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You need some reference. The latitude grid is available anywhere on your chart, with multiple subdivisions, so even when your boat is shaking like crazy you'll be able to draw a course ad hoc (which requires estimating drifts due to wind and tide etc., which are conveniently given in knots (so nautical miles per hour)).

The coordinate grid may be arbitrary (though having a lot of factors is nice), but the derived nautical mile is not.

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> which are conveniently given in knots (so nautical miles per hour)).

Yes. We may be misunderstanding each other.

It's convenient to have nautical miles per hour if you're using a base unit of nautical miles, and your map grid is nautical miles based. This isn't a surprise at all.

Estimating wind, tides, drift, etc, would be equally as easy if everyone was working in kilometres, and your maps were showing grids based on multiples-of-kilometres, and all speeds were given in kilometres per hour. Again, wouldn't be a surprise.

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