Sure you can draw a line between two points that's constant magnetic azimuth. It'll be a lot more work than drawing a rhumb line, since there's no reasonably simple formula for it (i.e. there's no reasonably simple formula giving the mag variation at every lat-lon).
|Quoting saafnav (Reply 10):|
if you are flying a Rhumbline Track (assuming no Wind), you will actually make heading changes
Just to clarify: if you trying to follow a course of constant true direction, your magnetic heading will need to change as you move along that intended "rhumbline" course. That "rhumbline" course will be a different line from the one that has a constant magnetic direction, but the latter still exists.
(I see at http://www.ngdc.noaa.gov/geomag-web/#declination
they claim half-degree accuracy for mag-minus-true. Do other sites claim to do better?
[Edited 2012-06-21 15:04:56]