[Jedd]

I think you may have missed my point.

The actual length of a nautical mile only makes sense because we have this weird way of measuring earth (based on how we measure circles).

NM's were obviously defined using that weird numbering system - 360 degrees in a circle, 60 minutes in a degree, 60 seconds in a minute - so it shouldn't be a surprise that they 'feel real' within that system.

Given a quarter of the circumference of earth is ~ 10,000km, it seems ripe for using base-10 and metric units ... but for the fact, obviously, that these arc-based-at-6-thousand-km-from-the-centre measurements are highly variable and not hugely useful.

> The point of calling nautical miles 'real' is that you can do easy mental math with them to express a distance in terms of latitude and longitude.

And that's not true either.

At best it works for latitude -- even a small way from the equator you'll suffer the effects of longitudinal meridians converging.

But at that point, why not just do your mental math in minutes rather than (yet another) mile variant?

[filmor]

The math is not done in ones head but using a plotter on a chart. As long as latitude is defined as it is, nautical miles have meaning.

[Jedd]

I didn't say 'in one's head', merely used the same phrasing as nmilo.

If you're using a plotter, then actual length is presumably irrelevant - any unit would do, yes?

I was not asserting that nautical miles don't have meaning - just that their meaning is even more arbitrary than most other units of measurement, except in this case they're extra useless because they align to what they ostensibly derive from only at or near the equator.

If you want to use something 'around that size', and you're very keen on avoiding metric at all costs, then why not just use the 'upstream' unit - a minute? Obviously it still suffers from converging meridians if you measure from a plane through the sphere, but I'm not sure what number of measurement problems you're happy to contend with.

[filmor]

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.

[Jedd]

> 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.