How accurate is CLLocation accuracy?

Question

I'd like to use reliable locations, even on an old iphone. However, many readings (particularly from cell towers) are too inaccurate. I think.

When I plot my position + accuracy radius (or look at google maps app), I notice the center of the estimated circle is generally close to my physical location. I'm guessing that if I cut the "accuracy" number in half, I'll still be in the circle 99% of the time.

I believe this is a probabilistic game - the location manager is trying to provide an estimate that's correct 99.99999% of the time, so they give a deliberately wide margin. Any thoughts/info?

Answer 1

When you're using CoreLocation, you're getting back "answers" that get better and better. I've noticed that the "best" answer is almost always accurate to within 100m, so theoretically you could probably cut down on the "buffer" that you're normally given. The only way to really know, though, and this is what I would do, is to test test test. Find iphones and ipods from all generations and see what types of accuracies you're getting and what types of results you're getting. In a lot of ways, it depends on the type of app you're making, but if you want to deliver sensitive or important information based on where the user is, you should really wait for the framework to give you a nearly exact location.

Answer 2

The CoreLocation framework gives you the radius of the circle for every CLLocation you get using the horizontalAccuracy/verticalAccuracy properties. You can specify to the CLLocationManager a desiredAccuracy property that use these types:

kCLLocationAccuracyNearestTenMeters, kCLLocationAccuracyHundredMeters, kCLLocationAccuracyKilometer, kCLLocationAccuracyThreeKilometers;

So you get notifications when you get inside your desired range. That said, when you use the CLLocationManager the first event is given to you ASAP, and then the proceeding events are the ones that satisfy your conditions.