tokens: IGRF 10 [ RN.29 ] shall be used as the Internal Geomagnetic Field Model . NOTE As a stand - alone model , the inputs required of the IGRF-10 are either position in geodetic coordinates , according to the reference ‘ World Geodetic Service 84 ’ standard ellipsoid , or position in a geocentric system , with respect to the centre of the Earth and for which the reference Earth radius is 6 371,2 km . The coordinates are therefore geodetic or geocentric latitude in decimal degrees , longitude ( same in both coordinate systems ) , altitude in km above the reference ellipsoid , or distance in km from the centre of the Earth . Time is input in decimal years , e.g. 2 007,5 . It is recommended that geocentric data are used , to avoid errors or confusion in respect of the reference ellipsoid .