An international geomagnetic reference field (IGRF 1975) has been recommended by a study group of Division 1 of the International Association of Geomagnetism and Aeronomy (IAGA). The recommendation was adopted on September 4, 1975, by IAGA at its general assembly at Grenoble. The reference field is given in the form of 80 main field coefficients and 80 secular variation coefficients, up to the eighth degree and order, of a spherical harmonic expansion. The coefficients are Schmidt quasi-normalized [Chapman and Bartels, 1940]. For values on the surface of the earth the international ellipsoid is used: equatorial radius 6378.160 km and flattening 1/298.25 [International Astronomical Union, 1966]. IGRF coefficients refer to a radius of 6371.2 km.