Chain

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Chain 

Chains are the linear distance measuring instrument used in surveying. Chains are formed by the 100 links of 4mm galvanized mild steel wire. These links are connected by 3 circular or oval wire rings. These rings between links provide the flexibility to the chains.  chain are available in both metric and foot system.
At the both end chain is provided with brass handle having swivel joint which make easy to roll or unroll the chain without twisting and knots.
At 10th link chain is provided with a tally of one teeth, 20th link with a tally of two teeth and so on till 40th link with tally of four teeth and at the center of the chain is provided with a circular tally. For every meter circular brass ring is used as tally. These tallies are provided for the easy reading of measurements.

  

 

fig. Chain

 

 

 


fig.  Tally

Types of Chains used in Surveying:

Depending upon the length of the chain and unit system these are divide into following types,

  1. Metric chains
  2. Gunter's chain or surveyor’s chain
  3. Engineer’s chain
  4. Revenue chain

Metric chains:
Metric chains are the most commonly used chain for surveying purpose. These types of chains comes in many lengths such as 5, 10, 20 and 30 meters. Among them 20m chain is most commonly in used.
For 20 meter metric chain:
NO. of link = 100 links
length of link = 0.2m. / 20cm.
tallies at every 2m. 
For 30 meter metric chain:
NO. of link = 100 links
length of link = 0.3m. / 30cm.
tallies at every 3m. 
NO. of link = 150 links
length of link = 0.2m. / 20cm.

Gunter's chain or surveyor’s chain:
English mathematician Edmund Gunter develop the chain at 1620A.D. and named as Gunter's chain.
length = 66 ft.
NO. of link = 100 links
length of link = o.66 ft. / 7.92inches.
10 G-chains = 1 furlong
80 G-chains = 1 mile
10 square G-chains = 1 Acre
It is also named as surveyor's chain.

Engineer’s chain:
length = 100 ft.
NO. of link = 100 links
length of link = 1ft. / 12inches.

Revenue chain
It is designed generally for the cadastral mapping.
length = 33 ft.
NO. of link = 16 links
length of link = 2 ft. 

Testing and Adjustment of Chain:

1. Chain length being shorten due to Bending of links or sticking of        mud in the rings.
2. Chain length being increases due to Opening of small rings or            wearing of surfaces.
3. Chains may be tested with respect to Steel tape, permanent test          gauge.

4. If chain is found too long, then; Close the joins of the rings,                reshape the elongated rings, remove one or two rings, and                  replace worn out rings.

5. If chain is found too short, then; Straighten the links, replace the        small rings with big one, insert additional rings, and flattening the      circular rings.

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