Influence of boundary points selection on the accuracy of spherical cap harmonic model of Mongolia magnetic field

During the period of 2011 to 2012， Institute of Geophysics， China Earthquake Administration has partnered with Research Center of Astronomy and Geophysics of Mongolian Academy of Sciences， and accomplished geomagnetic survey (total intensity F， magnetic declination D， magnetic inclination I) of 119 stations in Mongolia. We calculate nine spherical cap harmonic models of geomagnetic anomaly field in Mongolia of 2010.0 epoch by using the accurate geomagnetic data of 119 stations and adding selected boundary point values of nine groups calculated by EMM2010 model respectively. And then we give the contour maps of north component ΔX， east component ΔY and vertical component ΔZ， by which the nine models are compared. Root mean square (RMS) error of the difference between geomagnetic survey values and calculated values at 119 survey stations is calculated to represent the accuracy of the nine models so as to evaluate quantitatively the influence of boundary points selection on the accuracy of the models. The RMS analysis results indicate that， besides appropriate boundary points selection can improve model accuracy， the boundary points away from the measuring region 3°—5° can also improve model accuracy. Therefore， the appropriate boundary points should be introduced to improve accuracy of geomagnetic model in establishing model.