Hot tool used by Pro-pilots at MultiWiicopter.com
The Earth magnetic field is very weak - PARIS GPS cannot navigate unless the embedded electronic compass is able to correctly sense the earths field - this is because when motors are running a multicopter create strong EMI - magnetic interferance - So to use this tool -> go outside 10m from computers, cars, buildings etc... Take note of where North "N" is - then take the PROPS OFF - power on your aircraft and set all motors to 50% - then use this small compass to find the place on your Tricopter or Quad where the disturbance is lowest. This is a tool used to located the GPS/MAG pod in the best spot - for the least interference
Can instantly solve coriolis errors in diy DJI GPS/MAG autopilot setups - ie - the user has located his dji GPS/MAG in a magnetically influenced region of the multicopter - using this tool will show a 10° error for example in that same spot - commonly refered by noobs as toilet bowl effects - but correctly called coriolis effect.
Bear in your mind that magnetic disturbance can be static - ie ferrous metals - or electro-magnetic. So you will be moving this compass around both with a) power off and then b) power on engines running (PROPS OFF)