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Magnetic Variance

Magnetic Variance


Postby islayhawk » Sat Jan 26, 2013 12:58 am

Can someone tell me why I should get a different calculation for the Magnetc angle from different web sites. One gives the difference at 1' 3" west. While the other gives a difference of 4' 25" west.
Both for Lat 55.563 N and 6.25W

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Re: Magnetic Variance

Postby MacCookie » Sat Jan 26, 2013 8:44 am

Grid north verses true north?
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Re: Magnetic Variance

Postby IainG » Sat Jan 26, 2013 8:47 am

Out if interest, does anyone know the magnetic to grid north difference this year?
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Re: Magnetic Variance

Postby PhilTurner » Sat Jan 26, 2013 4:28 pm

It depends a little on your location - see http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html for a useful tool (based on July 2012).
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Re: Magnetic Variance

Postby Circles » Sat Jan 26, 2013 5:37 pm

Look at the general information section of your OS map at a section called North points...it will state the variance between grid and mag for the publish date as well as stating hw much it changes each year so if your map is a few years old, you can calculate the difference...most here say you should always have your own map...being able to convert grid to mag (and back again if taking back bearings or performing a resection) yourself is just as important...your correction must be based on the map you are using
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Re: Magnetic Variance

Postby Gavin99 » Sun Jan 27, 2013 12:04 pm

Hi Islayhawk , as MacCookie suggests one figure is the angle between magnetic and grid north(magnetic angle ) ,eg Islay Airport , 1deg 2 min west (July 2012)
The other figure is the angle between grid north and true /geographic north (magnetic declination) , at the Airport this is 4deg 25min west .
The magnetic / grid angle is the one to use when taking bearings and map reading
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Re: Magnetic Variance

Postby Gavin99 » Sun Jan 27, 2013 12:26 pm

Edit
Ooops , that should read 'The other figure is the angle between magnetic north and true ..'
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Re: Magnetic Variance

Postby islayhawk » Sun Jan 27, 2013 1:14 pm

Thanks everybody for your replies. It is the magnetic angle I use and I was thrown by the website giving the Declination. Problem solved
Thanks

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Re: Magnetic Variance

Postby Caberfeidh » Sun Jan 27, 2013 1:21 pm

There are alternatives. As Mrs Beaton would have it ~ "First catch your dung beetle...."
http://www.bbc.co.uk/news/science-environment-21150721
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Re: Magnetic Variance

Postby rabthecairnterrier » Mon Jan 28, 2013 11:13 am

The British Geological Survey have a page on their website which allows
you to calculate the current magnetic variation for any part of the uk:

http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html
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Re: Magnetic Variance

Postby ramiro darek » Thu Dec 12, 2013 2:19 pm

to make sure :) for my town just now is( MAGNETIC NORTH IS 3,02 W OF TRUE NORT AND 2.00 W OF GRID NORT) so the changes that I have to enter is 2.00 w on my compass ??THX
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Re: Magnetic Variance

Postby jmarkb » Thu Dec 12, 2013 4:01 pm

ramiro darek wrote:to make sure :) for my town just now is( MAGNETIC NORTH IS 3,02 W OF TRUE NORT AND 2.00 W OF GRID NORT) so the changes that I have to enter is 2.00 w on my compass ??THX


That's correct, though it's hardly worth bothering with in practice.
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Re: Magnetic Variance

Postby neilfromperth » Fri Dec 13, 2013 5:34 pm

That's correct, though it's hardly worth bothering with in practice.[/quote]

Oh, I would bother. Remind me not to follow you in a white out. :lol:
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