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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.
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]
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
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).
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.
The length of the chain chosen, 66 feet (20 m), being called a chain gives a unit easily converted to area. [9] Therefore, a parcel of 10 square chains gives 1 acre. The area of any parcel measured in chains will thereby be easily calculated. Table of Trigonometry, from the 1728 Cyclopaedia, Volume 2 featuring a Gunter's scale
Drawing of a circumferentor from the Cyclopaedia Circumferentor with Gunter's chain at Campus Martius Museum in Marietta, Ohio. A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century. [1]