The relationship between primary cosmic ray particles in space and the associated secondary particles observed using ground-based detectors can be described by a set of coupling coefficients. These coefficients depend, among other things, on the direction that the detector is pointing through the geomagnetic field. Along certain directions the coefficients may be highly sensitive to small changes in the field. The accuracy of the field model thus places restrictions on the accuracy of the coupling coefficients. Small errors in the field model can give large relative errors in the coupling coefficients. This will consequently give large relative errors in the interpretation of the space distribution of the primary particles inferred from measured data.