Abstract
To study the regional effects of the secular variation of the Earth's magnetic field, it is shown that the normal field in a region may be expressed as a limited development in spherical functions. The formulae obtained would be directly comparable from one region to another as well as for the geomagnetic field. As an example, the field has been calculated in France since 1948. Emphasis has been particularly put on the calculation of the precision obtained on the Gauss coefficients.
