Magnetic declination

[AJMD429]

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It seems to me I remembered as a kid that the topographic maps near where I lived were something like 3 degrees W declination, so I figured I'd check the current status of Indiana since I know the north magnetic pole kinda moves around. I got two totally different readings; 1 was -5 degrees which would be five degrees West, and the other was positive 10 degrees east...! Both appeared to be current data.

Then I realized that somehow when I looked for 'Indiana', one of the searches was giving me results for a town named 'Indiana' in Pennsylvania...

What struck me was I had figured the 'west' declination for Indiana meant that the magnetic north was somewhere west (relative to Indiana) of the rotational axis, sort of northern Alaska or something. Yet Pennsylvania data makes it look like more towards Greenland. Then if you go out to Wyoming it's also east, which at least sort of 'intersects' with the Indiana declination.

I guess the deal is that big mountain ranges affect it because of all the material there, But even that seems funny if you think about the fact that a mountain range is massive and maybe averages a mile higher than the surrounding terrain, but it's on a Planet with a diameter of 8000 miles or so, and a 'crust' that likely isn't smooth on the inside boundary either.

I saw one map that showed a bunch of wiggly lines representing declination. It was just a map of the central United States but the lines were certainly very wavy, probably relating to geologic features. I couldn't help but wonder if you got above the surface of the Earth a few dozen or 100 miles if the magnetic field then becomes much more uniform and geometric.

Supposedly the poles will soon (in geologic time) and rather abruptly (dunno how 'geologic' they mean by 'abrupt') switch north-to-south. THAT should be kinda interesting. Humans (if still around) will figure it out no doubt (and Al Gore's descendants will no doubt make money fear-mongering and offering 'solutions'... ), but the poor migratory birds may have some real issues and even extinctions. But you have to wonder what sorts of currently-stable geologic features (like iron-ore containing deposits in mountain ranges) will be suddenly tugged differently and trigger earthquakes or whatever.

[jeepnik]

Polar shift is still an unproven hypothesis. If Cussler hadn't written his book most folks would never have heard of it.

As to declination. Yes, it does slowly shift. If you have an old map on which the declination is marked (most topos) you can google it and get the current declination. I just write in in ink on the map. More importantly, declination can be effected by large ore deposits in the earth. Or even not so large hunks of metal not that far away. Those aren't always marked on maps. If you are using map and compass you need to check your map orientation with triangulation.

One of the things I get a kick out of is in the movies the hero lays his rifle down, then sets his compass next to it to get a bearing. Poor guys would actually just wander around in circles.

[Gobblerforge]

The semi-molten mantle rotates differently than the solid crust. The iron core moves and alters the magnetic poles.

[geobru]

My first exposure to declination was in 1978 when I started working as a forester in Washington's Cascade range. The declination was 21.5 degrees East. This thread made me wonder what it is now. I looked it up and its now about 14.6 degrees east. That got me to wondering how far the magnetic pole has moved and I ran across the following, which I thought is kind of interesting as it illustrates the impact of one degree on getting to where you want to go.

https://whitehatcrew.com/blog/a-mere-on ... ifference/

Consider this. If you're going somewhere and you're off course by just one degree, after one foot, you'll miss your target by 0.2 inches. Trivial, right? But what about as you get farther out?

After 100 yards, you'll be off by 5.2 feet. Not huge, but noticeable.
After a mile, you'll be off by 92.2 feet. One degree is starting to make a difference.
After traveling from San Francisco to L.A., you'll be off by 6 miles.
If you were trying to get from San Francisco to Washington, D.C., you'd end up on the other side of Baltimore, 42.6 miles away.
Traveling around the globe from Washington, DC, you'd miss by 435 miles and end up in Boston.
In a rocket going to the moon, you'd be 4,169 miles off (nearly twice the diameter of the moon).
Going to the sun, you'd miss by over 1.6 million miles (nearly twice the diameter of the sun).
Traveling to the nearest star, you'd be off course by over 441 billion miles (120 times the distance from the earth to Pluto, or 4,745 times the distance from Earth to the sun).
Over time, a mere one-degree error in course makes a huge difference!

[Twodot]

Grimmy.JPG

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[jeepnik]

Geobru is sort of right. But as you move the declination changes. So while a straight line from the original bearing might end up in Baltimore depending on how often you took a bearing it would be different.

So if you're traveling by foot and taking daily or better morning, noon, and evening bearings you'd come out in D C. A plane it might have a more significant effect if you didn't do course corrections.

If you think about it ship navigators, until the advent of really precise chronometers, did some pretty impressive feats of getting from point A to point B.

[AJMD429]

One of the things I get a kick out of is in the movies the hero lays his rifle down, then sets his compass next to it to get a bearing. Poor guys would actually just wander around in circles.

LOL yep - I've learned that when I'm sighting along a fence (i.e. to do a property-map sketch), I'd better be several feet from it if I want to avoid distortion.
Oddly enough though, I can usually get what appears to be a consistent reading pretty near the 765 kv transmission lines that cross our place...