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Re: [TowerTalk] Smart Phone Compass

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Smart Phone Compass
From: Jim Lux <jimlux@earthlink.net>
Date: Wed, 24 Dec 2014 15:59:52 -0800
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On 12/24/14 11:06 AM, James Wolf wrote:
I wouldn’t expect your smart phone compass to be more accurate than +/- 5 degs 
around the circle - at best.

On most quality, (read military) electronic compasses used for situational 
awareness,  there is an option to input the declination offset.  Or if it has 
GPS, then it can automatically look up the declination. In my experience this 
is rare and it changes over time.



Here’s a map of the world for declination values.  Some are very dramatic.



https://upload.wikimedia.org/wikipedia/commons/2/28/Mv-world.jpg



Bear in mind that the declination (variation) changes substantially with time. here in Southern California, it's in the degrees/decade sort of magnitude.

Most of the phone apps take this into account and use a reasonably up to date map of variation vs location (at least in the US). There are local anomalies (look how wiggly the lines of equal variation are), particularly when there are large ore bodies (you folks in Duluth...) or mountains around.

I doubt that the error in the variation is bigger than the uncertainty in the underlying magnetic measurement. In a relatively benign environment, I'd expect the magnetometer to give a reading in the +/- 1 degree uncertainty range.


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