INSTRUMENTS FOR CHAINING
The various instruments used for the determination of the length of line by chaining are as follows:
1. Chain or tape
2. Arrows
3. Pegs
4. Ranging rods
5. Offset rods
6. Plasterer’s laths and whites
7. Plumb Bob
1) CHAIN :
Chains are formed of straight links of gal vanised mild steel wire bent into rings at the ends and joined each other by three small circular or oval wire rings.
These rings offer flexibility to the chain.
The ends of the chain are provided with brass handle at each end with swivel joint, so that the chain can be turned without twisting.
The length of a link is the distance between the centres of two consecutive middle rings, while the length of the chain is measured from the outside of one handle to the outside of the other handle.
Following are various types of chains in common use :
(i) Metric chains
(ii) Gunter’s chain or Surveyor’s chain
(iii) Engineer’s chain
(iv) Revenue chain
(v) Steel band or band chain
i) Metric chains :
After the introduction of metric units in India. the metric chains are widely used.
Metric chains are generally available in lengths of 5. 10, 20 and 30 metres.
IS : 1492-1970 covers the requirements of metric surveying chains.
To enable the reading of fractions of a chain without much difficulty, tallies are fixed at every metre length for chains of 5 m and 10 m lengths and at every five-metre length for chains of 20 m and 30 m lengths.
ii) Gunter’s Chain or Surveyor’s Chain :
A Gunter’s chain or surveyor’s chain is 66 ft. long and consists of 100 links. each link being 0.6 ft. or 7.92 inches long.
The length of 66 ft. was originally adopted for convenience in land measurement since 10 square chains are equal to 1 acre.
Also, when linear measurements are required in furlongs and miles, it is more convenient since 10 Gunter’s chains 1 furlong and 80 Gunter’s chains 1 mile.
iii) Engineer’s Chain :
The engineer’s chain is 100 ft. long and consists of 100 links, each link being ft. long.
At every 10 links, brass tags are fastened, with notches on the tags indicating the number of 10 link segments between the tag and end of the chain.
The distances measured are recorded in feet and decimals.
iv) Revenue Chain :
The revenue chain is 33 ft. long and consists of 16 links, each link being 2 ft. long.
The chain is mainly used for measuring fields in cadastral survey.
2) TAPES :
Tapes are used for more accurate measurements and are classed according to the material of which they are made, such as follows :
i) Cloth or Linen tape
ii) Metallic Tape
iii) Steel Tape
iv) Invar Tape
i) Cloth or Linen Tape :
Cloth tapes of closely woven linen, 12 to 15 mm wide varnished to resist moisture, are light and flexible and may be used for taking comparatively rough and subsidiary measurements such as offsets.
A cloth tape is commonly available in lengths of 10 metres, 20 metres, 25 metres and 30 metres, and in 33 ft., 50 ft., 66 ft. and 100 ft.
The end of the tape is provided with small brass ring whose length is included in the total length of the tape.
A cloth tape is rarely used for making accurate measurements, because of the following reasons :
(i) it is easily affected by moisture or dampness and thus shrinks ;
(ii) its length gets altered by stretching ;
(iii) it is likely to twist and tangle ;
(iv) it is not strong.
Before winding up the tape in the case, it should be cleaned and dried.
ii) Metallic Tape :
A metallic tape is made of varnished strip of waterproof linen interwoven with small brass, copper or bronze wires and does not stretch as easily as a cloth tape.
Since metallic tapes are light and flexible and are not easily broken, they are particularly useful in cross-sectioning and in some methods of topography where small errors in length of the tape are of no consequence.
Metallic tapes are made in lengths of 2, 5, 10, 20, 30 and 50 metres.
In the case of tapes of 10, 20, 30 and 50 m lengths a metal ring is attached to the outer ends and fastened to it by a metal strip of the same width as the tape.
This metal strip protects the tape, and at the same time inspector’s stamp can be put on it.
In addition to the brass ring, the outer ends of these tapes are reinforced by a strip of leather or suitable plastic material of the same width as the tape, for a length of atleast 20 cm.
Tapes of 10, 20 , 30 and 50 metre lengths are supplied in a metal or leather case fitted with a winding device.
iii) Steel Tape :
Steel tapes vary in quality and accuracy of graduation, but even a poor steel tape is generally superior to a cloth or metallic tape for most of the linear measurements that are made in surveying.
A steel tape consists of a light strip of width 6 to 10 mm and is more accurately graduated.
Steel tapes are available in lengths of 1, 2, 10, 20, 30 and 50 metres.
The tapes of 10, 20, 30 and 50 metre lengths, are provided with a brass ring at the outer end, fastened to it by a metal strip of the same width as the tape.
The length of the tape includes the metal ring. It is wound in a well-sewn leather case or a corrosion resisting metal case, having
a suitable winding device.
iv) Invar Tapes :
Invar tapes are used mainly for linear measurements of a very high degree of precision, such as measurements of base lines.
The invar tape is made of alloy of nickel (36%) and steel (64%), and has very low coefficient of thermal expansion-seldom more than about one-tenth of that of steel, and often very much less.
The coefficient of thermal expansion varies a good deal with individual bands but an average value of 0.0000005 per 1° F may be taken.
The other great advantage of invar is that bands and wires made of invar enable base lines to be measured very much more rapidly and conveniently.
Invar tapes and bands are more expensive, much softer and are more easily deformed than steel tapes.
Another great disadvantage of invar tape is that it is subjected to creep due to which it undergoes a small increase in length as time goes on.
Its coefficient of thermal expansion also goes on changing.
It is therefore, very essential to determine its length and coefficient of expansion from time to time.
Invar tapes are normally 6 mm wide and are available in lengths of 20, 30 and 100 m.
3) ARROWS :
Arrows or marking pins are made of stout steel wire, and generally, 10 arrows are supplied with a chain.
An arrow is inserted into the ground after every chain length measured on the ground.
Arrows are made of good quality hardened and tempered steel wire 4 mm (8 s.w.g.) in diameter, and are black enamelled.
The length of arrow may vary from 25 cm to 50 cm, the most common length being 40 cm.
One end of the arrow is made sharp and other end is bent into a loop or circle for facility of carrying.
4) PEGS :
Wooden pegs are used to mark the positions of the stations or terminal points of a survey line.
They are made of stout timber, generally 2.5 cm or 3 cm square and 15 cm long, tapered at the end.
They are driven in the ground with the help of a wooden hammer and kept about 4 cm projecting above the surface.
5) RANGING RODS :
Ranging rods have a length of either 2 m or 3 m, the 2 metre length being more common.
They are shod at the bottom with a heavy iron point, and are painted in alternative bands of either black and white or red and white or black, red and white in succession, each band being 20 cm deep so that on occasion the rod can be used for rough measurement of short lengths.
Ranging rods are used to range some intermediate points in the survey line.
They are circular or octagonal in cross-section of 3 cm nominal diameter, made of well-seasoned, straight grained timber.
The rods are almost invisible at a distance of about 200 metres; hence when used on long lines each rod should have a red, white or yellow flag, about 30 to 50 cm square, tied on near its top.
RANGING POLES :
Ranging poles are similar to ranging rods except that they are longer and of greater diameter and are used in case of very long lines.
Generally, they are not painted, but in all cases they are provided with a large flag.
Their length may vary from 4 to 8 metres, and diameter from 6 to 10 cm.
The foot of each pole is sunk about 0.5 m into the ground, the pole being set quite vertical by aid of a plumb bob.
6) OFFSET RODS :
An offset rod is similar to a ranging rod and has a length of 3 m.
They are round wooden rods, shod with pointed iron shoe at one end, and provided with a notch or a hook at the other.
The hook facilitates pulling and pushing the chain through hedges and other obstructions.
7) PLASTERE’S LATHS :
In open level ground, intermediate points on a line may also be lined out with straight laths, 0.5 to 1 m long, made of soft wood.
They are light both in colour and weight, and can be easily carried about and sharpened with a knife when required.
They are also very useful for ranging out a line when crossing a depression from which the forward rod is invisible, or when it is hidden by obstacles, such as hedges etc.
8) PLUMB BOB :
While chaining along sloping ground, a plumb-bob is required to transfer the points to the ground.
It is also used to make ranging poles vertical and to transfer points from a line ranger to the ground.
In addition, it is used as centering aid in theodolites, compass, plane table and a variety of other surveying instruments.