"Gary" wrote

are always different, always! You can't swing a compass with a
typical GPS.

Why not? Pick a slack current time and set up a waypoint to distant
landmark. Stop the boat and check the compass. You need to do it on
multiple headings or else have landmark waypoints about every 30
degrees around the horizon. If you're in a good coverage period and
the landmarks are several miles away, it's going to be very accurate.

--

Roger Long