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A chain is a unit of length. It measures 66 feet, or 22 yards, or 100 links, or 4 rods (20.1168 m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in Britain and in some other countries influenced by British practice.
By extension, chainage (running distance) is the distance along a curved or straight survey line from a fixed commencing point, as given by an odometer.
The chain was commonly used with the mile to indicate land distances and in particular in surveying land for legal and commercial purposes. In medieval times, local measures were commonly used, and many units were adopted that gave manageable units; for example the distance from London to York could be quoted in inches, but the resulting huge number would be unmemorable. The locally used units were often inconsistent from place to place.
In 1620, the clergyman Edmund Gunter developed a method of surveying land accurately with low technology equipment, using what became known as Gunter's chain; this was 66 feet long and from the practice of using his chain, the word transferred to the actual measured unit. His chain had 100 links, and the link is used as a subdivision of the chain as a unit of length.
In countries influenced by English practice, land plans prepared before about 1960 associated with the sale of land usually have lengths marked in chains and links, and the areas of land parcels are indicated in acres. A rectangle of land one furlong in length and one chain in width has an area of one acre. It is sometimes suggested that this was a medieval parcel of land capable of being worked by one man and supporting one family, but there is no documentary support for this assertion, and it would in any case have predated Gunter's work.
In Britain, the chain is no longer used for practical survey work. However it survives on the railways of the United Kingdom as a location identifier. When railways were designed the location of features such as bridges and stations was indicated by a cumulative longitudinal "mileage", using miles and chains, from a zero point at the origin or headquarters of the railway, or the originating junction of a new branch line. Since railways are entirely linear in topology, the "mileage" or "chainage" is sufficient to identify a place uniquely on any given route. Thus a certain bridge may be said to be "at" 112m 63ch, meaning that it is at the location 112 miles and 63 chains (181.51 km) from the origin. In the case of the photograph the bridge is near Keynsham, that distance from London Paddington station. The indication "MLN" after the mileage is the Engineers line reference describing the route as the Great Western Main Line, so that visiting engineers can uniquely describe the bridge they are inspecting, as there may be bridges at 112m 63ch on other routes.
The chain is no longer taught in British schools, but has survived for these reasons:
- Railways need to keep permanent records of as-built drawings of structures, and of the topography of routes and junctions;
- Chains and links are in many survey and real estate records;
- Miles and chains remain values familiar to many people.
The chain also survives as the length of a cricket pitch, being the distance between the stumps.
Australian and New Zealand use
In Australia and New Zealand, most building lots in the past were a quarter of an acre, measuring one chain by two and a half chains, and other lots would be multiples or fractions of a chain. The street frontages of many houses in these countries are one chain wide—roads were almost always Script error: No such module "convert". wide in urban areas, sometimes Script error: No such module "convert". or Script error: No such module "convert".. Laneways would be half a chain (10.1 m). In rural areas the roads were wider, up to Script error: No such module "convert". where a stock route was required. Script error: No such module "convert". roads were surveyed as major roads or highways between larger towns, Script error: No such module "convert". roads between smaller localities, and Script error: No such module "convert". roads were local roads in farming communities. Roads named Three Chain Road etc. persist until today. An acre is nominally the area within a rectangle 1 chain by 10 chains.
North American agriculture
In North America the chain is still used in agriculture: measuring wheels with a circumference of 0.1 chain (diameter ≈ 2.1 ft or 64 cm) are still common and readily available in the United States and Canada. For a rectangular tract, multiply the number of turns of one of these wheels for each of two adjacent sides, then divide by 1000 to get the area in acres.
Also in the United States the chain is normally used as the measure of the rate of spread of wildfires (chains per hour), both in the predictive National Fire Danger Rating Systems as well as in after-action reports. The term chain is used by wildland firefighters in day-to-day operations as a unit of distance. Under the U.S. Public Land Survey System, parcels of land are often described in terms of the section (Script error: No such module "convert".), quarter-section (Script error: No such module "convert".), and quarter-quarter-section (Script error: No such module "convert".). Respectively, these square divisions of land are approximately 80 chains (one mile or 1.6 km), 40 chains (half a mile or 800 m), and 20 chains (a quarter mile or 400 m) on a side. The use of chains and links is also commonly encountered in older metes and bounds legal descriptions.
The use of the chain was once very common in laying out townships and mapping the U.S. 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"). Distances on township plats (US term for maps) made by the U.S. General Land Office are shown in chains.
American surveyors sometimes used a longer chain of 100 feet (30.48 m), also of 100 links, known as the engineer's chain or Ramsden's chain. The first such was constructed by Jesse Ramsden for the measurement of the Hounslow baseline at the start of the Anglo-French Survey (1784–1790). The term chain in this case usually refers to the measuring instrument rather than a unit of length; the distances measured with such an instrument are normally measured in feet (and usually decimal fractions of a foot, not inches). The unit symbol for a single Ramsden's link is lk. 
Also in North America a modern variant of the chain as a tool is used in forestry for traverse surveys. This modern chain is a static cord (thin rope), 50 metres long, marked with a small tag at each metre, and also marked in the first metre every decimetre. When working in dense bush, a short axe or hatchet is commonly tied to the end of the chain, and thrown through the bush in the direction of the traverse, to ease working in dense forest.
Another version used extensively in forestry and surveying is the hip-chain. A hip-chain is a small box containing a string meter, worn on the hip. The user simply ties the spooled string off to a stake or tree and the meter counts distance as the user walks away in a straight line. These instruments are available in both feet and meters.
- The Cassell English Dictionary, London 1990, p. 214, ISBN 0-304-34003-0
- Plane and Geodetic Surveying, A.L. Johnson (SPON)
- HS2 proposed alignment with chainages expressed in metres
- George Seddon (28 September 1998). Landprints: Reflections on Place and Landscape. Cambridge University Press. pp. 151–. ISBN 978-0-521-65999-4. Retrieved 28 May 2013.
- Lay, M. G. (July 2008). "Roads". emelbourne the city past and present. School of Historical Studies Department of History, The University of Melbourne. Retrieved 28 May 2013.
- Picture of Ramsden's chain
- Cardarelli, François Cradarelli (2003). Encyclopaedia of Scientific Units, Weights and Measures. London: Springer. p. 41. ISBN 978-1-4471-1122-1.