Distribution of Geomagnetic Field and Its Secular Variations Expressed by the Surface Spline Method in China (A Part) for 1900–1936

Based on the geomagnetic data at 426 geomagnetic sites and the geomagnetic data calculated at the 28 IGRF points for 1936.0, the surface Spline models for various components of the geomagnetic field and the geomagnetic anomalous field are calculated, and the corresponding geomagnetic charts and the geomagnetic anomaly charts are drawn. According to the secular variation charts of the geomagnetic declination D for 1909~1915, 1915-1920, 1920~1930 and 1930~1936, the horizontal intensity H for 1908~1917, 1917~1922 and 1922~1936, and the vertical intensity Z for 1908~1922 and 1922~1936 in China (a part), the surface Spline models of the secular variations are respectively calculated for D, H and Z for the above-mentioned 9 periods by using the surface Spline method, and the corresponding secular variation charts are drawn. According to these secular variation models, the absolute values of the three components at 426 sites for 1936.0 are reduced to the data for 1940, 1930, 1920, 1910 and 1900. These data have laid a solid foundation of calculating the geomagnetic field models for these 5 epochs.