If you do positioning in degrees, minutes, and seconds (or decimal minutes which I guess is more common nowadays), a 'natural' length unit for navigation is a minute of longitude, which is the original definition of the nautical mile. And then we get velocity as nautical miles per hour, or knots.
Blame the French for reinventing the wheel instead of, say, using a 1/1000th of a nautical mile as the definition of the metre. Alas, that's all water under the bridge by now.
If you want to do navigation in SI units, the UTM projections (see sibling thread) typically do positioning in metres inside each grid square.
Funnily enough the metre is defined by the globe as well: the arc from the equator to North pole is defined as 10,000 km. I think this might be where the alternative angle metrics came from, which went with the metre:
full circle is 400 grad, so a right angle (10,000 km) is 100 grad.
Then minutes and seconds are factors of 100, so a centesimal minute is 1 km and 1 centesimal second is 10 m.
Originally it was defined like that, but it was fairly quickly redefined as the distance between marks on a prototype platinum bar. It's had a couple of other definitions since then, but it's currently defined "by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit ms⁻¹, where the second is defined in terms of the caesium frequency ΔνCs".
And only 1852m at sea level; since it's an angular distance it's a bit longer in the air and a bit shorter underwater (~ 29 cm per km of elevation change).
If you do positioning in degrees, minutes, and seconds (or decimal minutes which I guess is more common nowadays), a 'natural' length unit for navigation is a minute of longitude, which is the original definition of the nautical mile. And then we get velocity as nautical miles per hour, or knots.
Blame the French for reinventing the wheel instead of, say, using a 1/1000th of a nautical mile as the definition of the metre. Alas, that's all water under the bridge by now.
If you want to do navigation in SI units, the UTM projections (see sibling thread) typically do positioning in metres inside each grid square.
Funnily enough the metre is defined by the globe as well: the arc from the equator to North pole is defined as 10,000 km. I think this might be where the alternative angle metrics came from, which went with the metre:
full circle is 400 grad, so a right angle (10,000 km) is 100 grad.
Then minutes and seconds are factors of 100, so a centesimal minute is 1 km and 1 centesimal second is 10 m.
Originally it was defined like that, but it was fairly quickly redefined as the distance between marks on a prototype platinum bar. It's had a couple of other definitions since then, but it's currently defined "by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit ms⁻¹, where the second is defined in terms of the caesium frequency ΔνCs".
https://www.bipm.org/en/si-base-units/metre
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Because nautical miles is an angle measurement which then happens to be 1852 meters when you break it down.
And only 1852m at sea level; since it's an angular distance it's a bit longer in the air and a bit shorter underwater (~ 29 cm per km of elevation change).