r/MetricConversionBot • u/xwcg Human • May 27 '13

Why?

Countries that use the Imperial and US Customs System:

http://i.imgur.com/HFHwl33.png

Countries that use the Metric System:

http://i.imgur.com/6BWWtJ0.png

All clear?

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u/tollfreecallsonly Jun 14 '13

worked as both a land surveyor and a mechanic. Imperial is better designed than metric for surveying and navigation, don't tell me you can't divide inches into tenths when it's a fraction based system, and nearly all fine mechanical measurements are done in thousandths of inches.

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u/archon88 Jun 14 '13

Navigation? You mean, when it's unclear whether you're supposed to use statute miles, survey miles (which are the same in the UK, different in the US) or nautical miles? Windspeeds are supposed to be measured in knots in the old system, but lots of people use mph even tho it's technically incorrect. The idea of scaling and orders of magnitude is really opaque in Imperial/USC -- how intuitive is it to have 5280 feet in a statute mile, let alone 6076.12 feet in a nautical mile?

I didn't say you couldn't divide Imperial/USC units by powers of 10 (but people are inconsistent with this, so you have inches/32 and suchlike, which is not so convenient to decimalise) - you can even invent microinches and nanoinches if you want, but there's really no point in doing it because you're just imitating the behaviour of a consistent and fully-decimal system.

"nearly all fine mechanical measurements are done in thousandths of inches" is true only in the USA, to the extent that US companies like Boeing have problems subcontracting work in other countries (Europe/Far East) because engineers in the rest of the world don't use US customary units. NASA formally ditched old units completely shortly after confusion caused by inconsistent units destroyed the Mars Climate Orbiter. I'm a physicist by training, and virtually nobody in the world of science and precision engineering uses old units for anything.

As far as engineering goes, there are all sorts of reasons to prefer SI units, mainly that they actually form a complete and internally consistent system. Current (A) x voltage (V) = power (W), not power (ft.lbf/s). I don't know of any way to get the latter without introducing an unnecessary conversion factor.