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I have been working on an electronic compass. It has become apparent the best way to calibrate the magnetometer is once out in the open with out any large metal object nearby.

Curiously, what I have found, any further calibration, for when the magnetic field changes, degrades the overall electronic compass effectiveness.

My use case is not inside a car or other similar metal-cage. And unlikely inside a building. I am expecting to use the compass out doors.

Many existing commercial applications (such as a phone) occasionally ask to be re-calibrated? If these devices were only to be used out of doors, would it not be better to only calibrate these devices once at the factory and never again?

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Yes, calibration inside a large ferromagnetic object, such as a car, does mean that the compensation is only correct at that spot inside that car. For a portable compass, compensation should be done far away from any metallic structure (or magnetic geologic feature, such as iron oxide deposits) -- but for a magnetic compass installed in a car, or in a boat's binnacle, compensation must be performed in place., to take into account the intrinsic field of the vehicle. That field also changes over time, e.g., if the vehicle has been left in one position, or if repairs have been performed.

Field calibration of magnetic compasses is needed because the North Magnetic Pole is not at the same location as the Geographic North Pole, 90°N. It is several degrees away, and a magnetic compass pointing "North" has to be compensated for that magnetic declination. In the United States, for example, compass compensation in the state of Maine is 40° degrees different from that in the state of Washington!

Yes, I've spent some time driving my car in circles and adjusting the two sets of compensators (E/W and N/S) on old magnetic compasses, halving the offset each time. This also compensates for the magnetic field of the car (and radio speaker magnets!). sigh.

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    \$\begingroup\$ Yeah. For me, it was the Cessna that had to be rotated around. See AC 43-215. ;) \$\endgroup\$ Dec 10, 2023 at 5:30
  • \$\begingroup\$ "Field calibration of magnetic compasses is needed because the North Magnetic Pole is not at the same location as the Geographic North Pole...". This can be corrected for by using this magnetic declination table from noaa.gov. I do not think calibration can help with magnetic declination. That is a good point about compasses inside of cars, boats and planes. I will re-word my question to exclude those. I want to know if an already calibrated electronic compass ever needs to be re-calibration if used out of doors on a person. \$\endgroup\$
    – st2000
    Dec 10, 2023 at 6:44
  • \$\begingroup\$ I was looking for compelling reasons in favor of the end user re-calibrating a compass who is meant to be used only out of doors. I hold the opposite opinion that re-calibration by the end user is a bad option which will likely degrade how well the compass works. I realize magnetometer calibration is necessary. I just what to know if there are good reasons (& what are they) for doing it more then just once at the factory. \$\endgroup\$
    – st2000
    Dec 11, 2023 at 18:15

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