On my large rover, compass calibration is impossible when the GPS puck and the PX4 are close to the vehicle. If I unplug them and move them a few meters away, everything is fine. In trying to determine where to put the GPS puck and/or Pixhawk (i.e. the compass or compasses), I wanted to know where around the vehicle the magnetic field is distorted by motors, batteries, steel tube framing, etc., I found this app Sensor Kinetics: https://itunes.apple.com/us/app/sensor-kinetics/id579040333?mt=8 It displays on my iPhone the magnetic fields in a real-time chart. Very cool. So it became a no-brainer to measure the mag fields around my rover and determine where (seems like 4-5’ away, vertically) the mag fields are not distorted and hence a safe place to mount the magnetic compass.
@ktrussell you should check it out!
Hopefully this lesson will be useful to someone else. Without that app, after all, it’s quite difficult to see the magnetic fields…