The other thing is on the Mercator, if you measure from point A to point B, and it gives you a compass heading, this is the correct compass heading to use to navigate to point B from point A. This is not true unless a map is designed for this purpose.

It won't necessarily be the shortest distance -- to plot that out, it is best to use a globe and chart the arc of a great circle path, which is a straight line on the curved surface of the earth. But that path will have the compass changing all the time.

If you are navigating by compass, which ships and airplanes had to do more often way back when, the mercator was great.

Posting for the benefit of @AAA@feddit.org and anyone else who didn't know.