Magnetic Field around step-down converters
The magnetic field near circuitry carrying about 1 amp could be a problem 20cm away, but for a compass application you can get there if a few steps are taken in the design. I would wind toroids for the buck inductors (distributed air gap) and keep them well out of saturation to keep the field contained in the core, and would lean toward continuous mode to keep discontinuities at a minimum.  Make sure your current paths and the corresponding return path follow close and parallel traces; keep the separation at a minimum so that you don't create large current loops.  Use twisted wire from your power source and out to any load.  If you take these precautions, you can easily keep the field associated with the circuit below 100 nT 20 cm away - a small but not insignificant error contribution since the earth's field is around 50 uT, about 2%.  You might be able to do better than this.