Following the derivation in the Freescale application note AN4248 very closely, it is possible to derive a system of equations for the yaw (psi) angle and the pitch (theta) angle, given the local magnetic field strength B and magnetic inclination angle delta. Here, Bx, By and Bz are the magnetometer readings in the usual orientation (north is X and Z is down). Unfortunately the system is highly nonlinear and after pondering it for a bit, I don't see a way to solve it for theta and psi. Any takers?
