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Magnetic North Pole
We talk a lot about using a topo map and compass when out in the wilderness but if you don't understand how the earth's magnetic poles work then a compass can actually get you in a lot of trouble depending on where you are. Let's look at this map I swiped off the internet.
http://i348.photobucket.com/albums/q...h-magfield.jpg
You can see the lines of magnetism radiating out from the south pole and into the north pole. If you look very very closely at the white needle on the compass, you can see that I'm pretty lucky because the agonic line passes almost perfectly through Indiana. The agonic line divides the east and west magnetic variations. It runs from the western shore of Hudson Bay, through Lake Superior, western shore of Lake Michigan, off the west coast of Florida, and through Cuba to name a few places. If you live along the agonic line then your deviation between magnetic north and geographic north is 11.5 degrees (see picture). But your magnetic north is very accurate. The further away from the agonic line you live the more deviation you have to account for when using a compass and a map. In Washington State or Maine your compass can be off by 20 degrees. In northern Canada it can be as much as 40 degrees! In Greenland over 50 degrees!!
Well that's great but what does it mean?
A decent compass used along the agonic line will usually give you a "true" reading to magnetic north. The further away the more you have to adjust your compass. For example, in southern California a compass will point north but actually be pointing in the direction of Nevada because the magnetic lines surrounding the earth bend so much. In order to travel in a geographic north direction by compass, a person in southern California would have to travel in a magnetic northwest direction. In other words, if you want to go to Oregon by compass you would have to travel in a northwest direction if you didn't adjust your compass. If you lived in Fairbanks, Alaska your compass will read magnetic north and actually point in a geographically southwest direction!!!! If you were lost and wanted to head geopraphic south and started walking without adjusting your compass you would actually walk geographic north!!!!
So what can I do?
First, be aware of just how much your compass will deviate based on where you are. Second, understand the difference between magnetic north and true north. All lines of longitude on a map are true north lines. True north is usually symbolized by a star on a map while magnetic north is symbolized by half an arrowhead on a map. Finally, look for the declination marked on a map. Not all maps have them but topographic maps and maps from the US Geological Survey do have them. They are marked MN for magnetic north. In our southern California example a topo map should be marked "14° W" indicating that magnetic north lies 14 degrees counter-clockwise from true north.
Understanding how a compass works and what your local declination is are critical elements in being able to navigate by compass.
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And be aware that any electronic equipment, motors, electric fields, alternators, or metallic masses close by will affect your compass as well.
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Yeah, that electromagnet at the scrap metal yard will give it fits. Hard to find your way around there.
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And...
Rick, old buddy, I think you should have put this in your "Things I need to know to survive in the wilderness" sticky, this is "Must Know Info!":cool2:
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I agree with Sarge. For those without the training (military or otherwise) this is a good primer.
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Im lucky here in Florida, the deviation is too slight to bother with unless maybe in an airplane. Magnets can reverse the polarity on compasses if they are stored togeather too. I have one. The red tip on the needle used to point north, now it points south. I had it in a fanny pack beside a fridge magnet thermometer. Thats the only thing I can think of anyway so beware.
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I find it fascinating that the field (and declination) changes over time.
http://upload.wikimedia.org/wikipedi...90_to_1990.gif
This kind of interesting about how the field appears to "flip" over time.
(That and the fact that I really love the area that this article speaks about).
http://scicom.ucsc.edu/scinotes/9501/Mountain.html
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It a shame it speeding up which mean it may flip soon.
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