(update) I changed the settings to higher field levels and then started working correctly. Now 4 stars. (initial review) The 3D arrow is using data from the accelerometer, not the magnetometer. So just pointing to the center of the earth. It could have 5 stars if fixed.
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Developer reply
The real magnetic fields lines, which are shown in this app, are tilted always toward the Earth's center. Everywhere except at the Equator. To confirm the magnetic data our app uses just move the phone near a speaker or a permanent magnet, and see how the vector changes. Waiting for your new rating, Microsys Team.