In measuring the length of a survey line called chain line, it is necessary that the chain should be laid out on the ground in a straight line between the end stations.
“The process of establishing intermediate point on a straight line between two end points is known as ranging”
Ranging must be done before a survey line is chained. It may be necessary to establish a number of intermediate points prior to chaining when chain line is much longer. Ranging may be done by direct observation by the naked eye or by line ranger or by Theodolite. Generally, ranging is done by naked eye with the help of three ranging rods.
Ranging is of two kinds:
Indirect or reciprocal ranging
1. Direct Ranging:
When intermediate ranging rods are fixed on a straight line by direct observation from end stations, the process is known as direct ranging. Direct ranging is possible when the end stations are intervisible.
Assume that A and B two end stations of chain line, where two ranging rods are already fixed. Suppose it is required to fix a ranging rod at the intermediate point P on the chain line in such a way that the points A, P & B are in same straight line. The surveyor stands about two meters behind the ranging rod at A by looking towards line AB. The assistant holds ranging rod at P vertically at arms length the rod should be held tightly by the thumb and forefinger. Now the surveyor direct the assistant to move the ranging rod to the left or right until the three ranging rods come exactly the same straight line. The ranging will be perfect, when the three ranging rods coincide and appear as a single rod. When the surveyor is satisfied that the ranging is prefect, he signals the assistant to fix the ranging rod on the ground. By following the same procedure, the other ranging rods may be fixed on the line.
2. Indirect or Reciprocal Ranging:
Indirect ranging is used when the end stations are not intervisible due to high ground or a hill or if the ends are too long. In such cases, intermediate points can be fixed on the survey line by a process known as reciprocal ranging.
Let A & B be the two stations with rising ground or a hill. Let two chainmen with ranging rods take up positions at M and P, such that, chainmen at M1 can see both rods at P1 and B and the chainmen at P1 can see the ranging rods at M1 and A. The chainmen at P1 directs the chainmen at M1 to shift the ranging rod at M2 in line with A and then chainman at M2 directs the chainmen at P1 to shift the ranging rod to P2 in line with B, by successively directing each other to be in line with the end points. Their positions will be changed until finally they are both in line with A & B exactly on line AB. Now the four ranging rods at A M P & B are on same straight line. This method may also be used in ranging a line across a valley or river.
Measurements of Areas:
There are two methods of measuring areas.
1. in Triangulation:
The lines of survey form a network of triangles. (It is the systems of surveying in which the area is divided into simple geometrical figures and the work is carried out by taking its measurements.)
2. A Traversing:
It is one in which the frame work consists of a series of connected lines, the length and directions of which are measured by chain or tape and angular instruments respectively.