Types of chain used in surveying
Chain surveying is the type of surveying in which only linear measurements are made in the field. Chain surveying derives its name from the fact that the principal equipment used in this process is the chain.
There are different types of chain used in surveying. They are,
- Metric chain
- Engineer’s chain
- Gunter’s or Surveyor’s chain
- Revenue chain
- Steel band or band chain
These chains are widely used among other types of chain in surveying and available in standard lengths such as 5m, 10m, 20m, and 30m respectively. Among these chains, 20m chain length is widely used. To ease the process of reading measurement, tallies are fixed at,
- Every 1m length – For 5m and 10m chain lengths
- Every 5m length – For 20m and 30m chain lengths
For 20m and 30m chains, a small brass ring is attached at every 1m length except where tallies are provided. The letter ‘m’ is indicated on each tally in a metric chain, indicating that it is a metric chain. The length of the chain is also imprinted on the brass handle.
As stated above, each chain is made up of several links. The length of one link may vary for different types of chains. For a metric chain, the length of one link is 0.2m. Hence, the numbers of links in each chain are,
- For 5m chain – 5/0.2 = 25 links
- For 10m chain – 10/0.2 = 50 links
- For 20m chain – 20/0.2 = 100 links
- For 30m chain – 30/0.2 = 150 links
For a longer chain like a 20m chain, we can easily read the length by recognizing the shape of the tallies. Different shapes of tallies are used at different lengths of chains. The shape of tallies at every,
- 10th link – Tally of one tooth
- 20th link – Tally of two teeth
- 30th link – Tally of three teeth
- 40th link – Tally of four teeth
- 50th link – Tally of circular shape used. (@middle of chain)
Let us understand this through an example,
For a 20m chain, we know that the length of one link is 0.2m.
- Length of chain @ 10th link = 10 x 0.2m = 2m, one tooth tally @2m
- Length of chain @ 20th link = 20 x 0.2m = 4m, two teeth tally @4m
- Length of chain @ 30th link = 30 x 0.2m = 6m, three teeth tally @6m
- Length of chain @ 40th link = 40 x 0.2m = 8m, Four teeth tally @8m
- Length of chain @ 50th link = 50 x 0.2m = 10m, circular tally @10m
The shape of tallies at different lengths of 20m chain are indicated in the following figure.
The engineer’s chains are 100 feet long and consist of 100 links, which means each link is 1 foot long. Readings are taken in feet and decimal. At every 10 links (10 feet), a brass tag with no of 10 links is present.
3.Gunter’s or Surveyor’s chain
This chain is 66 feet long and divided into 100 links, thus each link equals to 0.66 feet long. It enabled plots of land to be accurately surveyed and plotted for legal and commercial purposes. The entire process of land area measurements could be easily computed using a Gunter’s chain and then converted to acres easily by dividing the results by 10. Hence,
- 10 chains by 10 chains (10 square chains) = 660 ft. x 660 ft. = 435600 Sq.ft./10 = 43560 Sq.ft. = 1 Acre.
And, some of the easiest measurements done by Gunter chain are,
- 10 Square chains = 1 Acre
- 10 Gunter chains = 1 furlong (660 ft)
- 80 Gunter chains = 1 mile (5280 ft)
This chain is 33 feet long and divided into 16 links, thus each link equals to 2.06 feet long. This chain is mainly used in cadastral surveying.
The summary of all types of chain used in surveying and its chain lengths, number of links, length of one link are shown in the following table.
5.Steel band or band chain
It is available in 20m and 30m lengths. It has a uniform width of 12 – 16mm and a thickness of 0.3 – 0.6mm. They are numbered at every meter. A brass stud is dividing them at every 20cm. The first and last 20cm lengths are subdivided into cm and mm for precision. These steel bands are wound on a special steel cross from which they can be easily unrolled.
Steel bands are preferred over chains as they are more accurate and easy to use. But it should be handled very carefully. Unlike chains, these steel bands are not made up of several links to impart flexibility, hence the steel band has the highest possibility of breaking in the field, if not handled correctly. It is also difficult to repair in the field.