Chain (unit)

# Chain (unit)

Discussion

Encyclopedia
A chain is a unit
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

of length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

; it measures 66 feet or 22 yards or 100 links (20.1168 m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

). There are 10 chains in a furlong
Furlong
A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. The exact value of the furlong varies slightly among English-speaking countries....

, and 80 chains in one statute mile. An acre
Acre
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...

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.

## Origin

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
Edmund Gunter
Edmund Gunter , English mathematician, of Welsh descent, was born in Hertfordshire in 1581.He was educated at Westminster School, and in 1599 was elected a student of Christ Church, Oxford. He took orders, became a preacher in 1614, and in 1615 proceeded to the degree of bachelor in divinity...

developed a method of surveying land accurately with low technology equipment, using what became known as Gunter's chain
Gunter's chain
Gunter's chain is a measuring device used for land survey. It was designed and introduced in 1620 by English clergyman and mathematician Edmund Gunter long before the development of the theodolite and other more sophisticated equipment, enabling plots of land to be accurately surveyed and plotted,...

; 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 acre
Acre
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...

s. A rectangle of land one furlong
Furlong
A furlong is a measure of distance in imperial units and U.S. customary units equal to one-eighth of a mile, equivalent to 220 yards, 660 feet, 40 rods, or 10 chains. The exact value of the furlong varies slightly among English-speaking countries....

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.

## Contemporary use

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" 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
Keynsham
Keynsham is a town and civil parish between Bristol and Bath in Somerset, south-west England. It has a population of 15,533.It was listed in the Domesday Book as Cainesham, which is believed to mean the home of Saint Keyne....

, that distance from Paddington station. The indication "MLN" after the mileage is the Engineers line reference
Engineers line reference
An Engineers' line reference is a three or four-character code used in the British rail network to refer to a section of a track or route. These references are unique across the whole rail network, allowing identification of any part of the network...

describing the route as the Great Western Main Line
Great Western Main Line
The Great Western Main Line is a main line railway in Great Britain that runs westwards from London Paddington station to the west of England and South Wales. The core Great Western Main Line runs from London Paddington to Temple Meads railway station in Bristol. A major branch of the Great...

, so that a visiting engineer can uniquely describe the bridge he may be inspecting, as there may be bridges at 112m 63ch on other routes.

The chain is not taught in British schools, but it 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.

### Cricket pitches

The chain also survives as the length of a cricket pitch
Cricket pitch
In the game of cricket, the cricket pitch consists of the central strip of the cricket field between the wickets - 1 chain or 22 yards long and 10 feet wide. The surface is very flat and normally covered with extremely short grass though this grass is soon removed by wear at the ends of the...

, being the distance between the wickets.

### Texas chain

In Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

and elsewhere in the Southwestern United States
Southwestern United States
The Southwestern United States is a region defined in different ways by different sources. Broad definitions include nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas and Utah...

, the vara chain of 20 varas (16.9 m, or ~55½ ft.) was used in surveying Spanish land grants.

### Australian and New Zealand use

In Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, 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 city of Melbourne
Melbourne
Melbourne is the capital and most populous city in the state of Victoria, and the second most populous city in Australia. The Melbourne City Centre is the hub of the greater metropolitan area and the Census statistical division—of which "Melbourne" is the common name. As of June 2009, the greater...

is a classic example: surveyor Robert Hoddle
Robert Hoddle
Robert Hoddle was a surveyor of Port Phillip in the 1830s, and the creator of the Hoddle Grid, the street grid system upon which inner city Melbourne is based. He was also an accomplished artist and depicted scenes of the Port Philip region as well as New South Wales...

divided the city into 24 ten-chain blocks, which still serve as the basic grid of the city. The street frontages of many houses in these countries are one chain wide—roads were almost always one chain wide (20.117 m) in urban areas, sometimes one and a half or two chains (30.2 m or 40.2 m). Laneways would be half a chain (10.1 m). In rural areas the roads were wider, up to ten chains (201 m) where a stock route was required. Five chain (100 m) roads were surveyed as major roads or highways between larger towns, three chain (60 m) roads between smaller localities, and two chain (40 m) roads were local roads in farming communities. Roads named Three Chain Road etc. persist until today, particularly in Victoria, Australia. An acre
Acre
The acre is a unit of area in a number of different systems, including the imperial and U.S. customary systems. The most commonly used acres today are the international acre and, in the United States, the survey acre. The most common use of the acre is to measure tracts of land.The acre is related...

is nominally the area within a rectangle 1 chain by 10 chains, and a chain equals 20.1168 metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

s.

Chainage (running distance) is the distance along a curved or straight survey line from a fixed commencing point, similar to mileage
Milestone
A milestone is one of a series of numbered markers placed along a road or boundary at intervals of one mile or occasionally, parts of a mile. They are typically located at the side of the road or in a median. They are alternatively known as mile markers, mileposts or mile posts...

.

### North American agriculture

In North America the chain is still used in agriculture: measuring wheels with a circumference of 0.1 chain (Ø ≈ 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.
Under the U.S. Public Land Survey System
Public Land Survey System
The Public Land Survey System is a method used in the United States to survey and identify land parcels, particularly for titles and deeds of rural, wild or undeveloped land. Its basic units of area are the township and section. It is sometimes referred to as the rectangular survey system,...

, parcels of land
Lot (real estate)
In real estate, a lot or plot is a tract or parcel of land owned or meant to be owned by some owner. A lot is essentially considered a parcel of real property in some countries or immovable property in other countries...

are often described in terms of the section
Section (United States land surveying)
In U.S. land surveying under the Public Land Survey System , a section is an area nominally one square mile, containing , with 36 sections making up one survey township on a rectangular grid....

(640 acres or 259 ha), quarter-section (160 acres or 64.7 ha), and quarter-quarter-section (40 acres or 16.19 ha). 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 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
Public Land Survey System
The Public Land Survey System is a method used in the United States to survey and identify land parcels, particularly for titles and deeds of rural, wild or undeveloped land. Its basic units of area are the township and section. It is sometimes referred to as the rectangular survey system,...

) 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 made by the U.S. General Land Office are shown in chains.

Railroads in the United States have long since used decimal
Decimal
The decimal numeral system has ten as its base. It is the numerical base most widely used by modern civilizations....

fractions of a mile, but the New York City Subway
New York City Subway
The New York City Subway is a rapid transit system owned by the City of New York and leased to the New York City Transit Authority, a subsidiary agency of the Metropolitan Transportation Authority and also known as MTA New York City Transit...

continues to use a chaining system
New York City Subway chaining
New York City Subway chaining is a method to precisely specify locations along the New York City Subway lines. It measures distances from a fixed point, called chaining zero, following the twists and turns of the railroad line, so that the distance described is understood to be the "railroad...

using the 100 foot engineer's chain.

## Ramsden's chain

American surveyors sometimes used a longer chain of 100 feet (30.48 m), known as the engineer's chain or Ramsden
Jesse Ramsden
Jesse Ramsden FRSE was an English astronomical and scientific instrument maker.Ramsden was born at Salterhebble, Halifax, West Riding of Yorkshire, England. After serving his apprenticeship with a cloth-worker in Halifax, he went in 1755 to London, where in 1758 he was apprenticed to a...

's chain
. 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).

## Other instruments

Also in North America a modern variant of the chain as a tool is used in forestry
Forestry
Forestry is the interdisciplinary profession embracing the science, art, and craft of creating, managing, using, and conserving forests and associated resources in a sustainable manner to meet desired goals, needs, and values for human benefit. Forestry is practiced in plantations and natural stands...

for traverse surveys
Traverse (surveying)
Traverse is a method in the field of surveying to establish control networks. It is also used in geodesy. Traverse networks involve placing survey stations along a line or path of travel, and then using the previously surveyed points as a base for observing the next point...

. 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
Decimetre
A decimetre is a unit of length in the metric system, equal to one tenth of a metre, the SI base unit of length. In simple words there are 10 cm in a decimetre....

. When working in dense bush, a short axe or hatchet
Hatchet
A hatchet is a single-handed striking tool with a sharp blade used to cut and split wood...

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.