Chain Surveying – Instruments used in Chaining

Chain survey is suitable when Area to be surveyed is comparatively small and the Ground is fairly level. Chain survey can also be used when the Area is open and Details to be filled up are simple and less. In chain surveying only linear measurements are made i.e. no angular measurements are made.

Since triangle is the only figure that can be plotted with measurement of sides only, in chain surveying the area to be surveyed should be covered with a network of triangles. scheme of covering an area with a network of triangles. No angle of the network triangles should be less than 30º to precisely get plotted position of a station with respect to already plotted positions of other station. As far as possible angles should be close to 60º. However, the arrangements of triangles to be adopted depends on the shape, topography, natural and artificial obstacles in the field.

INSTRUMENTS USED IN CHAINING

The following instruments are required for measurements with chain and tape:

[1] ARROWS

When the length of the line to be measured is more than a chain length, there is need to mark the end of the chain length. Arrows are used for this purpose. Arrows are made up of 4 mm diameter steel wire with one end sharpened and other end bent into a loop. Length of an arrow is approximately 400 mm.

[2] PEGS

Wooden pegs are used in measuring a length of a line to mark the endpoints of the line. The pegs are made of hard wood of 25 mm × 25 mm section, 150 mm long with one end tapered When driven in ground to mark station points they project about 40 mm.

[3] RANGING RODS AND RANGING CHAINS

For ranging intermediate points along the line to be measured, ranging rods and ranging poles are used. Ranging rods are 2 to 3 m long and are made of hardwood. They are provided with iron shoe at one end They are usually circular in section with 30 mm diameter and are painted with 200 mm colour bands of red and white or with black and white. If distance is more than 200 m, for clear visibility they may be provided with multicoloured flags at their top.

The ranging rods are occasionally used to measure short distances since they are painted with alternate colour of band 200 mm. Ranging poles are similar to ranging rods except that they are longer. Their length varies from 4 m to 8 m and diameter from 60mm to 100 mm. They are made of hard wood or steel. They are fixed in the ground by making 0.5 m holes and then packed to keep them vertical.

[4] OFFSET RODS

These rods are also similar to ranging rods and they are 3 m long. They are made up of hardwood and are provided with iron shoe at one end. A hook or a notch is provided at other end. At height of eye, two narrow slits at right angles to each other are also provided for using it for setting right angles.

[5] LATHS

Laths are 0.5 to 1.0 m long sticks of soft wood. They are sharpened at one end and are painted with white or light colours. They are used as intermediate points while ranging or while crossing depressions.

[6] WHITES

Whites are the pieces of sharpened thick sticks cut from the nearest place in the field. One end of the stick is sharpened and the other end is split. White papers are inserted in the split to improve the visibility. Whites are also used for the same purpose as laths.

[7] PLUMB BOB

A typical plumb bob is used in measuring horizontal distances along sloping ground plumb bobs are used to transfer the position to ground. They are also used to check the verticality of ranging poles.

[8] LINE RANGER

It is an optical instrument used for locating a point on a line and hence useful for ranging. It consists of two isosceles prisms placed one over the other and fixed in an instrument with handle. The diagonals of the prisms are silvered so as to reflect the rays. To locate point C on line AB the surveyor holds the instrument in hand and stands near the approximate position of C. If he is not exactly on line AB, the ranging rods at A and B appear separated The surveyor moves to and fro at right angles to the line AB till the images of ranging rods at A and B appear in a single line . It happens only when the optical square is exactly on line AB. Thus the desired point C is located on the line AB. Its advantage is it needs only one person to range. The instrument should be occasionally tested by marking three points in a line and standing on middle point observing the coincidence of the ranging rods. If the images of the two ranging rods do not appear in the same line, one of the prism is adjusted by operating the screw provided for it.