Isopor [isoporic line]

An isopor [iso- and from the Greek pĆ³ros “passage” thus “passing (in the signification of “varies”) in the same way.”], or isoporic line, is an imaginary line connecting all points on the Earth’s surface that exhibit the same variation of one of the elements of the Earth’s magnetic field (declination, intensity, components, etc.) in a given time interval; usually annual variations are considered.