I have a GPS plotter to get position and course info wanted to build a system to run an linear actuator with reference pot to turn steering so need heading sensor etc
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.
I have a GPS plotter to get position and course info wanted to build a system to run an linear actuator with reference pot to turn steering so need heading sensor etc
I did auto GPS navigation for a two wheel robot using the Garmin GPS15-L using two serial ports a few years ago. Code is all in Java and its based on the NMEA $GPRMC string of coordinates.