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GPS and Magnetic Variation

 
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rothers
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Joined: Mar 28, 2007
Posts: 134

PostPosted: Sat Apr 19, 2008 9:31 am    Post subject: GPS and Magnetic Variation Reply with quote

How do GPS receivers obtain magnetic variation data ? Most Marine GPS units I've used have the ability to display bearings/headings as true or magnetic.

Does the magnetic variation for any particular position come from the GPS satellite data or from an inbuilt look-up table ?

Cheers

Neil
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Brickie64
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Joined: Dec 05, 2007
Posts: 158
Location: Lincoln (UK)

PostPosted: Sun Apr 20, 2008 4:50 pm    Post subject: Reply with quote

Neil,
I think you're right about the look up table.
It seems that most commercial devices do it this way:-

http://exchange.manifold.net/manifold/manuals/5_userman/mfd50Working_with_GPS_Receivers.htm

Look about 2/3 of the way down the page for the relevant paragraph.

I quote:-
"Most GPS devices calculate the magnetic bearing they display by consulting a look-up table within the unit that contains an approximate variation for various locations on Earth. Since the unit knows its location and the current date, it can find the current variation for that location in the table (interpolating if need be) and adjust the true bearing to the magnetic bearing that is displayed. "

Cheers

Gav
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"A good scientist is a person with original ideas. A good engineer is a person who makes a design that works with as few original ideas as possible"

Garmin Nuvi 3598 + GO 720 (+USA Map Card) + NavFree for iPhone in case all else fails ;-)
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rothers
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Joined: Mar 28, 2007
Posts: 134

PostPosted: Sun Apr 20, 2008 6:33 pm    Post subject: Reply with quote

Thanks Gav, great info.

Cheers
Neil
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