Hi, The Earth's magnetic field at the surface is roughly 0.5 Gauss or 0.05 mT. If the Earth's field strength diminishes in proportion to the inverse square of the distance, surely the field is going to be negligible at 10,000m altitude. Yet as far as I am aware it will still have an effect on the plane's compass traveling at such an altitude. Can someone please help me out with the maths to explain this scenario? Thank you.