[SierraDragon]

Does the GPS capability to geotag images work well when neither WiFi nor 3G are available?

Thanks.

-Allen

[P]

It works, but it's slower to find your location.

A GPS signal consists of three things: The time the signal was sent, the orbital information of the satellite sending it, and the position of all satellites in the system. The first two are quick to receive, but the third takes about minute to receive, and longer if you have bad reception. If you only have the first two and a rough idea of where you are, that is enough to locate you without the third piece of the puzzle. If you use the position of 3G towers to locate yourself, that's enough information. If you don't have it, you have to wait for that download to locate you once - after that, it works fine.

[ghporter]

The farther you are away from the last fix, the longer any GPS takes to determine a fix. And some systems don't do well at all without some sort of hinting. Last January I was in Central America and I could not get the GPS app (MotionX) on my 3Gs to get a fix until I turned on phone service (which I'd turned off to avoid international roaming charges). My hand-held unit (a Garmin eTrex) took longer than usual but got its fix fairly quickly. So I'd advise that you keep your eye on whether your device has a decent fix before starting to snap pictures you want accurately geotagged.

[Dork.]

I assume that the quality of the GPS tags is due mainly to the quality of the GPS fix when the picture is taken.

Most phone GPS units are designed to use the location information from fixed cell towers to provide rough location and time information, then they get the orbital information and position of all the satellites from the data connection. They use that so they "know where to listen" for the satellites and get a quicker fix. (This is Assisted GPS). Some phone GPS units may actually rely on this information to the point where they may not be able to lock on to the satellites without it. (But I don't know if the iPhone is one of these.)

As part of the math that any GPS receiver does to get its position and time, it also calculates position and time error. I don't know what apps exist for iOS, but on my Droid I have an app that displays all the satellites and their positions in the sky, and also displays the calculated position error in feet. If you check that when you have a fix, you'll know how far off your tags might be.

The last time I was in Europe, my CDMA Droid stubbornly refused to get a GPS fix anywhere, even when I had my rough position through wi-fi (and turning on data roaming wasn't an option!). I'm still not sure why. The server that my phone was going to for GPS assistance information may have refused to provide it to a location out of North America, or perhaps the phone's time has already wandered off after being out of CDMA range for a few days (which shouldn't really have mattered.). If I end up out of CDMA range anytime soon, I'm going to do some research on how to get my Droid's GPS to play nicely.

[TETENAL]

The iPhone 4S also supports GLONASS. I suppose that should help with accuracy and speed.

[Dork.]

Interesting, I didn't know that. (Upon googling it, I see I'm a few months behind the times). It won't necessarily help with time to first fix, as receivers only have so many channels, whether they're trying to find GPS or GLONASS satellites.
It will help in cities, though, because since there are more satellites to choose from, it's more likely to find ones with clear signals that aren't being reflected, and ones that are further apart in the sky (to decrease position and time error).

[Dork.]

Does Apple have any apps that give you access to the GPS and GLONASS data? Like which satellites are used in the fix, their locations, TDOP/PDOP, etc?

[SierraDragon]

Thanks, good info. The task is remote construction documentation. A few minutes to acquire should be OK.

What I do need is absolute confirmation that the 4s will access GPS data for geotagging when neither 3g nor WiFi are at all available for days. I do not want to recommend a multi-device purchase (probably iPads but maybe iPhone 4s devices) unless gps access is a sure thing.

Thanks.

-Allen