ISO 16695:2014 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. ISO 16695:2014 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. ISO 16695:2014 provides the formulae and a step-by-step computational procedure to evaluate a geomagnetic reference model for any desired location and date.
ISO 16695:2014 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.