There are two types of measuring chains, the engineer’s chain of 100’ with 100 links and the Gunther’s surveyor’s chain of 66’ with 100 links. Surveyors still call measuring use a metal tape chaining.
Here’s why the Gunther’s Chain is 66 feet:
The 66-foot (20.1 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 (201 mm) long. 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. Since an
acre measured 10 square chains in Gunter's system, the entire process of land area measurement could be computed using measurements in chains, and then converted to acres by dividing the results by 10.
[2] Hence 10 chains by 10 chains equals 10 acres, 5 chains by 5 chains equals 2.5 acres.
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