[TowerTalk] How accurate are iPhone Compass apps?

[K1TTT]

Yes, averaging over time does help.  Some gps's have built in time averaging to improve accuracy of saved waypoints.

The elevation is not usually as accurate as the lat/lon, that comes from the method of computing them using the time differences from the satellites, the vertical component has a much larger error than the horizontal ones do.

I'm not sure how the iphone compass works, if it is magnetic then it would be disrupted by nearby magnetic objects of course.  Gps can not resolve the orientation of a static object, it can only calculate the bearing between two points, so a moving gps can give you a direction of travel but it can't point north.  It may be possible to use the motion sensors in the phone to track it's motion like an inertial guidance system does and use that to orient the compass to gps while its moving.... but again I don't know enough about the iphone internals to know if the motion sensors are accurate enough for that.

David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net


-----Original Message-----
From: William Hein [mailto:bill.aa7xt at gmail.com] 
Sent: Friday, December 30, 2011 18:44
To: towertalk at contesting.com
Subject: [TowerTalk] How accurate are iPhone Compass apps?

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.

73
Bill AA7XT



Date: Fri, 30 Dec 2011 10:22:10 -0600
From: Richard Thorne <rmthorne at att.net>
Subject: Re: [TowerTalk] FAA & Towers, continued
To: towertalk at contesting.com
Message-ID: <4EFDE532.70506 at att.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

My new iPhone 4S has a compass application with GPS coordinates.

Rich - N5ZC

On 12/30/2011 8:51 AM, William Hein wrote:
> Thanks for all for the great advice and info!  TowerTalk really proving its value to me here.
> 
> I am filing out FAA form 7460-1 online right now.  It asks for the exact altitude ASL of my tower(s).  How do I find this out?  I can extrapolate from Google Maps but that won't be accurate to a foot, neither will be my consumer grade GPS.
> 
> 73
> Bill

-----
William Hein, AA7XT
(ex-AA4XT, NT1Y, AA6TT, KC6EDP)

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