  When optically pumped magnetometers are aimed for the use in Earth's magnetic
field, the orientation of the sensor to the field direction is of special
importance to achieve accurate measurement result. Measurement errors and
inaccuracies related to the heading of the sensor can be an even more severe
problem in the case of special operational configurations, such as for example
the use of strong off-resonant pumping. We systematically study the main
contributions to the heading error in systems that promise high magnetic field
resolutions at Earth's magnetic field strengths, namely the non-linear Zeeman
splitting and the orientation dependent light shift. The good correspondence of
our theoretical analysis to experimental data demonstrates that both of these
effects are related to a heading dependent modification of the interaction
between the laser light and the dipole moment of the atoms. Also, our results
promise a compensation of both effects using a combination of clockwise and
counter clockwise circular polarization.
