Indirect or Reciprocal Levelling

If the ranging rod at B is not visible from station A, Due to intervening ground, reciprocal ranging may be resorted. Given figure shows this scheme of ranging. It require 2 assistants one at point M and another at point N, where from those points station A and station B both are visible. It requires one surveyor at A and another at B. To start with M and N are around selected, say M1 and N1. Then surveyor near end A ranges person near M to position M2 like  AM2N1 are in a line. Then surveyor at B directs person at N, to move to N2 like BN2M2 are in a line. The procedure is repeated until AMNB are in a line.

                                       (b) Plan view

.                                             Reciprocal ranging