Gunter's chain at Campus Martius Museum
|Unit system||imperial/US units|
|imperial/US units||22 yd|
|metric (SI) units||20.1168 m|
|Unit system||imperial/US units|
|metric (SI) units||201.1680 mm|
Gunter's chain (also known as Gunter's measurement ) is a distance measuring device used for surveying. It was designed and introduced in 1620 by English clergyman and mathematician Edmund Gunter (1581-1626). It enabled plots of land to be accurately surveyed and plotted, for legal and commercial purposes.
The 66 feet (20.117 m) chain is divided into 100 links, usually marked off into groups of 10 by brass rings or tags which simplify intermediate measurement. Each link is thus 7.92 inches long. A quarter chain, or 25 links, measures 5½ yards and thus measures a rod (or pole). Ten chains measure a furlong and 80 chains measure a statute mile. 
Gunter's chain reconciled two seemingly incompatible systems: the traditional English land measurements, based on the number four, and decimals based on the number 10. Since an acre measured 10 square chains (or 100,000 square links) in Gunter's system, the entire process of land measurement could be computed using measurements in links, and then converted to acres by dividing the results by 100,000.
The method of surveying a field or other parcel of land with Gunter's chain is to first determine corners and other significant locations, and then to measure the distance between them, taking two points at a time. The surveyor is assisted by a chainman. A ranging rod (usually a prominently coloured wooden pole) is placed in the ground at the destination point. Starting at the originating point the chain is laid out towards the ranging rod, and the surveyor then directs the chainman to make the chain perfectly straight and pointing directly at the ranging rod. A pin is put in the ground at the forward end of the chain, and the chain is moved forward so that its hind end is at that point, and the chain is extended again towards the destination point. This process is called ranging, or in the US, chaining; it is repeated until the destination rod is reached, when the surveyor notes how many full lengths (chains) have been laid, and he can then directly read how many links (one-hundredth parts of the chain) are in the distance being measured. The chain usually ends in a handle which may or may not be part of the measurement. An inner loop (visible in the NMAH photograph) is the correct place to put the pin for some chains. Many chains were made with the handles as part of the end link and thus were included in the measurement.
The whole process is repeated for all the other pairs of points required, and it is a simple matter to make a scale diagram of the plot of land. The process is surprisingly accurate and requires only very low technology. Surveying with a chain is simple if the land is level and continuous--it is not physically practicable to range across large depressions or significant waterways, for example. On sloping land, the chain was to be "leveled" by raising one end as needed, so that undulations did not increase the apparent length of the side or the area of the tract.
Although Gunter's chain was later superseded by the steel tape (a form of tape measure), its legacy was a new statutory unit of length called the chain, equal to 66 feet (or 100 links). This unit still exists as a location identifier on British railways, as well as in some areas of America. In the United States (US), for example, Public Lands Survey plats are published in the chain unit to maintain the consistency of a two-hundred-year-old database. In the Midwest of the US it is not uncommon to encounter deeds with references to chains, poles, or rod units, especially in farming country. Minor roads surveyed in Australia and New Zealand were in the 19th and early 20th centuries customarily one chain wide.
A similar American system, of lesser popularity, is Ramsden's or the engineer's system, where the chain consists also of 100 links, each one foot (0.3048 m) long. The original of such chains was that constructed, to very high precision, for the measurement of the baselines of the Anglo-French Survey (1784-1790) and the Principal Triangulation of Great Britain.
The even less common Rathborn system, also from the 17th century, is based on a 200-link chain of two rods (33 feet, 10.0584 m) length. Each rod (or perch or pole) consists of 100 links, (1.98 inches, 50.292 mm each), which are called seconds (″), ten of which make a prime (′, 19.8 inches, 0.503 m).