Gentlemen,

Thank you for those links. The model of the magnetic field shows amazing complexity - it resembles the head of a rather tatty corn dolly. I was aware that the strength and direction of the field varies with time and place and the Geoshare site confirms that. In fact what that site says leads me to doubt whether an agonic line, other than a very short local one, could in fact be drawn. Perhaps the technology has developed since the space age and a more simplistic approach was taken in the '50s. I guess that it is of more significance in mapping, for example, than in surveying for the construction industry. Because the link is to a Geoshare site the information one can reach from it is connected to the oil industry. I'll look wider.

Thanks again. dxb.