where [psi] is magnetometer determinate value of magnetic course, but [phi] is true value of magnetic field angle.
Corrected course can be determined by summing measured value of magnetic course [psi] with compensation value for course [phi] calculated from GPS data [[psi].sub.GPS].
The FAA uses grid and magnetic courses
for approaches north of approximately the Arctic Circle, but all other published FAA courses are still referenced to magnetic north.
Slight differences in displayed magnetic courses
are not a significant issue because GPS units are direct-to devices.
A caveat is that occasionally the magnetic course
on an en route chart will differ slightly from the track value necessary to hold the desired course.
The GPS calculates the magnetic course
by conjuring the great circle route based upon the lat/long value of two VORs, and then adds the local magnetic variation at the aircraft's present position.
Likewise, a minor amendment could change charted magnetic courses
due to magnetic variation since the ground track remains the same.