Chains are the measuring instrument used in surveying formed by the 100 links of 4mm galvanized mild steel wire. These links are joined by 3 circular or oval wire rings. These rings provide the flexibility to the chains.
Every aspect of the life requires some measuring units. Measurements are used to do the work precisely and accurately. Let it be from kitchen to office, everywhere measurements are used. So as in engineering calculation or measurements holds a very greater role in construction or surveying or any other aspect.
There are various units of measurements such as meters, centimeters, feets, inches, acre, yards and the list goes on. Same as units there are various instrument used in the measurements of any entity. One of the instruments used in measurement are chains.
Parts of Chains used in Surveying
The chain consists of many small parts used for handling or reading the measurements.
- At the ends chain is provided with brass handle with swivel joint so that it can be easy to roll or unroll the chain without twisting and knots.
- At every 10th link is provided with a tally of one teeth, 20th link with a tally of two teeth and so on till 40th link. This is provided for the easy reading of measurements.
- At the center of the chain is provided with a circular talley used for easy reading.
Types of Chains used in Surveying
Depending upon the length of the chain, these are divide into following types,
- Metric chains
- Steel band or Band chain
- Gunter’s chain or surveyor’s chain
- Engineer’s chain
- Revenue chain
A. Metric chains
Metric chains are the most commonly used chain in India. These types of chains comes in many lengths such as 5, 10, 20 and 30 meters. Most commonly used is 20m chain. Tallies are provided at every 2m of the chain for quick reading. Every link of this type of chain is 0.2m. The total length of the chain is marked on the brass handle at the ends.
B. Steel band or Band chain
These types of chain consist of a long narrow strip of steel of uniform width of 12 to 16 mm and thickness of 0.3 to 0.6 mm. this chain is divides by brass studs at every 20cm or instead of brass studs, band chain may have graduated engraving as centimeter.
For easy use and workability band chains are wound on steel crosses or metal reels from which they can be easily unrolled. These steel bands are available in 20m and 30m length and the width of about 12-16mm.
C. Gunter’s chain or surveyor’s chain
Gunter chain comes in standard 66ft. These chain consists of 100links, each link being 0.66ft or 7.92inches. The length 66ft is selected because it is convenient in land measurements.
10 square Gunter’s chains = 1 Acre
10 Gunter chains = 1 Furlong
80 Gunter chains = 1 mile
D. Engineer’s chain
This chain comes in 100ft length. Its consist of 100 links each link being 1ft long. At every 10 links a brass ring or tags are provided for indication of 10 links. Readings are taken in feet and decimal.
E. Revenue Chain
The standard size of this type of chain is 33ft. The number of links are 16, each link being 2 ft. This chain is commonly used in cadastral survey.
Testing and Adjustment of Chain
As the chain is a metal made, it may undergo many changes due to temperature effect or human error and etc. So for all lengths of chain a tolerance is given,
5m chain = + or – 3mm
10m chain = + or – 3mm
20m chain = + or – 5mm
30m chain = + or – 8mm
Chain length shorten due to
- Bending of links.
- Sticking of mud in the rings
Chain length increases due to
- Opening of small rings.
- Wearing of surfaces.
Chains may be tested with respect to
- Steel tape
- Permanent test gauge
- Pegs driven in the field at required distances
- Permanent test gauge made with dressed stones
If chain is found long, then
- Close the joins of the rings
- Reshape the elongated rings
- Remove one or two rings
- Replace worn out rings
If chain is found short, then
- Straighten the links
- Replace the small rings with big one
- Insert additional rings
- Flattening the circular rings
Errors in chain Surveying
Errors in chaining may be classified as:
- Personal errors
- Compensating errors, and
- Cumulating errors.
Wrong reading, wrong recording, reading from wrong end of chain
etc., are personal errors. These errors are serious errors and cannot be detected easily. Care should be taken to avoid such errors.
These errors may be sometimes positive and sometimes negative. Hence
They are likely to get compensated when large number of readings are taken. The magnitude of such errors can be estimated by theory of probability. The following are the examples of such errors:
- Incorrect marking of the end of a chain.
- Fractional part of chain may not be correct though total length is corrected.
- Graduations in tape may not be exactly same throughout.
- In the method of stepping while measuring sloping ground, plumbing may be crude.
The errors that occur always in the same direction are called cumulative errors. In each reading the error may be small, but when large number of measurements are made they may be considerable, since the error is always on one side. Examples of such errors are:
- Bad ranging
- Bad straightening
- Erroneous length of chain
- Temperature variation
- Variation in applied pull
- Sag in the chain, if suspended for measuring horizontal distance on a sloping ground.
Errors (i), (ii), (vi) and (vii) are always +ve since they make measured length more than actual.
Advantages and Disadvantages of Chains in Surveying
Advantages of Chains in Surveying
- Chain survey is simplest and commonest method used in surveying exercises
- The equipment used to conduct chain survey are simple to use,
- The equipment used in chain survey can easily be replaced. For example measuring rods can be replaced with measuring tape.
- This method does not involve complicated mathematical calculation. I know this is the relief to those who are afraid of mathematics
- In chain survey few people are needed to conduct the survey. Normally chain survey team has three people Booker, leader and follower.
Disadvantages of Chains in Surveying
- Simple chain survey cannot be conducted in built up areas and large areas.
- Simple chain survey is subject to several chances of errors of accumulation which may cause by problem of chain. The chain linkage may fail to stretch up properly and result in inaccurate data. Also clogging of chain may read to error in reading.
- It is time consuming
- It may not be conducted in areas with steep slopes or water logged areas. Chain survey is usually conducted in dry areas with gentle slopes. It becomes more complicated when survey is conducted in areas that are too wet.
- Chain survey becomes more complicated method when there are raised points (obstacles) in between areas to be surveyed