The one thing holding me back with GPS tracking is the direction Hermione is pointing at launch; in particular, she can find out the flight direction to the target based on the difference between the GPS positions of takeoff and landing points, but Hermione’s compass only gives orientation angles based on magnetic north, and the difference between true and magnetic north varies over time and space.
After digging around, it turns out “Magnetic Declination” is the name of the angle between magnetic and true north, which for me is currently -1° 5′ i.e. at takeoff, when Hermione’s compass says she’s pointing magnetic north, she’s actually point +1° 5′ from true north. For me, that means it’s irrelevant for the short distance flights I’m implementing.
In the perfect world, I would like to have a magnetic declination table, mapping GPS longitude / latitude to magnetic declination angle, or use a GPS receiver which has one of these built in (they do exist apparently), but for now simply knowing the magnetic declination is negligible for my few meter flights is enough.
Just FYI, Cambridge’ magnetic declination is -0º 30′ which is even more negligible!