iPhone 101: Location data and GPS

gps data in a photo

Update: Look here for official word on the iPhone location data controversy from Apple.

Recently, the Apple community has become interested in location data as gathered by iPhones. Specifically, The Guardian has reported that researcher and former Apple employee Pete Wardensome and data visualization scientist Alasdair Allan believe that your iPhone's travel history is backed up to a file on your Mac, eliciting questions and concerns about iOS location services.

With that in mind, TUAW offers this brief primer so that you can better understand what's going on under the hood of your iOS device when it comes to location matters.

What are location services and how do they work?

Location services allow certain apps to determine your iPhone's approximate location and make use of that information. This is done through a combination of cellular network triangulation, Wi-Fi triangulation and the Global Positioning System, or GPS.

Here's how it works. Your iPhone will first attempt to communicate with GPS satellites to determine its approximate location. This is a series of medium Earth orbit satellites deployed by the US Department of Defense several years ago. For a more in-depth explanation, look here.

When a solid GPS connection is unavailable (the iPhone is indoors, amid many tall trees outside, etc.), the iPhone tries Wi-Fi triangulation. As our own Auntie TUAW recently explained, this works because Wi-Fi hotspots rarely move. Apple has amassed a database of known hotspots and, when your iPhone is connected to one of those, can use them to determine an iPhone's approximate place on the Earth. Of course, this method is less accurate than GPS.

Finally, determining location via cellular towers works in a similar fashion. Nearly every cell tower is built in a known, constant location (except for COWs). These fixed positions allow your iPhone to determine an approximate location by triangulating its distance from the nearest towers. Cell towers are less accurate because there are fewer of them than there are Wi-Fi hot spots. Therefore, you're dealing with larger distances.

The first time an app tries to access location data, it asks for permission. A dialog box asks to use your current location. If you're OK with that, tap Allow. Otherwise, tapping Don't Allow prevents the app from accessing your location data until you turn it back on as described below.

Can I turn it off?

Yes. In fact, you can disable location services on a case-by-case (or app-by-app) basis or in one fell swoop. Here's how.

First, tap the Settings app, then tap Location Services. You'll see a list of the installed apps that use location data. To disable services for an individual app, move its slider from On to Off. Or, move the Location Services slider at the top of the screen to disable them all at once. Note, however, that many apps (like Apple's Maps) become significantly less useful with location services disabled.

What does it record?

Location services record your iPhone's approximate location as well as a time stamp. Your iPhone does not record how long you spend at a given location.

Which apps use location services?

Each app uses location data differently. For example, Maps provides directions for traveling from Point A to Point B by car, public transportation or by foot. Camera+ allows you to geotag (the process of adding geographical identification metadata to images) your photos, and Tweetbot lets you do the same with your Tweets.

Speaking of photos, some people get freaked out by geographical data in their images. If that's you, here are two things you can do to calm your nerves. First, disable location services for photography apps, like Apple's Camera and Tap Tap Tap's Camera+ as described above.

You can remove location information from existing photos by converting JPG to PNG (this strips out EXIF data, save the date) or using an EXIF editor. Here's how to remove data with iPhoto '11.

First, select your image. Next, choose Export from the File menu. A pop-up screen appears. Click the File Export tab and ensure that the Location information checkbox is not selected. Finally, click Export. Your photo is exported without location information.

If iPhoto doesn't float your boat, try Reveal or ExifRenamer for editing this information.

What's the hubbub about location services?

As we pointed out the other day, it seems that the iPhone creates a cache of your iPhone's travel history on your Mac. As far as anyone can tell, there's absolutely no evidence that Apple or your carrier is accessing this information.

We hope this overview of location services was useful. For those of you interested in exploring EXIF and GPS data in your photos further, check TUAW later today.