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A quarter chain, or 25 links, measures 16 feet 6 inches (5.03 m) and thus measures a rod (or pole). Ten chains measure a furlong and 80 chains measure a statute mile. [1] 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.
A Gunter's chain showing the individual links. The link (usually abbreviated as "l.", "li." or "lnk."), sometimes called a Gunter’s link, is a unit of length formerly used in many English-speaking countries. In US customary units modern definition, the link is exactly 66 ⁄ 100 of a US survey foot, [1] or exactly 7.92 inches or 20.1168 cm.
Distances were always measured in chains and links, based on Edmund Gunter's 66-foot measuring chain. The chain – an actual metal chain – was made up of 100 links, each being 7.92 inches (201 mm) long. Eighty chains constitute one U.S. survey mile (which differs from the international mile by a few millimeters).
A longer chain of 100 feet (30.5 m), with a hundred 1 foot (305 mm) links, was devised in the UK in the late 18th century by Jesse Ramsden, though it never supplanted Gunter's chain. [7] Surveyors also sometimes used such a device, and called it the engineer's chain.
Europeans used chains with links of a known length such as a Gunter's chain, or measuring tapes made of steel or invar. To measure horizontal distances, these chains or tapes were pulled taut to reduce sagging and slack. The distance had to be adjusted for heat expansion. Attempts to hold the measuring instrument level would also be made.
In the case of roads or other linear infrastructure, a chainage (derived from Gunter's Chain - 1 chain is equal to 66 feet or 100 links) will be established, often to correspond with the centre line of the road or pipeline.
Their accuracy was adequate for cadastral surveying but they were deemed insufficiently accurate for the Anglo-French Survey (1784–1790), Britain's first high-precision survey. Roy asked Ramsden to prepare a new chain of 100 links, each one foot in length. [12] He also asked Ramsden to prepare three precisely-calibrated wooden rods 20 feet long.
American surveyors use a decimal-based system of measurement devised by Edmund Gunter in 1620. The base unit is Gunter's chain of 66 feet (20 m) which is subdivided into 4 rods, each of 16.5 ft or 100 links of 0.66 feet. A link is abbreviated "lk", and links "lks", in old deeds and land surveys done for the government.