Ranging is the process of establishing many intermediate points to measure the survey lines in linear measurement. This can be done by ranging rods, offset rods, and ranging poles. The first step in any construction work is Surveying. Ranging is one of the important aspects of Surveying.
When the length of the surveying line is longer than the length of the measuring chain, the line can be measured by using intermediate points along with it by ranging.
Instruments Used
Ranging can be done by using the following instruments:
Ranging rods
Offset rods
Ranging poles
Ranging rods are vertical rods used to mark survey points having 2-3 m height. it is painted with bands of red and white 20 cm each.
Types of Ranging
Ranging can be of two types
Direct ranging
Indirect ranging
Let us in detail learn about these types.
Direct ranging
Direct ranging is adopted when the two survey stations are inter-visible i.e. the two endpoints of the survey line are visible. It can be further of two types
Ranging by eye: It is done with the help of the eyes. For example, A and B are the two inter-visible points at the survey end line. Surveyor stands at point A concerning point B. Then, another person takes the ranging rod and fixes or establishes an intermediate point C between the line AB. The surveyor at point A then signals another person at the intermediate points so that the ranging rod is in perfect line with the endpoints A and B. Hence, the intermediate points are determined.
Ranging by line ranger: Here intermediate points are fixed directly utilizing an instrument known as line ranger. The major advantage of this method over the eye judgment method is that this method can be conducted easily by one person. The accuracy achieved by this method is also higher.
Indirect Ranging
It is also called reciprocal ranging. It is used when the two endpoints of the survey line are either not inter-visible or the two points are at a very long distance. For instance, let A and B be the points that are not visible. Now following steps are carried out
A1 and B1 are considered such that A and B1 are visible from A1 and A1 and B are visible from B1.
A1 is shifted to a position A2 such that A, A2, and B1 are in the same line
Similarly, B1 is shifted to B2 such that A2, B2, and B are in the same line
The process is repeated until all the points are in the same line and hence intermediate points are located.
Errors in Ranging
There can be errors in ranging while surveying. If the chain is stretched out of the line, the measured distance will always be more and hence the error will be positive. For each and every stretch of the chain, the error due to bad ranging will be cumulative and the effect will be too great a result.
This can be corrected by using the following correction:
If AB is supposed to be measured but AC has been measured (AB<AC), negative correction is applied.
C=h²/2l (or) C= l(1-cosθ)
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