OBSTACLES IN CHAIN SURVEYING

OBSTACLES IN CHAIN SURVEYING   The objects, which create obstruction or difficulty in continuing the chaining of a survey line, are called obstacles. There are many obstacles which create obstruction to the chaining or ranging, they may be pond, Hill, river, valley, building etc. Various methods are adopted to overcome these obstacles and to measure the distance, crossing obstructions.
The obstacles may be classified as follows:
1. Chaining is obstructed, but vision is free
2. Chaining is free, but vision is obstructed, and
3. Chaining and vision, both are obstructed.

CHAINING IS OBSTRUCTED, BUT VISION IS FREE:

Such a problem arises when a pond or a river comes across the chain line. The problem may be solved and chang can be continued in the following ways.
CASE 1:
When a pond create obstruction to chaining, it is possible to go around the obstruction.
If AB is the chain line, two points P and Q selected on the chain line, on either side of the obstacle. (Fig) Equal perpendiculars PR and QS are errected at Pand Q. The distance RS is measured. Then PQ = RS.

CASE II:
When a river creates obstruction to chaining, it is not possible to go round the obstruction.
If a small river comes across the chain Line. Let AB is the chain line. Two points P and Q are selected on opposite banks of the river. At Pa perpendicular PR is erected and bisected at T. A perpendicular is set out at R and a point S is so selected on it that S, T and Q are in the same straight line.
From triangles PQT and TRS, (Fig.) PQ = RS
Measure RS, and the distance PQ is obtained indirectly.

CHAINING IS FREE, BUT VISION IS OBSTRUCTED:

This type of problem comes, when a rising ground or a forest area interrupts the chain line. Here the end stations are not intervisible. There may be two cases of this type of obstacle.
Case 1: Both the ends of the chain line may be visible from intermediate points on the line. In this case, reciprocal ranging is adopted and chaining is done by stepping method (These are already discussed in Topics .
Case II: The end stations are not visible from intermediate points when a jungle comes across the chain line. In this case, the obstacle can be crossed over by using a random line.

CHAINING AND VISION, BOTH ARE OBSTRUCTED:

This type of problem arises when a building comes across the chain line.
Let AB is the chain line. Two points, P and Q are selected on it at one side of the building. Equal perpendiculars PP₁ and QQ₁ are erected through P and Q. Join P{1}*Q{1} and extend it until the building is crossed. On this extended line select two points, R{1} and S{1} Again perpendiculars R₁ R and S{1} S are erected from R{1} and S₁ such that, P*P{1} = Q*Q{1} = R*R{1} = S*S{1}
Thus, the points P, Q, R and S will lie on the same straight line AB. The distance Q{1} R{1} is measured, and it is equal to the obstructed distance QR. (Fig).