Methods of Ranging in Chain Surveying | Guide to Surveying and Levelling
In measuring a survey line, the chain has to be laid out on the ground between the stations.
If the line is short, the chain could be put in alignment easily but if it is long or the end station is not clearly visible, then intermediate points has to be established in line with end points to know the directions of the line by ranging.
Types of Ranging
There are two types of ranging:
- Direct ranging
- Indirect ranging
Direct ranging is possible when the stations are intervisible.
Ranging is done by eye-judgement. Ranging rods are erected vertically beyond each end of survey line.
The surveyor stands 2m beyond the ranging rod while the assistant folds the ranging rod vertically in the intermediate stations.
The ranging rod is held roughly in line by the thumb and fore-finger.
The surveyor directs the assistant to move the rod to the left or right until the three ranging rods appear to be in a straight line.
To avoid errors due to the ranging rods not being vertical, the lower end of the rod are cited for alignment.
1. Indirect Ranging is 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.
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.