Spherical cap harmonic analysis of the geomagnetic field in the Beijing-Tianjin-Hebei region

The three component absolute survey of the geomagnetic field at 45 field sites was carried out in the Beijing-Tianjin-Hebei region in 2002. The survey data were reduced, and the reduced time was 16:00similar to18:00(UT) on May 5, 2002. The RMS values after reducing are better than 1.5nT(for total intensity F) and 0.5'(for declination D and inclination I, respectively). Taking IGRF2000 as the main normal field in the region, the spherical cap harmonic models(BTHASCH)of the anomaly geomagnetic field in the region are established. The spherical cap harmonic models (BTHGRF) of the geomagnetic reference field in the region are calculated also. The pole of the spherical cap is at 39.5degreesN and 117. 0degreesE, and its half-angle is 4degrees. During the model calculation, the truncation order is taken from 1 to 10. According to comparison of results, the final truncation, order is taken as 5. Using this model, the geomagnetic charts (X, Y, Z, F, D, and 1) and the anomaly charts (DeltaX, DeltaY, DeltaZ, DeltaF, DeltaD, and DeltaI) in the region are produced.