Obstacles in Ranging:

Topography of land can create difficulties in ranging, i.e. ground undulations hindering clear visibility among main stations, e.g. existence of a hillock in the line of sight. These problems of obstacles in ranging can be solved by establishing mutually visible intermediate stations by reciprocal ranging as explained further.

If the two ends of a survey line A and B are not show from intermediate point on it, then reciprocal ranging cannot solve a problem. In such cases, a random line AB₁ is drawn in any convenient direction but as close to point B as possible (Figure 13). The point B₁ is selected such in which it is visible from B and BB₁ is perpendicular to the random line. Measure BB₁, select points C₁ and D₁ on the random line and erect perpendicular C₁C and D₁D on it. The value of C₁C and D₁D can be calculated as

C₁C = (AC₁/AB₁) × BB₁  and D₁ D = (AD₁/AB₁) × BB₁

After getting points C and D, join CD and prolong it.

Figure: Ranging when Ends not Inter-visible