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.