Collimation Method

As explained earlier, the height of instrument (HI), e.g. the height of line of collimation above BM (station of known level) at every instrument station is determined through adding the backsight of BM station to decreased level of BM. From this height of instrument at a particular instrument station, decreased levels of all the station points on ground are computed through substracting foresight of that particular station from HI, i.e.

HI of instrument = RL of Bench mark + BS of BM

RL of intermediate point = HI - FS at intermediate station

= HI - IS

While the instrument is shifted to its second position, height of instrument at new set up station is needed to be determined. This is achieved through correlating the levels of two collimation planes (first and second position) through foresight of change point from first setup station and backsight of same change point from second setup station, as given below:

RL of change point C = RL of A + BS at A - FS at C

HI (at second station O₂) = RL of C + BS at C

With instrument set up at second station (say O₂), staff readings at new system of intermediate stations are taken before shifting the instrument at next set up station (O₃). This process is continuously repeated till the levelling exercise is done, and all the required decreased levels are acquired.

A check could be applied on the mathematical correctness of calculation of decrease levels through collimation method as follows. The difference between the first reduced level (at starting station) and last reduced level (at end station) must be equal to the difference between summation of all foresights at change points and the summation of all backsights at change points.