Spherical Harmonic Analyses for the Spheroidal Earth

Most spherical harmonic analyses of the surface geomagnetic field assume that the earth is a sphere. Two methods are devised for correcting the coefficients derived from such analyses for the oblateness of the earth's surface. The corrections required are shown to be significant. The largest, the change in the g30 coefficients, is about 80γ. The appearance of an erroneous unipole term in spherical harmonic representations of the geomagnetic field is discussed. The calculation of such a term is shown to yield information regarding absolute errors in geomagnetic data.