Theory
It consist in finding the elevation of a plane of collimation (H.I.) for every return of the instrument and then obtaining the reduced level of points with reference to the respective plan of collimation.
Items required
1. Ranging rods
2. Cross staff
3. Theodolite
Diagram
Procedure
To begin with the elevation of the plan of collimation for the first set up of the level is determined by adding the back sight to the reduce level of the bench mark. The reduce level of the intermediate point and the first change point are then obtained by subtracting the staff reading taken on these points (I.S and F.S.) from the elevation of the plan of collimation (H.I.) when the instrument shifted to the second position a new plane of collimation is setup. The levels of the two planes of collimation (first and second) are corrected by means of the back sight and fore sight taken on the change point. The elevation of this plane from the second position of the level to the reduced level of the first change point. The reduced level of the successive points and the second change point are found by their staff readings from the elevation of this plane of collimation. The process is repeated until at the reduced level are worked out on completing the reducing of the levels the accuracy of the arithmetic work should be cheacked.
Result
Arithmetical check
AC = ∑ B.S ─ ∑ F.S. = Last RL ─ First RL
0.945 = 0.945
