Miles, Acres and Furlongs

A mile was originally 1000 (latin mille) paces--counted left to left, what we would today call 2000 steps.  Roman legions were trained to march all together, which led to a pretty consistent distance--5000 Roman feet or about 4851 modern English feet. There have been dozens of different miles through history, but most have fallen away, save for two:  the English mile of 5280 feet (1609.344 m) and the nautical mile of 1852 meters (about 6076.12 ft).  The nautical mile is a metric distance and is exactly 1/5400ths of the distance between the north pole and the equator, or one minute of latitude, which makes it very useful for navigation.  More on the mile in a minute.

An acre was meant to be the amount of land that a man and an ox could plow in a day.  Obviously this depends on the soil conditions and the strength and skill of the man and ox, and it was eventually standardized as an area of one chain (22 yards or 66 feet--20.1168meter) by 10 chains.  Thus an acre is 4840 square yards or 43560 square feet (4046.82 square meters).  An American football field, not counting sidelines or endzones, is 5333.333 square yards, pretty close to 1.1 acres.  A hectare is 10,000 square meters, 2.47107 acres.  It's fairly convenient to pretend that an acre is .4 hectares...it's off by just under half a percent.

A furlong is the length of a furrow in such a field--10 chains.  Thus an area one furlong square (660 feet by 660 feet) is ten acres.  Because it meant they didn't have to turn the plow and animal as often, farmers preferred to break up their fields, when necessary, into rectangles that retained the furrow-long length.

In 1593, during the reign of Queen Elizabeth I, Parliament passed a statute declaring the mile to be exactly 8 furlongs, or 5280 feet.  Thus we get the term statute mile.  A square mile is thus 64 square furlongs or 640 acres.

Virtually all navigation is done in nautical miles.  One of the advantages is that you don't need a scale of miles on the chart: you can get it from the latitude scale.     On some map projections, the scale of miles changes as you get closer or farther from the poles, but latitude allows you to measure correctly.  Mercator is such a projection.  You can't measure long distances with it, but you only do approximate navigation with such a map anyway.

A useful factoid:  13 nautical miles is 14.96 statute miles--so close to 13/15 that it's an easy conversion to do in your head.  11 kilometers is about 5.94 nautical miles.  Not quite so close to11/6 but still useful.