agonic line
From Glossary of Meteorology
The line through all points on the earth's surface at which the magnetic declination is zero; that is, the locus of all points at which magnetic north and true north coincide.
This line is a particular case of an isogonic line. The position of this line exhibits variations in time, but in 1995 was so located that it emanated from the north magnetic pole, trended southward through the Great Lakes region, left the American mainland near Mississippi, cut across South America to near Buenos Aires, thence through the south magnetic pole, and up in an irregular path on the other side of the earth to return to the north magnetic pole. At the present time, the North American segment of the agonic line is drifting very slowly westward.
Compare aclinic line.
