Ranging of a survey line: It is the process of locating intermediate points on the survey line when it’s longer than chain length, and it’s necessary to align intermediate points on the chain line so that the measurements are along the line.
Indirect ranging is a type of ranging possible when the ends of a line are not inter-visible as in the case when a hill ground or when the distance between the stations are so large that they are not clearly inter-visible. Consider the figure in the image attached.
Figure: Indirect ranging
Intermediate points are fixed by the process of reciprocal ranging as explained below.
Let A and B be the ends of a survey line to be measured as a rising ground between them. Two chain men with ranging rods take the positions M1 and N1 such that they are as nearly in line with A and B as they could judge the chain men at M1 could N1 and B. And the chain men at N1 could see M1 and A. First chain men at N1 directs M1 to M2 so that he comes in the line with A and N. Then the chain man at M2 directs N1 to N2 such that he comes in line with B and M2. The process is repeated so that they align each other successively directing each other until they are both finally in the line AB.
Chaining free, vision obstructed: In this type of obstacles, the ends of the lines are not inter-visible e.g. rising ground, hill or jungle intervening. Two cases may arise here;
1) Both ends may be visible from any intermediate point lying on the line such as in the case of a hill. The obstacle of this kind may easily be crossed over by reciprocal ranging and length measured by stepping method of chaining.
2) Both ends may not be visible from any intermediate point such as in the case of a jungle. The obstacle of this kind may be crossed over by “Random line method”.
Chaining obstructed, vision free: The typical obstacle of this type is a sheet of water, the width of which in the direction of measurement exceeds the length of the chain or tape. The problem consists in finding the distance between convenient points on the chain line on either side of obstacle. Two cases may arise here;
1) When the obstacle can be chained around, e.g. a pond, a thorny hedge etc.
2) When the obstacle cannot be chained around e.g. a river.
Both chaining and vision obstructed: A building is a typical example of this class of obstacles. The problem in this case consists both in prolonging the line beyond the obstacle and finding the distance across it.