So on the mission planner’s Flight Data screen, there’s a status tab near the bottom left. a huge bank of labels and numbers is shown, the mx, my, mz shows the individual values for the first compass, also the “magfield” shows the length of the vector made up of those three value.
there are similar fields for the 2nd (mx2, my2, my2, magfield2) and 3rd compass (mx3, my3, myz3, magfield3). It’s also possible to see these numbers more easily from the Quick screen by clicking on any of the existing numbers in the Quick screen and then selecting the appropriate checkbox on the window that pops up.
The magfield values in particular are useful for seeing how bad the compass interference is in real time. So you can move the compass closer/further from the body of the vehicle and see at what point is stabilizes. Ideally we should aim to place the compass somewhere that it’s magfield doesn’t change by more the 30% from it’s “natural” lenght. I.e. take it 10 feet from the vehicle and it’ll probably give a lenght of something like 450… move closer and closer but once it’s length is over 600 then it’s too close. These numbers are just guesses and examples and the length can be different depending upon the compass hardware itself, the environment in which it was calibrated and your location on Earth. Anyway, hopefully that gives some ideas about how to determine a good place to mount the compass.
By the way, ArduPilot only ever uses one compass at a time and by default it’ll use the first compass (which is normally an external compass). We can add support for blending compasses, it’s not particularly hard we just haven’t done it yet.