On the basis of an analysis of the 7/8 December 1982 ground-level cosmic ray event the paper specifies the limitations of any mathematical model of the main geomagnetic field in its use for cosmic ray analyses, presents procedures and problems of combining models of the Earth's internal field with models of the external geomagnetic field, and discusses the significance of the external geomagnetic field for calculations of cosmic ray particle trajectories in near-Earth space.