The magnetic field of the Earth originates in the motions of the fluid core of the planet. By good fortune it is predominantly dipolar and so has a simple topography. The core motions cause the field to change with time, and once charted it has to be charted again and again. Mathematical models can be made which represent the geomagnetic field to an accuracy of about one degree globally. Higher accuracy demands advanced techniques such as satellite surveys, and also attention to the detail of external fields caused by solar activity which disturb the main field, restricting the accuracy of mapping and producing magnetic deviations from the mean value from day to day. Understanding such effects is essential in preparing magnetic reference data for navigation.

Though now losing ground to other methods, magnetic compasses still have a valuable role to play in many situations, so long as the limitations are fully appreciated and taken into account. Modern technology in standard geomagnetic observatories offers ways of updating a datum very accurately in automatic navigational systems and even of forecasting disturbances, and may bring magnetic reference back to the forefront of many navigational systems.