Harmonic Analysis for a Spheroidal Earth.

by Anne B. Kahle

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A study of the errors in geophysical data produced by using uncorrected spherical harmonic series over the nonspherical surface of the earth. The coefficients derived do not represent true potential functions: One cannot accurately be derived from another, nor can the field be correctly extrapolated inward and outward through source-free regions. Formerly, these errors were small compared with the errors in the data. Now that the field is being determined more accurately, the effect of the oblateness of the earth becomes more significant: An error of up to 120 gammas may result from failure to consider it. Of several recent analyses suggested for consideration in selecting an International Geomagnetic Reference Field (IGRF), only one was originally referred to as a true sphere. As the sets are not consistent, the selection of a median would not be desirable. The corrections required for the spheroidal shape of the earth and their implications are factors to be considered in selecting the IGRF. 12 pp. Refs.

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