How accurate is the iPhone GPS?

I have written in the past about the inaccuracy of location data provided with "Significant Change" background location updates. The solution to this problem is simply to use Significant Change as a trigger (ignoring the data it gives you), then use CLLocationManager startUpdatingLocation to get the quality location data.

So how accurate are the updates that you get from this approach?

At goCatch we track the locations of thousands of Taxis driving all over the world. The data gives us some useful insight about how to measure the proximity of passengers and drivers, hopefully you will find it useful too.

For the 650,704 location updates I used in my tests, I found the average accuracy radius was 246m. 85.1% of updates had an accuracy of less than 100m. Indeed if you eliminate the wildly inaccurate values (>1km) the average accuracy radius is only 91m.

In this last chart you can see the spread of accuracy updates over the first km. I am guessing the outlying (>1km) values are for phones with no GPS and no nearby Wifi that are relying on the cell tower triangulation (same as Significant change) to get a location.

N.B. When I say "GPS" in the title I am actually talking about all the sources of location data that the iPhone passes to the app (GPS, Wifi and Cell proximity).

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