A discussion of sources and description of the earth's magnetic field and its secular variations

The problem of collecting data for making geomagnetic charts including secular change is reviewed. Satellite data give excellent coverage, but the satellite total field intensity data are not sufficient to properly define the field components. In constructing charts, the specific time and space filtering that is used should be specified so the user can make needed corrections in the use of the charts. An attempt should be made to include time variations having periods of one year or greater. In the space domain spherical harmonic coefficients up to degree 12 or 13 should probably be included. Adequate charts in the future will likely require the use of some external coefficients.