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The total magnetic field near the surface of the Earth is a sum of several constituent fields. Part of the total field consists of fields that are transient or rapidly varying. These fields are caused, either directly or indirectly, by electric currents in the upper atmosphere and beyond. The part of the total field that is more permanent arises from sources that are located inside the Earth. Evidence suggests that this part has two principal constituents: the main field and the crustal field.

A new model of the near‐surface geomagnetic main field and its secular variation has been derived. The main field part, a degree 12 spherical harmonic series of 168 internal source terms, was derived from 1248 representative values having a nearly even distribution over the earth. These were created by an intermediate analysis of approximately 100,000 reduced original observations made since 1939. The secular variation part, a series of 80 linear time terms, was derived from observatory data exclusively. The model is the basis for the 1975 edition of the U.S. World Magnetic Charts.

A mathematical description of the geomagnetic field for 1970 has been derived by using 360,000 land, sea, and airborne measurements accumulated since 1939. The model is expressed by 168 spherical harmonic coefficients gnm, hnm to m = n = 12. It was used as the basis for the 1970 edition of the U.S. World Charts of Magnetic Variation.

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