MAGNETIC-FIELD WORK AND IGRF MODELS FOR MEXICO, 3 EXAMPLES FOR THE 20TH-CENTURY

The historical field work inside the current Mexican area since the XV II century is presented as an introduction to the comparison study don e on three examples of these magnetic surveys during the XX century, i n particular the years 1907, 1952 and 1990, in order to find how well the IGRF fits this area. The magnetic declination was used because of the better availability of the magnetic declination maps for the perio d studied. The values given by the IGRF were compared with the values from magnetic surveys in the Mexican Republic. Results of these compar isons show RMS differences for the magnetic declination of 52.5' for 1 907, 115.4' for 1952 and 18.7' for 1990. Means and standard deviations of the residuals are discussed. The isolines shows a possible underes timation of the surface field in 1907 and an overestimation in 1952 by the IGRF. Long-wavelength features were found to be correlated with a eromagnetic and satellite magnetic anomalies, but, in any case, the IG RF seems to be a satisfactory geomagnetic reference model for the area and epochs studied.