[Moderator Edit: This is information is not accurate.] Forgive me if I am covering material that has already been covered many times, but I have searched and can't seem to find the information that I am going to place here. DJI has stated many times that you only need to calibrate your compass if you move a good distance from where it was calibrated the last time, and it flew just great. There are other ways that have been reported, that could possibly effect compass calibration, like having it exposed to a strong magnetic field, or stored in a vehicle next to the door mounted speaker. But in reality, even those type of events should not influence the stored calibration from the last successful calibration. The Compass Calibration is stored numerically in the firmware when you do the calibration. So, if that is indeed true, the calibration location, stored in firmware, should not have any way of being changed, short of your doing a Compass Calibration and storing new variables that replace the existing ones from the last calibration. So if all the above is really the way it works, and I am not claiming that it is, then the simple way to know when to calibrate your compass, after the first successful calibration, is to only consider calibration when moving, as DJI has stated, to some distance from the last calibration place. In order to know where to do that, you need a Magnetic Compass Calibration Zone Map. Attached below is a Magnetic Declination Map that covers our Hemisphere. As long as you stay within your "numbered" area, where you did the last calibration, you should always be good to go without doing a Compass Calibration before that next flight.