Main Field, Ellipticity Correction
The first spherical harmonic analyses of the geomagnetic main field ignored the ellipticity of the Earth. This was not a serious omission, since the radial departures of the Earth's surface from a sphere never amount to more than 0.17%. However, as the quality of data improved, it was thought that they might justify inclusion of a correction for ellipticity.
The Earth's surface is closely approximated by an oblate spheroid with eccentricity 0.082. This affects both the position of an observation and the observation itself, since the horizontal component, for example, is measured tangential to the spheroid rather than perpendicular to the line joining the site to the Earth's center. Early analyses that took account of this ellipticity (Schmidt, 1898; Adams, 1900; Jones and Melotte, 1953) first determined coefficients for a spherical Earth, then adjusted these for ellipticity. A better method (Cain et al., 1963) was based directly on the original observations.
Adjustment from geodetic to...
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