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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.
The only exceptions to this rule are special requirements for measurement in feet in mineral surveys and townsite surveys. [13] Linear Measurement 1 Chain = 100 links or 66 feet 1 Mile = 80 chains or 5,280 feet. Area Measurement 1 Acre = 10 square chains or 43,560 square feet 1 square mile = 640 acres
This law defined a survey grid system of 6–mile–square townships divided into 1–mile–square sections, with the defining unit being the chain, specifically Gunter's chain. This was the first unit of measurement designated into law by Congress. This law and the way it defined the survey grid ended debate over Jefferson's decimal system. [2]
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).
Gunter's chain was introduced in 1620 by English mathematician Edmund Gunter. It enabled plots of land to be accurately surveyed and plotted for legal and commercial purposes. Leonard Digges described a theodolite that measured horizontal angles in his book A geometric practice named Pantometria (1571).
The practice of using surveyor's chains, and perch-length rods made into a detachable stiff chain, came about a century later when iron was a more plentiful and common material. A chain is a larger unit of length measuring 66 feet (20.1168 m ), or 22 yards , or 100 links , [ 11 ] or 4 rods (20.1168 meters ).
Eighteenth-century surveyors used Gunter's chains which were 22 yards long (one chain with 100 links of 7.92 inches). 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.