ending the work this is the last reading taken from the instrument station. It is also a minus sight.

(v) Change Point (CP): This is also called as turning point (TP). It is a point on which fore sights and back sights both are taken. After taking fore sight on this particular point instrument is set at some other suitable point and back sight is taken on the staff held at the similar point. In establishing the new plane of collimation the two readings help with respect to the earlier datum. Since between taking the two sights on the change point, there is time gap it is advisable to choose change point on a well defined point.