"Sims" wrote in message ...

As Steve has noted there are two things happening:
(1) The bearing to point B along a great circle/ellipsoid path
from Point A to B varies along the path. (A computer/autopilot
can be set to follow the ever varying bearing.)

Thanks

I was looking for a formula that does the same, so that i can tell my
heading at a certain point on the course.

I will look at the links given.

Again many thanks all for the help.

Sims

As others have pointed out, it's all there on my web-site, but you
have to piece it together.

http://williams.best.vwh.net/avform.htm

To get the magnetic heading at each point:

(1) determine the great circle true course, TC, at each point on the
path. It varies as you progress.
(2) Correcting for magnetic variation (aka declination), you can
determine the magnetic course, MC by MC = TC +- VAR. You can get VAR
from a fit. It varies with time. If you want something quick and
dirty, use the polynomial fit. If you want to use an official model,
such as WMM2000 or IGRF2000, there's source code on my site that will
compute it.
(3) Lastly, to get magnetic *heading* from magnetic course, you need
to solve the standard flight planning wind triangle to find the wind
correction angle, WCA, using the known or forecast wind at that point
on your flight. Coding for this is also on my site. MH = MC +- WCA
(4) Even then you are not strictly finished. You need to correct
the magnetic heading to get a compass heading, CH using the listed
deviation, DEV, on the specific airplane's compass correction card.
CH = MH +- DEV