Magnet for picker/dropper

[slup]

...is it right that if before take off, ensure the drone direction on the ground, matches that of the map, it *should* avoid the compass / IMU error ?

Yes, if the drone icon on the app map is pointing equal to what the drone does in reality ... you know that the IMU have been initialized correctly.

During take off if it gets closer, it may upset things, once airborne should be ok ?

Both the compass & the gyros will start to constantly feed in corrections to the IMU immediately after the IMU initialization. Meaning that if the drone gets into a magnetic interfered environment some time after the IMU initialization, the IMUYaw will slowly start to change.

Magnetic fields during flight is somewhat bad, if in them for a shorter period no harm but constantly is a no, no ... magnetic disturbance during powering on is always catastrophic.