METHODS OF RANGING

While measuring the length of a survey line, the chain or tape must be stretched straight along the line joining two terminal stations.

If the length of the line is less then the length of the chain, there will be no difficulty.

If the length of the line exceeds the length of the chain, some intermediate point will have to be established in line with the two terminal points before chaining is started.

The process of establishing intermediate point on a straight line between two end points is known as ranging.

TWO METHODS:1. Direct Ranging2. Indirect Ranging

RANGING

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 instrument such as line ranger or a theodolite.

1. DIRECT RANGING

1. DIRECT RANGING

Let A and B be the two points at the end of a survey line.

First, fix the ranging rod at known point A and B and its ranging rod should be fixed at point A up to completion of work.

Third ranging rod must be established at point P (or any) approximately on the line of point AB (by judgment) and it’s not greater than one chain length from point A.

Ranging by Eye:

Measure the distance of AP by chain and move ranging rod at point P to its next position and establishing a wooden peg or arrow at point P.

Third ranging is established at point Q (or any) approximately on the line of point AB (by judgment) and it’s not greater than one chain length from point P.

Measure the distance of PQ by chain and move ranging rod at point Q to its next position and establishing a wooden peg or arrow at point Q.

Its procedure repeats up to reaching point B.

Ranging by Eye:

Third ranging rod is established at known point C (or at known object) and a ranging rod should be fixed at point C up to completion of work.

Ranging by Eye:

A line ranger consists of either two plane mirrors or two right angled isoceles prisms placed one above the other.

The diagonals of the two prisms are silvered so as to reflect the incident rays.

A handle with a hook is provided at the bottom to hold the instrument in hand to transfer the point to the ground with the help of a plumb bob.

Line Ranger

Line Ranger

To range a point P, two ranging rods are fixed at the ends A and B, and the surveyor holds the line ranger very near to the line AB (by eye adjustment).

One of the prism receives the rays from A and the other prism receives the rays from B which are reflected towards the observer.

Ranging by Line Ranger

Thus the observer views the images of ranging rods at rods at A and B, which may not be in the same vertical line.

The surveyor moves the instrument sideways till the two images are in the same vertical line.

Direct and Indirect Ranging

Indirect Ranging or Reciprocal ranging is followed 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 (by judgement) in such a way that from M1, both N1 and B are visible and from N1, both M1 and A are visible.

Indirect Ranging

The person at M1 then directs the person at N1 to move to new position N2, so that the person at M1 can see the point N2 in the line M1B.

The person at N2 then directs the person at M1 to move to a new position M2 in line with N2A.

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.

Indirect Ranging

Indirect Ranging