Chain (unit)
|
|
As a unit of measurement within the Imperial system, the chain is defined as 22 yards, 66 feet, or 4 rods. Ten chains made one furlong, and 8 furlongs to a mile means there are 80 chains to a mile. In metric units, a chain equals 20.1168 metres. A chain is divided into 100 links.
The term 'chain' derives from the device commonly used for measurement of land in the past - a chain of 100 links, the Gunter's chain being the most common. The links were about 6 inches long, made of heavy gauge wire, with a loop at each end. The links were joined end to end to create the chain by three rings between the links. This enabled the chain to be folded up, link by link, until all 100 were in a bundle which could be held in the hand. At each end were brass handles and the full chain measurement was between the outside extremities of the brass handles with the chain at full stretch on flat ground. If the chain had been folded correctly, an experienced chainman (surveyor's assistant) could fling the bundle out and it would unfold neatly with no snags. Another chainman would grab the handle, flick the chain to get it straight and then be ready to take the measurement. Long distances would be measured in bays of 1 chain, the actual chain being dragged forward for each bay.
With so many links in the chain there were many wearing surfaces and chains commonly were longer than the designated length. Also some surveyors added an extra link, so that their surveys always included a greater physical area than the actual measurements indicated (the landowners weren't going to complain!). When retracing old surveys with modern equipment a surveyor will almost always find his measurements between monuments are longer than the originals.
The unit was once important in everyday life, being one of the fundamental units of Imperial system in the United Kingdom and its colonies, and was used to some extent in engineering and surveying in the U.S.
In Britain, it was commonly used in the railway industry (where the measure is still in widespread use). Mapping by the Ordinance Survey (Britain's national mapping organisation) began in the early 19th century using the chain as the basic unit of measurement. All map scales at that time were expressed as a relative fraction of a chain or a mile (e.g. a one inch to ten chain scale was equivalent to 1:7920)
The use of the chain was once very common in laying out townships and mapping the US along the train routes in the 19th century. In the U.S. a federal law was passed in 1785 (The Public Land Survey Ordinance) that all official government surveys must be done with a Gunter's chain (also referred to as the 'surveyor's chain').
American surveyors sometimes used a longer chain of 100 feet, known as the engineer's chain or Ramsden's chain. In Texas, the vara chain of 2 varas (55.556 ft) was used in surveying Spanish land grants.
In agriculture, measuring wheels with a circumference of 0.1 chain are still common and readily available in the United States and Canada, at least. For a rectangular tract, multiply the number of turns of one of these wheels for each side, then divide by 1000 to get the area in acres.
The chain also survives, in fact if not always in name, in two other specific contexts.
- It is the length of the pitch, between the wickets, in cricket.
- It lies at the origin of the definition of an acre as 4840 square yards. The original acre was an area of land suitable for ploughing in a defined time, and was therefore not square; it measured one chain by one furlong (10 square chains).
In the laying out of towns in Australia and New Zealand, most building lots in the past were a quarter of an acre, measuring 1 chain by 2 and a half chains, and other lots would be multiples or fractions of a chain. As a consequence, the street frontages of many houses in these countries are one chain wide — roads were almost always one chain wide (20.117m) in urban areas, sometimes one and a half (30.175m) or 2 chains (40.234m). Laneways would be half a chain (10.058m). In rural areas the roads were wider, up to 10 chains (201.17m) where a stock route was required.
External links
- Math Words (http://www.pballew.net/arithm16.html#gunter)
- How Many? A Dictionary of Units of Measurement (http://www.unc.edu/~rowlett/units/introd.html), Russ Rowlett
- How to make a Gunter's Chain (http://www.orbitals.com/self/survey/chain/chain.html)de:Chain