I have a Point on the surface of the earth which I am converting to a Vector from Earth Center.

I have a True North Heading in degrees describing the path the point will travel on the surface of the earth.

I need to calculate a Vector which is perpendicular to the plane created by the path of this point along the earths surface.

I have tried calculating an arbitrary point along the path using the method described [here](http://www.codeguru.com/Cpp/Cpp/algorithms/general/article.php/c5115/)
and then taking the cross product of the two vectors however it does not seem to be quite accurate enough and seems like more overhead than is necessary.

This is related to my other post [ray-polygon-intersection-point-on-the-surface-of-a-sphere](https://stackoverflow.com/questions/628335/ray-polygon-intersection-point-on-the-surface-of-a-sphere).