[seanalltogether]

Its interesting that the nautical mile is based on a more abstract number that would define when looking at maps and charts, rather then something more immediate and physical like the distance to the horizon when out on the ocean. Most imperial units seem to be based on simple laymen measurements.

[codeduck]

Distance to the horizon is variable. Sea state, height of deck over water, visibility etc will all affect it. Whereas, with a sextant and a decent maritime chronometer it is very easy (*) to determine longitude and latitude.

[292355744930110]

The length of people's feet vary and so does the amount of land a yoke of oxen could plough in one day.

[throw0101a]

> Most imperial units seem to be based on simple laymen measurements.

I would not label the nautical mile as "imperial". It was based on mathematical properties of the planet:

> Historically, it was defined as one minute (1/60 of a degree) of latitude.

Just like the metre originally was:

> The metre was originally defined as 1⁄10,000,000 of the meridian arc from the North pole to the equator passing through Dunkirk.

* https://en.wikipedia.org/wiki/Nautical_mile

[SiempreViernes]

I guess this mostly means that Nautical Mile is a unit that traces back to the introduction of the spherical coordinate system in cartography.

[rocqua]

As a nice bonus this means your coordinate grid also gives a scale bar.

This makes measuring distance in nautical miles using a chart a lot more practical.

[bdjewkes]

You can pick a NM right off the latitude scale on a marine chart with a pair of dividers. Its an eminently practical measurement for navigation.

edit: of course you could pick off distance with any kind of measurement; but my point is that it converts directly into minutes of latitude