|Quoting mawingho (Thread starter):|
Isogonic lines are where the declination has the same value, and the lines where the declination is zero are called agonic lines."
Sounds like isogonic lines are the set of continuous points where the magnetic variation stays constant.
So if you start somewhere with a magnetic variation of, say, +5 degrees, then the line you follow to maintain a magnetic variation of +5 degrees is an isogonic line.
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".