Mag / compass calibration method

I describe method that provides magnetometer values on all points of a sphere, for calibration of an electronic magnetometer, or compass.
Some type of movement is needed when calibrating a magnetometer in Betaflight, for example. The intent is to have each of the X, Y and Z axes pointing to every possible point on a 'sphere' which represents the magnetic field detected on each axis.

In this case I already know roughly where magnetic North is, both in terms of declination (12 degrees East) and inclination (about 65 degrees upwards). So I start with one axis pointing roughly directly into that field. In this case, the X axis.

I then rotate the quad back and forth, so that the target axis swings over 180 degrees. Then I walk around, 'on the spot', in a 360 degree circle, while that axis goes up and down. This rotates my 'swinging' quad 360 degree, pointing the target axis to nearly every possible spot on a sphere. The 'stable' points are approximately directly North and directly South, so we get lots of North and South in the data, and that is good.

Once I've done the X axis, I yaw the quad 90 degrees and point the Y axis into the North of the magnetic field, and do the up/down 180 moves while turning 360 degrees again.

Finally I point the Z axis up into the field, and repeat for Z.

I need to do all this in 30s, so more quickly than in the video. It takes some practice but does return extremely reliable calibration values when used with Pichimeter's spherical calibration code (not yet merged).