Feet
In 1959 the international yard and pound agreement (between the United States and countries of the Commonwealth of Nations) defined a yard as being exactly 0.9144 metres, which in turn defined the foot as being exactly 0.3048 metres (304.8 mm).
ch =
ft * 0.015151
Chains
Unit of length equal to 66 feet, used especially in the U.S. public land surveys. The original measuring instrument (Gunter's chain) was literally a chain consisting of 100 iron links, each 7.92 inches long. Steel-ribbon tapes began to supersede chains around 1900, but surveying tapes are often still called "chains" and measuring with a tape is often called "chaining". The chain is a convenient unit in cadastral surveys because 10 square chains equal 1 acre.
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