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Re: [TowerTalk] How accurate are iPhone Compass apps?

Subject: Re: [TowerTalk] How accurate are iPhone Compass apps?
From: Jim Lux <>
Date: Fri, 30 Dec 2011 11:27:55 -0800
List-post: <">>
On 12/30/11 10:43 AM, William Hein wrote:
> I have the Commander Compass (paid version 3.3.6) app on my iPhone 4S.  It's 
> an impressive looking app - says its "Mil Spec" - however I am wonder how 
> accurate it is.  The manual says "The precision of the readings of the 
> built-­‐in hardware sensors affects all of the Spyglass features. Spyglass 
> averages the input from the sensors over time and uses the basic motion 
> dampening in order to achieve a better precision."  So I guess its down to 
> the accuracy of the iPhone 4S GPS.
> Sitting on my kitchen table it indicates an elevation of between 6893 and 
> 6909 feet.  If I average elevation readings over time will I improve the 
> accuracy?  I used the app's Location feature - it overlays an arial photo 
> with crosshairs of the devices position and it looks to be VERY accurate, I'd 
> say within a few feet (it has a cross hair over my kitchen an I'm sitting at 
> the kitchen table).  Is the elevation measurement equally accurate.
> I also wonder how good this would be for finding true north and aligning Yagi 
> antennas and rotators.  Again, looks pretty accurate and seems to take 
> declination into account.

The combination of accelerometers and magnetic sensors means that you 
can compensate for the non-level-ness, which is a huge advantage over a 
conventional card compass.

Based on some of the sensors I've fooled with over the years, I'd guess 
you can probably get within a degree or so. Certainly good enough for an 
HF Yagi. probably not good enough to orient the owl<grin>.

A good check is to mark out 8 directions evenly spaced (compass and 
straightedge, bisection of angles, etc.) on a piece of paper.  Tape that 
to the table. Then put your phone down and see if they're all 45 degrees 
apart as expected.

That will give you a check on the "linearity" or "circularity"

(there are some clever algorithms where you can spin the phone/sensor 
unit through several directions rotating it, and use the accelerometer 
(which can always find "down") to calibrate the compass sensor uniformity.

There's always the problem of local magnetic field distortions, of course.

But you still need to know if it's aligned to "true north", and for 
that, you need some astronomical comparison.  Sun at local noon or 
Polaris is easy to get 1 degree accuracy.  Naval Observatory 
Astronomical Applications web page has useful tools to figure out what 
local time will be solar noon for you (crossing the meridian), or, for 
that matter, what azimuth the sun has at a given time.

I've thought about seeing if you could write a celestial nav application 
using the camera on the front of an iPhone 4 to see the sun.

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