This International Standard defines reference models representing the geomagnetic field. It closely mirrors and clarifies specifications which have been in use for many decades.

The approach is to represent the corresponding scalar magnetic potential by a spherical harmonic expansion having specified numerical coefficients, called Gauss coefficients. This International Standard covers models in which, at any one time, changes of the magnetic field are modelled by a linear time-dependence of each Gauss coefficient. A model might consist of a succession of sub-models, in each of which the coefficients change linearly with time. For such a step-wise linear model, the coefficients are continuous in time. This International Standard provides the formulae and a step-by-step computational procedure to evaluate a geomagnetic reference model for any desired location and date.

This International Standard does not specify the interpretation of the geophysical content of a geomagnetic reference model. It is left to the producer of a model to specify which internal and external magnetic sources are included in (or excluded from) their model.