Some comparisons among geomagnetic field models, observatory data and airborne magnetometer data - Implications for broad scale anomaly studies over Canada

The secular change characteristics of several geomagnetic field models have been compared with the observed secular changes at sixteen magnetic observatories in Canada and nearby regions, for the period 1960 to the present. The significance of inadequacies in the secular terms of the field models when used as reference fields for the compilation of large scale magnetic surveys is discussed. Errors of the order of 50 nT/year are encountered with some models. A model derived primarily from satellite data shows the best description of the secular change in the vertical component in western Canada during the interval 1969 to 1973, which includes the epochs of several airborne surveys. The revised IGRF75 describes the secular changes more closely than does the original IGRF (International Geomagnetic Reference Field), but cannot account for recent secular accelerations. Comparisons between field models and an extensive airborne data set show good agreement in the vertical component between the airborne survey data and two satellite-based models. Correction surfaces are developed to reduce the survey data to a common epoch, by considering differences between observatory monthly means and the corresponding values predicted by the field model.