Different types of Chains used in Surveying
Chains are made up of galvanized mild steel wire of 4mm diameter having number of links in series. The ends of links were bent like loop. The links were connected by three small circular rings. Brass handles were provided on each end of chain with a groove to hold an arrow. The length of links is measured from the center of middle ring to the center of the next middle ring. The length is 200 mm. The total length of chain is measured from outside of one handle to outside of other handle. Normal tension is that pull which neutralizes the effect due to pull and say. If the length of a chain is found to be short on testing, it can be adjusted by straightening the links.
Different types of chains:
- Metric chain: These are generally available in 10, 20 and 30 m. A 10 m chain have 50 links means each link is 0.2m(200mm) in length. So 20 m length chain have 100 links. Metallic tallies are fixed in chain at 5 m, 10 m, 15 m, 20 m for quick and easy reading of the chain.
- Gunter’s Chain: length is 66 feet with 100 links means each link is of length 0.66 feet.
- Engineer’s chain: chain length is 100 feet having 100 links means each link is of length 1 feet.
- Revenue chain: chain length is 33 feet having 16 links. Used in cadastral survey.
- Steel band: it is available in 20 or 30m length. Used for high accuracy. For accurate work the steel band should always be used in preference to chain because the steel band is practically inextensible and is not liable to kinks when in use.
The maximum tolerance in a 20 m chain is ±5mm.
Important Points On Chains and tapes:
- If a chain is found too short on testing, it can be adjusted by straightening the links, inserting additional circular rings, and flattening the circular rings.
- During chaining along a straight line, the leader of the survey party has three arrows and while follower has five arrows, the distance of the follower from the starting point will be five chains.
- Compensating errors in chaining are proportional to the square root of the length of the line.
- Chainage is the distance measured along a chain line.
- In a chain survey execution, the first step taken is reconnaissance.
- Invart tapes are made of an alloy of Nickel and steel.
Plumb bob is a metal bob shaped like cone attached with string at the top circular surface. When freely suspended its bottom tip always points towards the gravity and shows the direction of vertical line.Plumb bob also used for centering of surveying instruments like dumpy level, theodolite, compass etc. Masons generally prefer plumb bob for the vertical alignment of walls.
The limits were provided to check whether the work performed is correct or is within permissible limits.
Permissible Limits of errors in chaining for measurements:
- For normal/plane conditions: 1/500
- For rough work or hilly terrain conditions: 1/250
- When a measurement is done with steel tape/steel bands permissible error: 1/2000
- For measurement done with tested chain: 1/1000.
