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post #1 of (permalink) Old 12-18-2007, 06:47 PM Thread Starter
 
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Default Magnetic Declination

Hello everyone, I am relatively new to Canada from the UK which is what my experience of compass work is based on. When in the UK a familiar term to factor in the declination/ variation is "Mag to Grid - Get rid" ( Meaning subtract whatever the declination value is from your bearing when taking a magnetic bearing and transferring it to a map. ) or "Grid to Mag - Add" ( Meaning add the declination value to your bearing when taking a bearing from your map and shooting it in the field. ) Seem ok so far ?
Well the issue I have over here is that the position of MAG North has now changed for me since moving to western Canada. Rather than being a few degrees to the west of my European maps the declination now is around 17 degrees to the east. This obviously makes less room for error ( with the declination difference being much more severe.)
My main question is this :-
Would I be right in saying my old method of applying magnetic variation / declination should be reversed to accomodate the change in my relation to MAG North ?
Sorry for not explaining it very clearly ? Anyone able to wade through all that enough to give me a definitive answer ? Thanks in advance !
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post #2 of (permalink) Old 12-18-2007, 07:16 PM
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Ground onto map add = GOMA. Map onto ground subtract = MOGS.

It's also pretty easy to work out by looking at the diagram on the bottom of the map and remembering that you are basically measuring an angle.

Or buy a compass with adjustable declination and forget about the whole thing.
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post #3 of (permalink) Old 12-18-2007, 07:23 PM Thread Starter
 
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Thats perfect Sandy......it is the opposite of what I have been used to but it makes total sense...guess I just wanted someone to confirm what I suspected. I dont want to get rid of my trusty old compass just yet ! Thanks for the prompt reply.
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post #4 of (permalink) Old 12-18-2007, 07:23 PM
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post #5 of (permalink) Old 12-19-2007, 06:01 PM Thread Starter
 
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Thanks Pdops..much appreciated.
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post #6 of (permalink) Old 12-20-2007, 09:52 AM
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If you like, you can see that there is only one right way to do it, and of course it is the way we do it out here .

If something is a 'few degrees west of north', then its bearing is something like 357 degrees. So in the UK, the true bearing of magnetic north is 357. In other words when your compass says 0, you know the true bearing is 357, and in general you need to add 357 (modulo 360) to your compass bearing in order to get the true bearing. You just chose to think about it as subtracting 3.

Whenever I have to deal with declination I think about what happens at zero degrees; it will be the same for any other bearing. I inevitably confuse myself.
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post #7 of (permalink) Old 12-20-2007, 10:41 AM
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Just remember that the north magnetic pole is somewhere on Ellesmere Island. So from the UK, that's up and left a bit, but from here, it's up and a long way right. Now, your compass always wants to point to the north mag pole, so when you sight along your compass's north arrow, just figure that true north is ~20 degrees left and it's all good.

Or, use a GPS.
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post #8 of (permalink) Old 12-20-2007, 11:15 AM
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If you want to really simplify things, just move to Thunder Bay, where the magnetic declination is currently zero.
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post #9 of (permalink) Old 12-20-2007, 11:35 AM
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When's the next pole flip predicted to happen? Won't that be fun!
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post #10 of (permalink) Old 12-20-2007, 05:07 PM
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Quote:
quote:Originally posted by hafilax

When's the next pole flip predicted to happen? Won't that be fun!
We'll be too busy getting irradiated by cosmic rays and solar flares to notice a little thing like that [XX(]
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post #11 of (permalink) Old 12-20-2007, 05:29 PM
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Quote:
quote:Originally posted by Dru

Quote:
quote:Originally posted by hafilax

When's the next pole flip predicted to happen? Won't that be fun!
We'll be too busy getting irradiated by cosmic rays and solar flares to notice a little thing like that [XX(]
I've wandered about this too. Does the magnetic pole "flip" or does it die and reemerge. Let's hope it flips quickly... when it's night in this part of the world.
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post #12 of (permalink) Old 12-20-2007, 05:32 PM
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The paleomagntic record indicates field strength declines and polar wander increases. When the mag poles wander well away from the actual poles (like into the subtropics), the field strength dies away altogether and then the field reasserts strongly, but flipped polarity, in a geologically short interval (which might be as long as a decade or two [] )
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post #13 of (permalink) Old 12-20-2007, 05:37 PM
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post #14 of (permalink) Old 12-20-2007, 06:04 PM
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From the Wiki:
The Earth's magnetic north pole is drifting from northern Canada towards Siberia with a presently accelerating rate — 10km per year at the beginning of the 20th century, up to 40km per year in 2003. It is also unknown if this drift will continue to accelerate.

and

At present, the overall geomagnetic field is becoming weaker at a rate which would, if it continues, cause the dipole field to temporarily collapse by 3000–4000 AD.

More information and an online program that predicts the current declination at your location:

http://www.ngdc.noaa.gov/seg/geomag/geomag.shtml
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post #15 of (permalink) Old 12-20-2007, 06:57 PM
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A possible reversal in 3000-4000 AD. That's good news; I wont have to buy a new compass for a while.
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