Concept: An obstacle in Chaining: It is desirable to select stations so as to avoid obstacles, occasionally the obstacles are unavoidable. Various obstacles to chaining may be grouped into: ● Obstacles to ranging (chaining free-vision obstructed) ● Obstacles to chaining (chaining obstructed-vision free) ● Obstacles to both ranging and chaining (a) Obstacles to ranging: ● These obstacles can be further classified into the following categories: ◎ Both ends of the line are visible from some intermediate points. The intervening ground is an example of such an obstacle. By resorting to reciprocal ranging this difficulty can be overcome. ◎ Both ends of the line may not be visible from intermediate points on the line but maybe visible from a point slightly away from the line. Intervening trees and bushes are examples of such obstacles. ● A hill is an obstacle to ranging but not chaining. (b) Obstacles to chaining: ● Cases often occur in the field where the distance between two points is required, but direct chaining from one point to the other is impossible because of some sort of obstacle. ● A river is an obstacle to chaining but not ranging. (c) Obstacles to both ranging and chaining ● The building is a typical example of this obstacle.