Mostly an aviation thing but descended from the fact that many early airplanes were American/British. IIRC only China, Russia and the DPRK use metric units.
Technically, the lowest flight level is FL200 or 20000'. Below that, altitudes are reported in feet. I'm not really familiar with the "angels" thing. I suspect that is specifically military.
EDIT: there was a reply (since deleted) that indicated I may not be completely correct on stating FL200 as the lowest number used. I contend that I am still basically correct for North America, but there is some subtlety:
Technically, a flight level isn't a distance measurement at all. It's defined by atmospheric pressure, so that all planes use the same "ground" barometer by definition. The feet are expressed nominally relative to a standard sea level pressure.
That is absolutely correct. I thought about mentioning it, but did not. Flight levels are definitely expressed as "pressure altitudes" rather than absolute altitudes. The air pressure (or rather density) is the more important number when it comes to keeping an aircraft in the air, anyway.
Generally, you'll use flight levels until either you hit the transition level specified in the approach plate or ATC clears you beneath that level, at which point they'll switch to using feet and give you the QNH (atmospheric pressure at sea level) to calibrate your altimeter.
Depending on conditions, FL180 could be below the transition altitude at 18000'. I doubt, therefore, that you would ever be given anything below FL200. That is the subtlety to which I'm referring (well that and the fact that different transition altitudes are used in other countries). I'll admit that this is stretching my knowledge. Prior to this thread, I merely "had it on good authority" that FL200 was the minimum.
1. The nautical mile is an SI derived unit, now fixed at 1852 meters.
2. It's a more useful unit than the kilometer for long-distance navigation due to its historical definition in terms of arc along a line of latitude making it easy to work with on charts.
Aviation. 500 & 1000 ft intervals are very convenient discrete altitude levels for coordinating traffic, and that's probably a key reason why it's held up universally. In meters, the interval will either awkward and hard to communicate numbers or spaced too widely to be efficient.
Are the imperial units of measurement an aviation thing, or an American thing, or a bit of both?