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To be precise, a nautical mile is always a minute of latitude, everywhere on earth. It is only a minute of longitude at the equator.

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Ah, good point. In that case expand the niche cases to traveling due north or south.

And, at sea level. At altitude this no longer applies either because the circumference at altitude is greater.

Well, it still applies in aviation, as distance and ground speed used in navigation are measured at sea level irrespective of altitude.

...by around 0.1%. Which is why nobody is bothering with it, considering how susceptible to wind speed distances and velocities are up there.

In practice, when navigating by pen and paper, what you do is you measure a distance on the chart with a divider, then you put the divider (with the previously measured distance set) to the north-south scale on the side of the chart to read out the distance in nautical miles.

Now, of course, in principle charts could have a separate scale for distances in km, and we could use km for navigation just fine. But, well, I've never seen a nautical chart with such a scale.

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