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Comment by eloisant | Hacker News Reader

To be fair, non-metric units are also use worldwide in some specific fields. In aeronautics, height is in feet. In a boat, distance are nautic miles (not even the same miles as Americans on the road).

Quick quiz: what's the diagonal size of your laptop screen in centimeters? Your mobile phone?

So it doesn't matter if you use completely unrelated metrics for different things. When you want to buy a laptop you know how many inches you want. When you swim, you know what a 50 meters pool means. But do you care how many laptop diagonals you're swimming? Probably not.

Now back to US switching to metrics: I think the real argument in favor of sticking to imperial is the cost and risk of the switch. Risk because confusion between metrics could lead to a catastrophe (example: filling the tank of a plane with liters when it should be the same number of gallons).

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You understand, though, that the reason laptop screens are measured this way is because of America's continued use of the imperial system.

Yes, TV screens are measured in cms, no reason why computer screens couldn't be too.

Automobile tires are an interesting case of measurement system compromise - the mounting diameter is measured in inches, but the section width is measured in millimeters. Sidewall height is even stranger - it's represented as a percentage of the section width.

Example: 195/60R15 - 15" diameter, 195 mm wide, and 117 mm sidewalls (60% of 195)

To my understanding, diameters are almost always measured in inches, even in countries that follow the metric system.

> In aeronautics, height is in feet.

Note that this is only because of US influence. USSR and its satellites used meters-graduated altimeters, and so did most of the rest of the world until the end of WWII at least.

Nautical miles are used in aviation as well. In both cases, the use actually has a very good, albeit obsolete, reason: one nautical mile is equal to one arc minute of latitude on the Earth, to within reasonable accuracy. Very handy for navigation, at least when you didn't have a magic black box to do it for you.

I previously had a flight computer with two displays, each of which showed the distance to the next waypoint. One showed the distance in nautical miles, the other in statute. I could never remember which was which, and always had to look at both and see which number was bigger.

Silly measuring systems. We should measure everything in planck units.

Personally, I think the cost argument goes in the other direction. It's true that there would be costs to switching, even potentially lives lost. But there are constant, ongoing costs to having mixed units, potentially including lives lost (and certainly very expensive space probes lost). To me, that says that we should switch as quickly as possible to stop that ongoing cost. It may cost more up-front, but the cost of switching eventually ends, while the cost of staying as we are never stops.

> But there are constant, ongoing costs to having mixed units, potentially including lives lost (and certainly very expensive space probes lost).

In engineering there is never an excuse for not knowing the units of measure of a value in use. That probe would have been brought down just as easily by mistaking a 'kg' parameter as 'g' as it would have been by mistaking a 'kg' parameter as 'lbs'.

What's worse, in engineering there are multiple metric standards of measure (c-g-s, m-k-s, subatomic measures, etc.) There's no way around it, you have to know your units.

Excuses don't matter. The fact is that these mixups happen. And the probe would not have been brought down just as easily by mistaking 'kg' for 'g', because that would result in a number that was off by a factor of a thousand, and the result would be unlikely to make any sense. The meters/feet mixup that actually did happen could only happen because being off by a factor of 3.3 resulted in a number that still made basic sense, even though it was incorrect.

There are documented instances of metric/imperial mixups crashing space probes and forcing airliners to land after running out of fuel in midair and such things. Are there any documented instances of similar metric/metric mixups?

> There are documented instances of metric/imperial mixups crashing space probes and forcing airliners to land after running out of fuel in midair and such things.

As I recall, the Gimli glider came about because they were trying to switch to include metric units and had to change their procedure to do it, but didn't make the change in all places where it was necessary. This resulted in pilots using an Imperial conversion factor to try to calculate a metric fuel loadout, with disastrous results. http://en.wikipedia.org/wiki/Gimli_Glider#Refueling

... this isn't exactly selling me on the idea that switching up all the units used in the U.S. is actually a good idea.

This sort of error can happen any time you have mixed units, though, and as long as some countries insist on not using metric, the potential is there. The potential goes away once you switch, so as long as the costs for switching and for staying mixed are both finite and non-zero, it makes sense to switch now and get it over with.

I wonder if the Gimli glider incident had anything to do with imperial units being the preferred unit in the US. In any case I think the aviation industry would be well across this issue by now, given they would have to deal with conversion all the time on international flights between the US and metric countries. There is the benefit of hindsight with the Gimli incident, so the same mistakes need not be repeated.

Screen sizes are actually always in mm height and width. The diagonal size is just an approximate size.

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