direct and indirect ranging during chain surveying

  

 Ranging : In measuring the length of a survey line, it is necessary

that the chain should be laid on the ground in a straight line between the end

stations. If the line is short, it is easy to put the chain in true alignment. But if the

line is long, it is necessary to place intermediate ranging rods to maintain the

direction. Fixing of intermediate points in a straight line between the two end

stations is known as ranging.

Ranging is of two types: a) Direct ranging b) Indirect ranging.

(a)  Direct Ranging

Direct ranging is adopted when the two end stations are mutually visible.

The ranging is carried out by an eye or a line ranger.

Ranging by Eye

The ranging by eye is done by the following steps.

1) Fix ranging rods at each end of the line.

2) Stand about 1.5m beyond the first ranging rod.

3) Direct the assistant to hold the ranging rod vertically where the intermediate

point to be fixed.

4) Direct the assistant to move left or right using code of signals until the

three ranging rods are in straight line.

5) Check the verticality of the rods by sighting the lower ends of the rods.

6) As and when the intermediate point is in straight line, signal the assistant

to fix the ranging rod.

The following code of signals may be used in directing the assistant into

line.

Rapid sweep with right hand   – move rapidly to the right.

Rapid sweep with left hand     – move rapidly to the left.

Slow sweep with right hand    – move slowly to the right.

Slow sweep with left hand      – move slowly to the left.

Right arm extended               – move continuously to right.

Left arm extended                           – move continuously to left.

Right arm up and moved to the right – plumb the rod to the right.

left arm up and moved to the left      – plumb the rod to the left.

Both hands up and brought down     – correct.

Both arms extended forward

horizontally and the hands depressed  – fix the ranging rod in position.

briskly.

Ranging by Line Ranger

Line ranger is an optical instrument used for fixing intermediate points on a

chain line. It consists of two right angled isosceles triangular prisms placed one

above the other.

For fixing an intermediate station P on the line AB, the observer stands as

near P as possible and holds the instrument at his eye level. Rays of light coming

from the ranging rods at A and B are reflected by the upper and lower prisms

respectively and reach the eye. If the images of A and B are in separate lines as

shown in fig (b), the observer moves a little perpendicular to AB such that both

images will be in the same line as in fig(c).

The required position of P will be then exactly below the center of the

instrument. One of two prisms can be adjusted by a screw. To test the instrument

it is held at the mid point of a line and the ranging rods at the end station observed.

If both rods appear in the same line, the instrument is in adjustment. Otherwise,

the fixing screw of the movable prism is slackened and the prism slightly rotated

so that both ranging rods appear in one line. Then the prism is fixed by tightening

the fixing screw.

Indirect Ranging

Indirect ranging is adopted when the ends of the line are not mutually visible

due to high intervening ground or the distance is too long. The process is also

known as reciprocal ranging.

Let A and B are the ends of a chain line which has a rising ground intervening  between them. Two chainmen with ranging rods take the position M such that they are as nearly is line with A and B as they could judge and such that the chainman at M1 could see N1 and B and chainman at N1 can see M and A. First chainmen at N1 direct M1 to M2 so that he comes in line with A and N1 Then the chainman at M2directs N1 to N2 such that he comes in line with B and M1

This process is repeated so that they align each other successively directing each other until they are in the line AB.