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I started using the Nike+ GPS app on my iPhone 4 (not 4s) to track my running in summer 2011. The GPS measurement of runs was pretty consistent: runs on the same course would vary as much as a tenth of a mile from run to run, but I know GPS is not always as accurate as it is precise, so that doesn't bother me.

Sometime in September of this year (2012) the accuracy of my run tracking went way down. I had a few where the distance was under-measured by as much as 50%. I thought it might have been Nike+, but I installed Strava Run and had another run massively over-measured. Last weekend a 6-mile run (measured using gmap-pedometer.com) was measured as 0.17 miles on Nike+. (The Nike+ app, to its credit, has been regularly warning me that the GPS signal is "weak", so even it is dubious about the quality of its data.)

However, in non-running applications, location services seem to be just fine. Maps places me in the right place. Weather apps (Dark Sky is my best example) know exactly where I am. Are they using something else to get my location? Reading this question and this one make me think this could be a hardware problem, but the inconsistency between apps puzzles me.

ETA: I have not yet installed the iOS 6 update, so this is still iOS 5.

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Did you try completely rebooting your iPhone? –  Gerry Oct 12 '12 at 12:46
    
By "completely rebooting" you mean a power cycle, right? No, actually that hadn't occurred to me, which is a little embarrassing considering my tech-support background. I'll try it now. –  pjmorse Oct 12 '12 at 12:57
    
Are you running iOS 6? I've been experiencing problems with the GPS in Waze and MotionX Drive recently (two apps I use regularly in the car). I am running iOS 6 and I think this is when the problems started. –  EmmEff Oct 12 '12 at 14:28
    
I'm resisting the iOS 6 upgrade for a few weeks (relatedly, to avoid the new Maps app) so no, not iOS 6. I'll edit the question to provide that data. –  pjmorse Oct 12 '12 at 14:37
    
Have you opened a support ticket with the developer of your apps to be sure they are actually using GPS when the screen goes dim or perhaps are calling the less power hungry cell tower location mechanism. I've had great responses from the developer of Runmeter on the best settings to smooth my run location data and get better distance measurements when location data is noisy (as it often is). –  bmike Oct 12 '12 at 15:01

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up vote 7 down vote accepted

I regularly experienced similar problems with RunKeeper on my iPhone 4S where distance was over-measured. When reviewing the plotted map of my run I could see the coordinates jumping a bit erratically from one point to the other, accounting for the extra distance, and this for entire outdoor runs, not just sporadicly on spots with bad GPS reception.

Normal run:

enter image description here

Bugged run:

enter image description here

While I cannot give a definitive answer as to why this problem occurs, and why the iPhone seems to get stuck in this bugged state when it does, completely rebooting the iPhone (and possibly additionally toggling Flight Mode to reboot the antennas as well) seemed to always resolve it for a few days or even weeks.

On a sidenote, since I upgraded to my iPhone5 (with iOS 6 ofcourse), the problem has not presented itself again so far. Perhaps the problem was fixed in either iOS or in the hardware meanwhile.

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I've done the reboot. I'll see if it helped on tomorrow's run. –  pjmorse Oct 12 '12 at 13:30
    
Flight mode will turn off the GPS radio also, but if you temporarily just turn off wifi and cellular data it will stay running - you may want to try this to see it it improves things, and rule out any potential antenna interference. –  stuffe Oct 12 '12 at 15:00
    
My guess is the location API is using less accurate location calls when the screen is off. Most GPS apps have settings to prevent the screen from dimming or offer smoothing of the data so that you can explicitly burn more battery when you absolutely need GPS level details or handle the less accurate location data that arrives when the device is in a lower power mode after the screen turns off. Basically, iOS has changed and the app hasn't changed to handle less accurate (but more economy-minded) data. –  bmike Oct 12 '12 at 15:03
    
@bmike In the case of RunKeeper, I always turn the screen off to prevent accidental input, and that usually has no negative impact on the accuracy at all. From my observings, the iPhone occasionally gets stuck in a state where it's feeding RunKeeper inaccurate data, which everytime it occured for me was resolved immediately after a complete reboot. –  Gerry Oct 12 '12 at 15:30
    
@Gerry Very interesting. Do you suppose some background task is taking the time away from the GPS / location and the app you care about which is causing the sample times to delay? I'm glad I haven't had to reboot to get good location data, but you might be on to something if you can reproduce this bad results / good results with a reboot in between. –  bmike Oct 12 '12 at 15:40

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