I downloaded a couple compass apps for my iPhone so I could check the readings I’m getting in mission planner against readings from an app that has been known to be very accurate to within a fraction of a degree. Most of the compass apps that I came across have had a few variables that you can use to calibrate your compass in a certain manner. The two that were present in all the apps were a designation for either true north or magnetic north. Which north should I choose for the purpose s of checking the compass offsets in mission planner?
The compass in your aircraft uses magnetic north, it however also knows the magnetic declination from the GPS coordinate so can calculate true north if needed for navigation. Not a direct answer I know but may explain the use better.