The following instruments are used while chaining: 1. Chain 2. Arrows 3. Tapes 4. Ranging Rods 5. Offset Rod 6. Pegs.
Instrument # 1. Chain:
It is composed of a fixed number of pieces of galvanised mild steel wire 4 mm (8 s.w.g.) in diameter known as links. The ends of each link are bent into a loop and connected together by means of three oval or circular rings which provide flexibility to the chain and make it less liable to become kinked.
Both ends of the chain are provided with brass handles in swivel joint so that the chain can be turned round without twisting. The chain length is the distance from outside surface of one handle to outside surface of the other. To facilitate counting the number of links, brass tags or tallies are provided at various distinctive points of the chain.
Kinds of Chains:
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There are two main kinds of surveying chains viz:
(1) Metric surveying chains, and
(2) Non-metric surveying chains.
The 20m and 30 m surveying chains have been prescribed by I.S.I, for use on all engineering surveys.
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1. Metric Surveying Chains (Figs. 2.1, 2.2 & 2.3):
The metric surveying chain is 20 or 30 metres in length each metre length is divided into 5 links, each being 20 cm long. To facilitate counting the number of links, tallies are provided at every 5 metre length and small brass rings at every metre length except where tallies are provided (Figs. 2.1& 2.2).
The tallies are of different shapes depending upon their position in the chain (See figs. 2.1 & 2.2) and are marked with letters “मी” and “m” as shown in fig. 2.3. To facilitate holding the arrows in position with the handle of the chain, a groove is cut on the outside surface of the handle of the chain as in fig. 2.3. The full length of the chain, 20 m or 30 m as the case may be, is marked over the handles.
Steel Band:
It is also called the band chain and is used for accurate survey work. It is 20 or 30 m long and is made from ribbon of steel 16 mm wide. The brass handles in swivel joint are provided at the ends. It is wound either upon an open steel cross (fig. 2.4) or on a metal reel in a closed case (fig. 2.5). The graduations are marked either by etching with acid or by brass rivets.
Comparison between Chains Composed of Link and Band Chain (Steel Band):
The following are the point of comparison between the two:
I. Chain composed of Links:
1. It can withstand rough treatment.
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2. It can be easily repaired with a hammer in the field
3. It can be easily read.
4. It is heavier and sags considerably when suspended.
5. It is easily liable to increase or decrease in length due to continuous use and bending of links respectively.
6. It is heavier and takes more time to spread.
II. Band Chain (Steel Band):
1. If carelessly used, it gets broken.
2. It can be repaired only by riveting or soldering which is a difficult process.
3. It cannot be read so easily.
4. It is lighter and does not sag to the extent to which chain sags.
5. It maintains length very much better than the chain.
6. For the same length, it is lighter time than chain and easier to open.
2. Non-Metric Surveying Chains (Fig. 2.6):
Chains used in countries where foot is the unit of linear measurement are:
(i) Gunter’s Chain, and
(ii) Engineer’s Chain.
(i) Gunter’s Chain:
It is named after its inventor and is also called Surveyor’s Chain. It is 66 ft. long and is divided into 100 links. It is very suitable for measuring distances in miles and furlongs and areas in acres on account of its simple relation with the furlong and the acre (10 chains = 1 furlong, and 10 x (chain)2 = 1 acre). In England, the word chain is understood to mean Gunter’s Chain.
(ii) Engineer’s Chain:
Also called 100 ft. chain, it is 100 ft. long and is divided in to 100 links. It is commonly used on flat country where the ground is fairly even and in all Engineering Surveys for alignment of roads, railways, canals etc.
Testing the Chain:
The length of a chain changes due to wear, mud- sticking and change in temperature etc.
It may increase in length due to:
(i) Stretching of links and joints,
(ii) Opening out of the rings,
(iii) Wear of the wearing surfaces, and
(iv) Rough handling in pulling it through hedges and fences etc.
On the other hand, it may decreases in length due to bending of the links and mud-sticking etc. It, therefore, becomes necessary to check its length before commencing the survey work and also at frequent intervals otherwise the distance measured with the chain will become unreliable. Before testing the chain, the bent up links should be straightened and mud should be removed from the joints.
According to the Indian standard specifications every metre length should be accurate to within ± 2 mm when measured with a tension of 8 kg and checked against a certified steel tape or steel band which has been standardised at 20°C.
The overall length of the chain should be within the following limits:
20 m chain: ± 5 mm
30 m chain: ± 8 mm
The chain can be tested by any of the following methods:
i. By Comparing It with a Chain Standard or with a Test Gauge:
The Chain standard is formed by marking the standard length of the chain, with the steel or invar tape, on the verandah floor or apron of some large public building or on the railway platform etc. If such places are not available at hand, a test gauge may be established by driving two pegs the required distance apart and inserting nails into their tops to mark the exact points (fig. 2.7).
A third peg may also be driven in the middle to serve as a check for the central tag of the chain. But it is preferable to establish a permanent test gauge by using dressed stones about 20 cm square instead of pegs (Fig. 2.8)
ii. By comparing it with the Levelling Staff laid down successively.
iii. By comparing it with the Steel Tape reserved specially for this purpose.
Adjusting the Chain:
If the chain is found to increase in length than the standard length, it may be adjusted:
(i) By closing up the joints of the opened out rings.
(ii) By hammering back to the shape, the flattened out rings.
(iii) By replacing some of the larger rings by smaller ones.
(iv) By removing some of the rings, and
(v) By the adjustable links at the handles.
If on the other hand, the chain is found to decrease in length than the standard length, it may be corrected:
(i) By straightening the bent up links,
(ii) By replacing some of the smaller rings by larger ones.
(iii) By inserting the new rings as required and
(iv) By the adjustable links at the handles.
The adjustment should as far as possible be symmetrical on either side of the middle point so that the position of the central tag remains unchanged.
Instrument # 2. Arrows:
These are also called marking or chaining pins. Accompanying each chain are ten arrows, which are used to mark the end of the chain during the process of chaining. They are pointed at one end have a ring or eye at the other for the facility of carrying (Fig. 2.9).
They are about 40 cm long and are made of good quality steel wire 4 mm (8 s.w.g.) in diameter. It is usual to tie a piece of red cloth or tape round the handle of the arrows so that they may be more easily distinguishable when struck in the midst of grass, plants etc.
Instrument # 3. Tapes:
The measuring tapes are usually found in the following four different kinds according to the material from which they are manufactured:
1. Linen or Cloth Tape:
It is made out of linen cloth with brass handle at the zero end whose length is included in the tape length. It is very light and handy. But it cannot withstand much wear and tear. After much use, it elongates and the figures get indistinct. It is easily affected by dampness.
If wet, it shrinks. It stretches very easily and also shortens by sagging in a windy weather. Moreover it is an intolerable nuisance to keep winding up or to have it in folds in hands. The hands get dirty and filthy. It should not be used for accurate work. It is little used in surveying except for taking subsidiary measurements such as offsets.
2. Metallic Tape (Figs. 2.10 and 2.11):
The linen tape is reinforced with fine brass or copper wires to prevent stretching or twisting of fibres and is then called a metallic tape. The wires are interwoven into the varnished strips and are not visible to the naked eye.
These are available in several lengths but the tapes of 20 and 30 metre lengths are more common and are supplied in leather cases Fig. 2.10. Measuring tape fitted with a winding device. Each metre length is divided into ten parts (decimetres) and each part is further sub-divided into ten parts (centimetres). It is commonly used for taking offsets in chain surveying. But still it cannot be used for very accurate and important measurements.
3. Steel Tape (Figs. 2.10 & 2.12):
It is made of steel ribbon varying in width from 6 mm to 16 mm and is available in lengths of 1, 2, 10, 30 & 50 metres. Each metre length is divided into 200 parts, each being 5 mm long. But the first 10 cm length of the tape is graduated upto millimetres. It is chiefly used for testing chain lengths and measurements of important buildings. It is very delicate and is easily breakable. It cannot withstand rough usage and therefore it should be used with great care.
4. Invar Tape:
It is made of an alloy of steel (64%) and nickel (36%) which is known as invar steel. This reduces the error due to temperature in the precise measurements to a minimum. It is 6 mm wide and is usually available, in lengths of 30 m, 50 m and 100 m. It is very much costly and extremely delicate.
It is likely to be easily kinked and broken and must, therefore, be used with greatest care. It is used for measurements of highest precision such as measurement of base line in triangulation work and in city work. It should not be used for ordinary work.
Instrument # 4. Ranging Rods (Fig. 2.13):
Ranging rods are made of light, thin and straight bamboo or of well-seasoned wood such as teak, pine or deodar etc. They are shod with iron at the bottom and preferably, surmounted with a flag about 25 cm square in size of red and white or yellow and white bunting.
These are usually 3 cm in diameter and 2 m or 3 m long, painted alternately either red and white or black and white in lengths of 20 cm i.e. one link length of a metric chain. These are used for marking the position of stations conspicuously as well as for ranging the straight lines.
Note:
1. Ranging rods of more length i.e. 4 m to 6 m, are called Ranging Poles and are used in the case of very long lines.
2. When the number of ranging rods is limited, thin sticks or twigs 40 cm to 1 m length with white papers in the cuts at tops can serve their purpose. Such stricks which are pointed at bottom and are cut from a wood or hedges are called whites.
Instrument # 5. Offset Rod (Fig. 2.14):
The offset rod is similar to the ranging rod with the exception that instead of a flag, a recessed hook or stout open ring is provided at top for pulling or pushing the chain through a hedge or other obstacle. It is also used for measuring small offsets when the work is of ordinary nature.
Instrument # 6. Pegs:
These are rods made from hard timber and tapered at one end. The length varies from 15 cm to 60 cm and the cross-section from 3 cm to 5 cm square. These are driven into the ground to mark the instrument- stations.