Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
With GPS it should also be possible to hold true ground track. Constant heading might take you somewhere.
Also motor boat's true ground track is a result of heading and drift angle (caused by wind and water movement)
If you use heading you'll have to decide weather to use magnetic heading (referenced to magnetic north) or true heading (referenced to geographical north).
I've not been in flying business quite a while, but the magnetic variation is considerably high in the USA. There is about 34deg west variation in California.
If you use a GPS system as it is used for small type aircraft the course displayed is always a great circle course (the shortest connection between two geographical points) and the autopilot will always steer into the needle.
Using a military type autopilot you'll hit a target of DINA-4 size.