Aquaculture Engineering

Experiment No 3 - Ranging

Practical - 1
Experiment 3 - Ranging

Ranging

While measuring the length of a ‘Survey Line’ or ‘Chain line’, the chain or tape must be stretched straight along the line joining its two terminal stations. If the length of the survey line is less than the length of the chain, there will be no difficulty in doing so. If the length of the survey line exceeds the length of the chain, intermediate points have to be established in line with the two terminal points before chaining is started. “The process of fixing or establishing such intermediate points is known as Ranging”.

Ranging Code Signals

Direct ranging by eye

Direct ranging is done when the two ends of the survey lines are intervisible. In such cases ranging can either be done by eye or through some optical instruments such as line ranger or Theodolite.

Let A and B be the two points at the ends of a survey line. One ranging rod is erected at the point ‘B’ while the surveyor stands with another ranging rod at point ‘A’ holding the rod at about half meter length. The assistant then goes with another ranging rod and establishes the rod at a point approximately in the line with AB, at a distance not greater than one chain length from ‘A’. The surveyor at ‘A’ then signals the assistant to move transverse to the chain line, till he is in line with A and B. Similarly other intermediate points can be established.

Indirect ranging or Reciprocal ranging

Indirect ranging is resorted to when both the ends of the survey line are not intervisible either due to high intervening ground or due to long distance between them. In such a case, ranging is done indirectly by selecting two intermediate points M1 and N1 very near to the chain line in such a way that from M1 both N1 and B are visible, while from N1 both M1 and A are visible.

Two surveyors station themselves at M1 and N1 with ranging rods. The person at M1 then directs the person at N1 to move to a new position N2 in the line with M1B. The person at N2 then directs the person at M1 to move to new position M2 in line with N2A. Thus the two persons are now at M2 and N2 which are near to the chain line than the positions M1 and N1. The process is repeated till the points M and N are located in such a way that the person at M finds the person at N in line with MB and the person at N finds the person at M in line with NA. After having established M and N other points can be fixed by direct ranging.