Reciprocal Ranging:

The vision ranging and line ranger can be adopted only when the end stations are inter-visible. However, in many real life situations, the line of sight between two stations is obstructed by natural or man- made objects (Figure 9), or they being too far apart to be clearly visible. Under such conditions, indirect or reciprocal ranging is resorted to. In this method, two intermediate points C₁ and D₁ are selected which are not along the line of sight AB (survey line). Stations C₁ and D₁ are approximately in line such that line C₁ D₁ is approximately paralleled to AB. C₁ is so situated that both D₁ and B are visible from it, while from D₁ both A and C₁ can be sighted.

The surveyors are situated at C₁ and D₁ originally. Surveyor at C₁ directs surveyor at D1 to move such that he is aligned in direction C₁ B to occupy new position at D₂. Next surveyor at D₂ directs the surveyor at C₁ to align along line AD₂ to occupy new position C₂. This process of alignment and realignment continues till both the surveyors occupy positions C and D which are situated along line AB ensuring that survey line is aligned along ACDB as shown in Figure.

Figure: Indirect or Reciprocal Ranging