Abstract
This study presents some characteristics of geomagnetic models computed from magnetic observatory data. We first analyse the covariance matrix of the Gauss coefficients for different data distributions and different truncation degrees of the spherical harmonic expansion. We then compute the ratio R of the mean square value of the diagonal terms of this covariance matrix to the mean square value of its off-diagonal terms. Finally, we estimate the accuracy of two main-field models, computed using annual mean values from 132 magnetic observatories and two weights series. The effect of the uneven distribution of observatories on global modelling of the geomagnetic field and its secular-variation is underlined. The accuracy of such models is good enough in the areas where the density of observatories is high enough (Europe, North America, East Asia), but it decreases dramatically in poorly covered areas.
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