Why do boats use nautical miles and why are they different than normal miles? Couldn't you just convert it to normal miles or km?

[zoidberg]

Let's not even start on Irish Miles....

 

[AntarcticPilot]

Interesting, I always thought they used a standard 1 km square.

Does this mean that all the recorded kms walked in the past might be wrong?

The 1 km squares are defined on a map, not on the surface of the earth. The OS uses a transverse Mercator projection, so distance measurements are only exact in along the central meridian (2°W). Because the whole of the UK is not too far from the central meridian, the distortion isn't great - but it exists. Further, grid north is not the same as true north (true north is indicated on OS sheets, I think).

 

[billskip]

Let's not even start on Irish Miles....

Don't they depend on how long the car is?

 

[Momac]

Does this mean that all the recorded kms walked in the past might be wrong?

If there were any deviations from level ground on the route of a walk and you measured off the map then yes .

 

[Jerbro]

This is how long a nautical mile is here. If I ever forget, I motor by to check…

 

[B27]

For nautical use, it is still a minute of arc and changes in length with latitude. 1852m is actually a middle value in the range that it can take (I once calculated the range but it's about 10m different between equator and pole). It is a unit of distance in other fields (aeronautics being one) but for navigation it's a measure of arc.

For nautical use, worrying about small differences is pretentious most of the time.
How often do you need to talk about a distance and need to care whether it's 100NM as in 185.2km or 100 minutes on the chart? and what would it actually mean if the first of those 100NM were longer or shorter than the later ones along a great circle?

I've worked a little with satellites and stuff, in their world for every mile of altitude, an orbit is 2pi more miles.

I enjoy sailing more when there are enough waves that you sail maybe 2km to the NM anyway...

 

[B27]

Let's not even start on Irish Miles....

Best not, he's a good chap, I sailed with him ages ago.

 

[AntarcticPilot]

For nautical use, worrying about small differences is pretentious most of the time.
How often do you need to talk about a distance and need to care whether it's 100NM as in 185.2km or 100 minutes on the chart? and what would it actually mean if the first of those 100NM were longer or shorter than the later ones along a great circle?

I've worked a little with satellites and stuff, in their world for every mile of altitude, an orbit is 2pi more miles.

I enjoy sailing more when there are enough waves that you sail maybe 2km to the NM anyway...

Of course - but my work was in mapping and intimate knowledge of datums, projections and the figure of the earth was all part of it. I had to understand how maps distort reality - which all flat maps do; the map-maker's skill is in choosing projections and parameters that minimise the distortion for users of the map.