World Magnetic Charts for 1980 -- spherical harmonic models of the geomagnetic field and its secular variation

The 1980 World Magnetic Charts, published by the UK Hydro-graphic Department and by the US Defense Mapping Agency, Hydrographic Topographic Center, have been produced jointly. They are based on spherical harmonic models of the main geomagnetic field (168 coefficients) and of the secular variation (80 coefficients). These models are presented here with a discussion of the methods used to derive them and of the data on which they are based. Positions of the dip-poles, the geomagnetic poles and the eccentric dipole, together with their rates of change, are derived from the models.