Here is a picture of the 3 degree east isogonic line just north of Tulsa, OK. enter image description here

Here is a picture of the 4 degree west isogonic line just east of Nashville, TN. enter image description here

They both trend westward in relation to true north as you follow the lines northward. Why do they not go in opposite directions if one is to the east and the other to the west? This also implies that the 0 degree isogonic line is not parallel to true north.

I checked, and sure enough, it also goes west as you follow it to the north. enter image description here

Why don't the lines tilt in different directions after crossing over from zero degrees?


Because the present position of the magnetic pole pole is such, that in central and eastern US states the isogonic lines are tilted to the West. But in western parts of the U.S., those lines are tilted to the East...

enter image description here

  • $\begingroup$ I thought the isogonic line variation was intended to correct the offset from true north? If they all tilt the same direction then how could they change from east to west without having paralleled true north at some point? $\endgroup$ – Ryan Mortensen Sep 21 '19 at 4:23
  • $\begingroup$ @Ryan Mortensen There are points where the magnetic declination is zero. Those points define the isoline of declination zero, but that line does not necessarily has a zero inclination in respect of the (geographical) North-South line... $\endgroup$ – xxavier Sep 21 '19 at 4:41
  • $\begingroup$ Could you elaborate on this in the answer? That is what I do not understand. I was always taught, or at least thought I was taught that the declination was needed to correct to the angle from magnetic north to true north. Is that not so? That's my point of confusion. This answer feels to me to be very distal to my question rather than proximate. $\endgroup$ – Ryan Mortensen Sep 21 '19 at 15:14
  • $\begingroup$ @Ryan The isogonic lines do run directly north–south at certain points. (Just pick a line of latitude, and scan east or west along that line until you find a point where the isogonic lines are running north and south.) But the set of all such points is not, itself, an isogonic line, and in particular, it's not the 0° isogonic line. $\endgroup$ – Terran Swett Sep 22 '19 at 3:19
  • $\begingroup$ @TannerSwett I seem to have a fundamental misunderstanding of the purpose of their existence. $\endgroup$ – Ryan Mortensen Sep 22 '19 at 3:53

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