Since it is thought that the magnetic field is produced mostly 5000 kilometers below the surface, an altitude increase of 50 kilometers (1%) should produce a 10% decrease in magnetic field strength. Occasionally this is the amount we measure, but variations in magnetite just below the surface can produce larger variations, especially within a few meter of the surface. We likely have extensive, but clasified data on magnetic field at various altitudes for missile tracking purposes, even though the magnetic compass is rarely used for navigation now that GPS is available cheap. Before GPS, we had loran and lorac which were also far more accurate than a compass. Neil