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There seems to be some dispute on whether the iPhone can still get a GPS signal when out of cell tower range, see this archived Apple support thread, for example.

I had a friend out in the woods hunting this weekend, and he said that he couldn't get a location via gps, it was just giving errors.

Does this depend on how the app is coded? Are there specific location-aware mapping apps (like the built-in maps tool) that someone can firm either do or don't work properly with no wireless signal?

Update: I guess its kind of obvious that it entirely depends on how the app is designed. The built-in maps app tries to download map tiles from the internet, and won't be able to do that, so that probably explains his errors.

So my underlying question really is: If I'm going camping, do I need to buy a dedicated "wilderness" GPS like one of these? Or is there an iPhone app that can do the same things when away from a wireless/cell signal?

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Did he try the compass? It shows the position alongside the direction (if it can get the position). –  Martin Nov 15 '10 at 15:05
    
Not sure what apps he tried. I expect he probably tried the built-in Maps app, which (as Kyle mentions) downloads map tiles from the internet. –  BradC Nov 15 '10 at 15:21
    
that really depends what do you want the GPS for. If you want it to know where you are, how to get back if you get lost, etc. then you might be better served by a product specifically dedicated to that. The iPhone’s GPS is nice, but not as fast/good as a dedicated device. Plus, the battery life of the iPhone really goes down fast when the GPS usage is intensive. –  Martín Marconcini Nov 15 '10 at 17:38
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6 Answers

up vote 9 down vote accepted

This summer I took some photos out of cell range and they were all geotagged so it's definitely possible to use GPS out of cell range, but some apps (like the Maps app) use a data connection to get map tiles and routing data, so those features wouldn't work if you're not in range of either cellular data or wifi.

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Some of the GPS applications like Tom-Tom or Navigon store the data on the phone and will work without any data signal. However, they're optimized highly for roads and may not help in the wilderness. Also the dedicated "wilderness" GPS might manage better battery life, the iphone doesn't last long when running the GPS in my experience.

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I browsed the "Navigation" category on the app store, and "MotionX GPS" and "GPS Kit" are two of the popular apps that claim to support this kind of use, including offline downloading of map packs. (Anyone have any experience with these apps?)

So it appears that this is at least hypothetically possible using the iPhone hardware. Obviously, battery life for an extended trip could be an issue.

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Most phones use the cell network to get a rough a idea of where you are (I say rough, but really it cuts it down to a tiny area compared to the size of the Earth aka the coverage of GPS) then use GPS to get the exact location. So if your out of cell range it can take something like up to 20 minutes to get the location (although that was on a Samsung phone and I noticed the iPhone is faster at GPS anyway)

The Apps have a accuracy setting so it's possible that if the accuracy is not accurate enough to use GPS it won't find your location, however I would've thought it would find your location with GPS once it realizes there's no data connection.

Turning my iPhone onto AirPlane mode (effectively taking it out of range of any data connection) and then going onto the default maps app and finding my location takes a while, and switching app while it takes this long makes it timeout.

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I was under the impression that AirPlane mode turned off all antennas/receivers, including the GPS. Found it: support.apple.com/kb/ht1355 –  BradC Nov 15 '10 at 21:57
    
You can turn GPS on again afterwards. –  Jonathan. Nov 15 '10 at 22:16
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Provided the Location Services are enabled on the phone, GPS will always be able to be used for locating your phone.

Outside of connectable cell tower and wifi range you'll be limited to GPS accuracy (approx 50 to 100 metres) because you won't have internet access for your phone to find out where the towers/wifi access points are to improve your position accuracy (down to say 10 metres).

The problem your friend had is they were in the woods, but GPS is line of sight only. When you're inside (and not next to a window) your phone is only using cell towers and wifi to locate itself, likewise when you're under trees or otherwise cannot see the sky, you lose the GPS signals.

GPS signals are very easily reflected by buildings, trees, etc, so in a city with buildings all around you or with trees or mountains around you, you may either lose sight of the GPS satellites or receive signals that have bounced off nearby buildings/mountains/etc which will be misleading/confusing for the GPS locator in the phone. In such cases, it either won't give you a location, or may give you the incorrect location due to what is called "multipath" error in GPS (i.e. the signal hasn't just travelled directly to you like the algorithm assumes).

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I know it's only anecdotal, but I've been very impressed by the accuracy of the iPhone 4's GPS. Given clear skies, I'd say the margin of error is less than 10 meters even without the cell tower assistance. –  Kyle Cronin Nov 15 '10 at 22:07
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Yes, GPS still works, but that only helps if you preload a map

Knowing where you are by GPS depends on your phone being able to see the satellite signals; cell towers are irrelevant. GPS signals can be blocked by tall objects (so might not work in a ravine or between tall buildings) and possibly in dense tree cover.

Being able to download a map while on the move depends on being connected to WiFi or the phone network so you won't be able to do that if you are far from a cell tower. But some apps (I have RouteBuddy atlas) allow you to download full maps to the phone before you travel so they can still tell you where you are when you are otherwise out of contact.

GPS is much more accurate that cell tower location but is also slower. Celltower approximate location speeds up the GPS but doesn't make it more accurate.

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