Whether it is twips for typographic screen measurements, hands for horses’ heights or furlongs for horse racing, many imperial units are used for specific purposes. By contrast, the International System of Units (SI), the modern version of the metric system, is based on the principle that each measurable phenomenon has one basic named unit and all the multiples and subdivisions of the unit then follow the same logical structure using prefixes.
Miles, yards, feet and inches are used on British road signs. Fathoms, cables and nautical miles are maritime units in the imperial system – these units were designed to be used at sea. Then we have Gunter’s survey units of links, rods and chains – these units were designed for surveying.
For volumes, the imperial system has specific units for pharmaceutical purposes (i.e., apothecaries’ volume measures). For mass and weight, the imperial system has troy weight for precious metals, apothecaries’ weight for pharmaceutical purposes and avoirdupois weight for everything else. The stone is a unit of avoirdupois weight. These days, it is only used for body weight.
By contrast, SI has universal units for each phenomenon that can be used with the same set of defined prefixes for multiples and subdivisions of units:
- metre for length
- cubic metre for volume
- kilogram for mass (and weight) though prefixes apply to the gram
These are all general-purpose units used for everything. There are compatible metric units that are not strictly part of SI but are approved for use with SI such as the litre (equal to one cubic decimetre) and the tonne (equal to 1000 kilograms). However, this does not change the fact that SI has just one unit for each measurable phenomenon.
Metric Views 
Table 8 of the SI Brochure presents the “non-SI units that are accepted [by the CIPM] for use with the SI”. These units are used world-wide and to name a few, include the hour, minute, litre and hectare. The litre and hectare are of course survivors from the original metric system of 1795. Many of the units of measure identified in the article are peculiar to the English-speaking world.
Looking at the article, it should be noted the apothecary’s system of units is no longer in use in the United Kingdom while the Troy pound was declared to be illegal for use in trade in the United Kingdom during Queen Victoria’s reign. While the stone is in common use in the United Kingdom as an unofficial measure of body weight, health professionals record patients’ weights using metric units. Furthermore, prior to the Second World War, the value of the stone depended on its use: until 1936 the stone, when used at Smithfield market for animals on the hoof had a value of 14 pounds, but for dressed carcases, it had a value of 8 pounds. The stone is not used outside the United Kingdom (and possibly Ireland) – I certainly never saw it in use in South Africa when I lived there in pre-metric days – during my student days I participated in judo in the under 175 lb class (which looks horribly like the under 80 kg class).
Links to various presentations of the SI Brochure can be found at https://www.bipm.org/en/publications/si-brochure/ .
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Can anyone explain why “nautical miles” are still used in navigation for ships and airplanes? I understand it has something to do with alignment with longitude or something like that?
I ask because I am wondering if there is some challenging impediment to switching over from “nautical miles” to “kilometers” and from “knots” to “km/h”.
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@punditgi: One nautical mile is approximately one minute of latitude. It shoud be noted that due to the earth being ellipsoidal, degres of latitude ar not all the same.
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As far as I’m aware, the only link between the imperial land mile and the nautical mile is the name ‘mile’. They are completely different distances. It’s as though there wasn’t a choice of any other word! As usual, no consistency in imperial measures.
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Thanks for the comments. Is there a way to replace “nautical miles” and “knots” with SI units that does not upset the navigation and supply chain apple cart?
–Ezra
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Ezra,
It shouldn’t be that difficult. First you will have to replace degrees, minutes and seconds of arc with gradians. Since there are 100 gradians in a quarter arc of the earth, and a quarter arc is 10 Mm, each gradian would be equal to 100 km. But, I think you would still run into some problems.
1.) Since the earth is a not a perfect sphere, each gradian of latitude would vary somewhat.
2.) Due to it not being a perfect sphere it isn’t exactly 10 Mm from the pole to the equator. The actual distance is 7.56 km longer. This would make each gradian 75.6 mm longer than it should be. But, I’m sure this can be compensated for.
3.) Digital equipment I’m sure can be easily and with zero cost be converted with a firmware update if the instruments are not presently capable of being switched. Older, mechanical equipment would need to be replaced.
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@Daniel, @Ezra: During the French Revolution, a decimalised system of angular units was devised. The right angle was divided into 100 grads and the grad into 100 centigrads. This system continued in use in France until after the Second World War. However, the British nautical maps were the best in the world, so the rest of the world used degrees. One of the impacts that the French system had was in 1948 to change the names for units of temperature from degrees centigrade to degrees Celsius, thereby removing confusion between an angle and a temperature having the same name.
For the record, the SI unit of angular measure is the radian and for solid angles, the steradian. Just to remind you, one radian is the angle subtended at the centre of a circle of constant radius by an arc that is equal in length to the radius.
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One strange thing I have noticed in the various news feed video clips I get on my phone via YouTube is the use by some farmers in Australia of “acres” instead of “hectares” even today. The reporters actually use “hectares” and the farmers are fine using “kilometres”, “litres”, etc. But when it comes to land area, they seem to be stuck in using “acres”, and it seems not to be confined to older farmers.
Does anyone have a clue as to why this might be the case? Seems like a pretty strange anomaly!
— Ezra
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Just a news report from Sky News that was entirely in metric (e.g. the bullet heading for Donald Trump was only a centimetre away from striking him fully plus another mention of some metric unit that I cannot remember).
Is this typical for Sky News? If only the BBC did likewise.
Ezra aka punditgi
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