Calculating the True Bearing Between Two Points

Date: 9/20/95 at 8:43:4
From: Anonymous
Subject: Calculation of true bearing

How can I calculate the true bearing between two points on the 
earth's surface?  I know their grid coordinates, and their lat-
long coordinates, and how to get from the grid coordinates to a 
grid bearing, but how can I calculate the true bearing between the 
two points?

Thanks in advance,

David

Date: 11/19/95 at 16:13:59
From: Doctor Jonathan
Subject: Re: Calculation of true bearing

To find the true bearing from point A to point B on a sphere, 
project the chord AB on to the tangent plane of A. Call this 
projection AB'. The angle between AB' and the line segment 
pointing north in the tangent plane will be the true heading. The 
latter segment may be found by projecting the chord between A and 
the north pole onto the tangent plane of A. In fact, with this 
method you may find the heading relative to any point, not just 
the "north" pole. This would be useful for finding the heading 
according to a real life compass which gives headings relative to 
magnetic north, not true north. For example, if you know the 
coordinates for magnetic north (which I don't, offhand) N, you 
could would find the angle between the projection of AN and AB in 
the tangent plane of A--this would be the heading as measured by a 
"real" compass.

-Doctor Jonathan,  The Geometry Forum