HMC6343 Compass module

i trying find angle of change on device rotates , forth (picture door rotating , forth door frame), not move on fixed axis. find angle of change, using these compass sensors.
one sensor (reference) fixed , other on end of moving device. theory if sensors starting positions oriented in same direction, initial difference should 0 , subtracting moving sensor heading fixed sensor heading should give accurate difference when device moves. works in fixed location, move entire system location (the sensor orientations in relation each other not changed)  difference between sensors changes anywhere 5-15 degrees.
i have tested indoors , outdoors , calibrated many times, , still have issue. have fundamental theory magnetometers wrong? don't care getting accurate heading, difference between 2 sensors assume should same no matter if oriented same way.

any current carrying wires, magnetic fields or iron-containing materials near magnetometer introduce local distortions in earth's magnetic field, , cause orientation errors of 5-15 degrees (and worse).

after calibration, compass work outdoors (that is, track orientation changes +/- 1 degree, full circle)?

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