The most frequent reason for UAVs crashing during magnetic surveys is collisions with trees. Alexey Dobrovolskiy offers a workflow that can help stop that.
UAV mission planning for magnetic surveys typical for exploration geophysics/minerals prospecting is noticeably different from another large class of magnetic surveys that detect small/artificial objects such as UXO (unexploded ordnance), utilities and archaeological items, requiring extremely low altitudes (starting from a few dozens of centimetres) and usually using UAVs capable of following terrain using radar or laser altimeters.
Magnetic surveys for geophysics/minerals prospecting usually have a flight altitude of a few dozen metres above the ground and are very often conducted over forested areas in rough terrain. There are a few key factors for successful magnetic data collection:
Elevation of the sensor (UAV) above ground level should be more or less the same.
The flight altitude should be as low as possible to collect high-resolution data but high enough to not crash the UAV into tall trees.