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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