Thursday, April 15, 2010

The Origin of the Statute Mile

Measurement trivia can be interesting for one with a formal education in surveying and later career in electronic navigation. What is even more interesting is the anthropological rationale behind why we as a people chose our measures. Land measurement in England before the 18th century was measured in rods. For some reason the rod was 16 and a half feet long and land was measured by laying down the rod end to end. Astronomer Edmond Gunter came up with a novel new measure called the chain. Compared to the rod, this tool could be folded up and easily stored and could measure distances by stretching it out straight. He chose the chain to be four rods long which made it 66 feet, and his choice for that was that he figured that it could be easily manipulated by two operators and be of a reasonably useful length I suppose. Distances were then measured by a chain crew including a forward chainman and a rear chainman. The forward chainman would start out with a metal loop on his belt from which hung 11 chaining pins. Starting from a survey marker, the rear chainman would hold the trailing end on the marker and the forward chainman would press a chaining pin into the ground at the exact chain distance along the desired line. The pair would move forward a chain length the rear chainman holding the trailing end of the chain on the chaining pin left by the forward chainman and the process would proceed until the 10 chains were measured.

The only oddity is that the chain was divided into 100 links which makes each link an odd length of 7.92 inches. Old survey plans will show lots,  for example the original King's grants, measured in chains and links.

And so the 66' chain became important for land measure in the British Colonies such as Australia, Canada and the US. This measure still has a very strong influence in our lives today. For example a road allowance for all the older roads -- width of a road from boundary to boundary -- is 66 feet or one chain. This is the case for all the streets in Ottawa for instance, yet Wellington Ave, which was initially intended to be a boulevard, is actually 2 chains or 132 feet wide.

Today with the conversion to metric in Canada, newly built road allowances are now typically a convenient 20 metres or 65.6168 feet wide, a measure still strongly influenced by the chain.

In the 1880's with the very significant invention of spring steel, the tape measure took over from the chain. Yet I still remember in survey school seeing a steel windable tape measure that was marked in chains and links on one side and feet and tenths of a foot on the other. Inches were never used in surveying. This was probably very handy for retracing old surveys before the advent of computers that automated conversions.

The acre by definition is 10 square chains. The township lot that our property is a part of was originally 200 acres or 20 chains wide by 100 chains deep, the original grant from the king. In Ontario, for military service, officers were typically given 100 acres by splitting the front and back half of the lot, and soldiers were typically offered 50 by cutting a lot into quarters, one in each corner.

Many Ontario farmers kept a survey chain hanging in the barn, in case they wanted to divide their land into fields or put in fence lines. They are a lovely instrument, a steel chain with nice brass handles at both ends, and brass tabs that mark every ten links in the chain. I picked one up years ago from Ebay and still have it around here somewhere.

Finally the statute mile is by definition 80 chains long or 5280 feet.This is not to be confused with the nautical mile.

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