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Mac User #001 Nov 26, 2013 04:16 AM
GPS Photo Geotag Without Cell Service
Hi,

I have a trip to Europe planned in March and plan on using my iPhone 5 as my main camera. I won't have service overseas, but what I'm wondering is will the photos still be tagged with a location?

I realize it may literally be GPS coordinates until I establish a data connection and it can fetch a city or something.

Really hoping this will work!
Mac
 
P Nov 26, 2013 11:06 AM
Sort of. GPS-tagging works, but without network access, it takes about a minute to find your location (same as any car GPS if you move it while turned off). With network access, the phone can use so-called A-GPS to improve that time to seconds, which is what you usually see. The first photos you take might get the wrong location until the phone figures out where it is.

What is stored in the photo is always the GPS-coordinates - the location for them is then figured out in the viewer app.
 
Mac User #001 Nov 26, 2013 11:51 AM
Thanks man. So quick point and snaps aren't going to end well? Is there a way I can have it constantly tracking me, even if there's a loss in battery life?
 
P Nov 26, 2013 01:46 PM
I'll admit to never testing this in practice, but I believe that what happens is that the first photo or so gets the coordinate of the last time the phone located itself. A-GPS will work if you simply leave the phone connected to the cell phone network even if you completely disable mobile data and never use it for phone calls and texts, so I suggest you just do that.
 
powerbooks Nov 26, 2013 06:56 PM
My suggestion is also leave your sim card in your phone but turn off roaming or anything related to data. The phone will be able to find roaming partner cell signals and display it on your top left screen corner. That way, you can still get A-GPS coordinate without using any data, plus plenty of free msg from those carriers to promote their service, in some countries.

Not sure if phone call will be blocked though, if that's what you want to avoid.

Best approach is to have your iPhone unlcoked and use a cheap local prepaid sim card in many European countries.
 
Mac User #001 Nov 27, 2013 11:35 AM
Thanks guys. I'll probably do a test run here at home.
 
P Nov 27, 2013 02:46 PM
Hm. I think that if you pull the SIM, your phone will see the networks but not connect. That could be a solution.
 
mduell Nov 28, 2013 06:27 AM
Quote, Originally Posted by P (Post 4259183)
Sort of. GPS-tagging works, but without network access, it takes about a minute to find your location (same as any car GPS if you move it while turned off). With network access, the phone can use so-called A-GPS to improve that time to seconds, which is what you usually see.
GPS cold start takes 15 minutes. 30 seconds for a warm start.

Quote, Originally Posted by powerbooks (Post 4259258)
Best approach is to have your iPhone unlcoked and use a cheap local prepaid sim card in many European countries.
Unless you want your phone to work so people can text and call you at your normal number they all know.
 
P Nov 28, 2013 07:57 AM
Quote, Originally Posted by mduell (Post 4259385)
GPS cold start takes 15 minutes. 30 seconds for a warm start.
Interesting, but my experience suggests otherwise. If I take my dedicated GPS on a flight to somewhere far from where it was last turned on, it figures things out after about a minute. In that case it has the almanac and the time, but no idea of the position. There is a note in that article that some devices can shorten that cold start time by using multiple channels - perhaps that's what's happening here.
 
ghporter Nov 28, 2013 09:49 PM
Cold start is very worst case. Most GPS units have sufficiently accurate internal clocks that being off for even a few days won't get you to that bad a situation if the location is still relatively near the last fix. There's where the "unknowns" creep up. The GPS unit uses the current time to look up what satellites should be visible from the last known location. With an accurate clock but a large distance between current location and the last fix, the unit will look for the wrong satellites, and that can slow things down immensely.

Cold start means the unit (or phone) has NO time information, and a GPS unit will start essentially polling ALL satellites for a time signal. There are at least 30 GPS satellites, counting hot spares, and they broadcast their time signal every 30 seconds, so depending on when in that 30 second window the unit starts looking, it could have to go through the whole list more than once before getting lucky and polling a satellite that should be visible.

Having a fairly recent cell connection, or a WiFi connection with time information, an iPhone should be able to find satellites more quickly than worst case, and if the WiFi provides the phone with sufficient location data, it can help the GPS system in the phone look for the right satellites at the right time.*

A couple years ago I was at Roatan Island in Honduras, and I had my international roaming option turned off to avoid charges; my GPS app, MotionX GPS, couldn't even get a clue where I was, since the phone had been turned off for a couple of days. But when I turned on the cell service, MotionX locked on in 20-30 seconds. I would assume that this would be an indication of how the internal GPS system reacted without, and then with cell service as a "seed" for coarse location determination.

*I'll admit a complete lack of knowledge of how basic WiFi networks provide either time or location information that iPhones can use, and I'd appreciate pointers to where I can read up on this.
 
P Nov 29, 2013 05:33 AM
Quote, Originally Posted by ghporter (Post 4259437)
*I'll admit a complete lack of knowledge of how basic WiFi networks provide either time or location information that iPhones can use, and I'd appreciate pointers to where I can read up on this.
Skyhook Wireless - Wikipedia, the free encyclopedia

Basically, someone sits in a car with a GPS and a WiFi antenna, driving along all streets and logging which networks can be "seen" from any location. These days the data comes from many sources - in Apple's case, I believe that iPhones report data like this in anonymized form to build Apple's database.
 
ghporter Nov 29, 2013 08:40 PM
Much simpler than I had thought. Instead of some extra stuff coming out of certain networks, it's just a remote database thing.
 
Martin84Z Dec 15, 2013 05:18 AM
the coorinates might be having issues,if they can be correct
 
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