[电] 无偏线

[ noun ]
an imaginary line connecting points on the Earth's surface where the magnetic declination is zero
<noun.location>

Agonic \A*gon"ic\, a. [Gr. ? without angles; 'a priv. + ? an
angle.]
Not forming an angle.

{Agonic line} (Physics), an imaginary line on the earth's
surface passing through those places where the magnetic
needle points to the true north; the line of no magnetic
variation. There is one such line in the Western
hemisphere, and another in the Eastern hemisphere.