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I am trying to understand some plots of the earth's magnetic variation (declination) by trying to correlate what I see in a figure like the one below with the variation reported for some airports. I'm particularly interested in those locations where the figure seems to show an extreme variation of up to 90 degrees. But when I look up nearby airports, I don't see any variations values outside of 22 degrees.

Some locations I have checked are

  • SOG, Sorong, Indonesia (1.47 degrees)
  • BWN, Brunei (0.13 degrees)
  • CMB, Bandaranaike International Airport , Sri Lanka (2.15 degrees)
  • REC, Recife, Brazil (22.12 degrees)
  • IVA, Ambanja, Madagascar (9.8 degrees)

Am I misreading the figure? What are some airports with very large magnetic variations? I'd like to compare them with this figure and try to understand what I am missing.

enter image description here Source: Wikipedia

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    $\begingroup$ Those lines are named isogonal lines, meaning lines of equal magnetic declination (from iso=same like in isotherme, and gone=angle, like in goniometer). $\endgroup$
    – mins
    Nov 3, 2016 at 22:15
  • $\begingroup$ I think that gone=angle would be recognized by more people from trigonometry, the measurement of bodies with three angles. $\endgroup$
    – dotancohen
    Nov 5, 2016 at 14:54

4 Answers 4

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How do you think "the figure seems to show an extreme variation of up to 90 degrees"?

I don't think you're understanding that diagram correctly.

There is nothing close to a variation of 90° anywhere that is populated.

The angle of the curve has nothing to do with how strong the variation is. All points on a single curve share the same variation. If you find a 10° variation on a curve, and trace that curve a thousand miles away, that point will also have a 10° variation.

Look at about Washington DC, with -10° variation. Follow that curve all the way down to the middle of South America. That point also has -10° variation.

Look near Denver, CO, where it is +10°. Follow that curve out to near eastern Hawaii. The curve there is almost due East-and-West, but that doesn't matter. That area also has +10° variation, because it is on the same curve.

The bold curves are at 10°, 20°, 30° etc variation.
The red curves have a positive (east) variation.
The blue curves have a negative (west) variation.

All of this information is in the legend at the bottom of the map.

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    $\begingroup$ Actually, down by the bottom right of the chart there are some areas of 90 degrees.... (Everything else is right though!) $\endgroup$
    – Lnafziger
    Nov 3, 2016 at 21:43
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    $\begingroup$ I had a completely different idea that this represented the field line projected onto the map. Thanks for clearing that up. $\endgroup$
    – Jim
    Nov 3, 2016 at 21:44
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    $\begingroup$ @Lnafziger : Any active cities or airports there? I didn't think so. :) $\endgroup$
    – abelenky
    Nov 3, 2016 at 21:44
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    $\begingroup$ @abelenky No, however before your edit (made 30 seconds ago), you said "There is nothing close to a variation of 90° anywhere." which was patently false, and even misleading.... :) For that matter, it actually clouds the issue of how to read the chart and would probably be better stated as "There are areas of 90 degree variation close to the poles, however not in the areas in which you are looking..." $\endgroup$
    – Lnafziger
    Nov 3, 2016 at 21:48
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    $\begingroup$ @abelenky look carefully and you can find an active airfield there. It is extreme case though. $\endgroup$
    – vasin1987
    Nov 4, 2016 at 20:30
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Take a closer look at the map. The contour lines show the places where the magnetic deviation is the same. The green lines specifically show where the deviation is zero, so in Indonesia the deviation will be close to zero. The green line runs roughly East-West, but that's indication that there is a region roughly East-West where the declination is zero.

The more extreme deviations occur close to the magnetic poles which are, in global terms, close to the geographic poles, and there aren't many airports in those regions.

I live in Wellington, New Zealand. Take a look at the southern magnetic pole and find the nearest contour line that runs through the lower North Island - it's 22deg East, which is what I set my compass to.

Now look below that on the map to find the 90deg contour. It loops around mostly within the confines of Antarctica.

That close to the magnetic pole strange things happen. At McMurdo Station the declination is 144deg (not sure whether that's East or West but it doesn't matter for this). Before adjustment their compasses actually point South!

More info on McMurdo Station here

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The lines in your plot indicate paths along which the magnetic deviation is constant.

If one plots the direction of north as displayed by a magnetic compass, one gets the blue arrows in the following image:

enter image description here

I've also marked the regions where the direction of magnetic north deviates less than +/-0.1° from the geographic north in green, and you can clearly see the similarity to the green lines in your plot.

In addition, a deviation of more than +/-45° is marked red, and +/-90° cyan.

If you are looking for larger airports with large deviations, check for example Greenland, Canada, South Africa and New Zealand. Extreme cases could also be found on Antarctica, though they don't have so much traffic...

Just as reference: I used what I still have on disk from this answer.

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In fact, magnetic variation (often called declination) can be as much as 180°. If you walk from the axial south pole towards the magnetic south pole, or from axial north to magnetic north, your compass will be backwards.

Sketch of north compass needle pointing south when placed between the magnetic and geographical south poles.

Of course, the real magnetic field is rather "imperfect", as your diagram shows, so standing exactly between the two south poles might not give you exactly 180° declination. But anywhere along the line on your diagram right between the red and blue areas (your diagram shows it as the final red line) is at 180° declination.

You can count the bold lines to confirm this on your diagram.

Crop of original diagram with isogonal lines labeled up to 180.

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