|Unit system||imperial/US units|
|1 chain in ...||... is equal to ...|
|imperial/US units||22 yd|
|metric (SI) units||20.1168 m|
A chain is a unit of length that measures 66 feet, 22 yards, 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.
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The chain was commonly used with the mile to indicate land distances and in particular in surveying land for legal and commercial purposes. Starting in the 19th century, the chain was used as a subdivision with the mile to show distances between railway stations, tunnels and bridges. In medieval times, many practical measures were in everyday use, often defined locally with inconsistencies from place to place.
In countries influenced by English practice before about 1960, land parcel sales usually specified lengths in chains and links, and areas in acres. A rectangle of land one furlong long and one chain wide has an area of one acre.
In 1620, the clergyman Edmund Gunter developed a method of accurately surveying land using a simple chain 66 feet long with 100 links. From his system, the chain and the link became standard surveyors' units of length.
American surveyors sometimes used a longer chain of 100 feet (30.48 m), also with 100 links, known as the engineer's chain or Ramsden's chain. This was devised 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, and distances measured are normally given in feet and decimal fractions of a foot (not inches).
Also in North America, a variant of the chain 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 meter, and also marked in the first meter every decimeter. 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.
Another version used extensively in forestry and surveying is the hip-chain: a small box containing a string meter, worn on the hip. The user ties off the spooled string 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.
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 location may be indicated as 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 112 miles 63 chains on other routes.
The chain is no longer taught in British schools, but has survived as a legacy unit in:
- Permanent railway records of structures, routes, and junctions;
- Many survey and real estate records;
- Everyday use among many older 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 1 chain (20.1 m) wide in urban areas, sometimes 1.5 chains (30.2 m) or 2.5 chains (50.3 m). Laneways would be half a chain (10.1 m). In rural areas the roads were wider, up to 10 chains (201.2 m) where a stock route was required. 5 chains (100.6 m) roads were surveyed as major roads or highways between larger towns, 3 chains (60.4 m) roads between smaller localities, and 2 chains (40.2 m) roads were local roads in farming communities. Roads named Three Chain Road etc. persist today. An acre is nominally the area within a rectangle 1 chain by 10 chains.
The "Queen's Chain" is a concept that has long existed in New Zealand, of a strip of public land, usually 20 metres (or one chain in pre-metric measure) wide from the high water mark, that has been set aside for public use along the coast, around many lakes, and along all or part of many rivers. These strips exist in various forms (including road reserves, esplanade reserves, esplanade strips, marginal strips and reserves of various types) but not as extensively and consistently as is often assumed.
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, multiplying the number of turns of one of these wheels for each of two adjacent sides and dividing by 1000 gives 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 System 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 (640 acres or 259 hectares), quarter-section (160 acres or 64.7 hectares), and quarter-quarter-section (40 acres or 16.19 hectares). 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 plat maps made by the U.S. General Land Office are shown in chains. In Canada, road allowances were originally 1 chain wide and are now 20 metres.
Railroads in the United States have long since used decimal fractions of a mile, though some operations such as the New York City Subway and the Washington Metro continue to use a chaining system using the 100-foot engineer's chain.
- Mathematics Dictionary (p 453), R.C. James, ISBN 9780412990410
- Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue 233 U.S. Department of Commerce, National Bureau of Standards, 1960
- Picture of Ramsden's chain
- 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.
- "Queen's Chain". Oxford Dictionaries – oxforddictionaries.com. Retrieved 4 July 2017.
- "Truth behind the Queen's Chain". NZ Herald. 12 August 2003. Retrieved 4 July 2017.
- "Te Ara, Encyclopaedia of New Zealand".
- Lakey, Jack (21 June 2017). "Turns out there is a standard to determine where a homeowner's property ends: The Fixer". Toronto Star. Retrieved 22 June 2017.