The maximum spherical harmonic degree of the expansion is ” a,,rr)(cosmnPnm1nnm.N
“The IGRF models for the main field are truncated at (120 coefficients) which represents a compromise adopted to produce well-determined main-field models while avoiding most of the contamination resulting from crustal sources. The coefficients of the main field are rounded to the nearest nanotesla (nT) to reflect the limit of the resolution of the available data. The IGRF models for the secular variation are truncated at (80 coefficients). This time the coefficients are rounded at the nearest 0.1 nT/year, this time to reduce the effect of accumulated rounding error. Coefficients for dates between the 5-year epochs are obtained by linear