How accurate is the triangulated GPS of the non-3G iPhone?

Question

Does anyone have any experience with the triangulated GPS used by the non-3G iPhone? How does it compare with 3G positioning? Does the iPhone 3G use triangulation in the event that there is no GPS signal available? Is there anyway to determine the accuracy of the non 3G coordinates? Thanks.

Answer 1

I have done a lot of mobile software with a bunch of different devices including 3G iphones and 3G blackberry's and here is what I have found.

The blackberry and iPhone GPS is really good when you have clear line of sight and at least 9 satellites present. In some dense residential or urban areas you might only get 5-6 satellites which can take a while to converge.

If you do not have a signal, GSM phones like the iPhone will try and find your position using cell tower signal strength but it is NOT as accurate as GPS... not by a long shot.

I have heard, though this has not been confirmed that the iPhone also uses some server side machine learning when it can't find a GPS lock meaning that it takes the average all of the cell towers, plus the average of all the users who have used GPS in your area to try and find your best position. This is sometimes called AGPS or assisted GPS where the GPS information and cell tower strength are used together.

Also, the only thing I can think of for finding the accuracy of the non 3G coordinates would be to programmaticly switch providers in your code, or simply go into preferences and turn off 3G and write an application that does some tests.

The Pragmatic Programmers have a great iPhone SDK book that just added a chapter on using the Location API, so that might be a great place to start.

Hope this helps.

Answer 2

The CLLocation class has the properties 'horizontalAccuracy' (for latitude/longitude) and 'verticalAccuracy' (for elevation).

In addition to cell towers and GPS, locations may also determined by Skyhook Wireless, which has a database of Wi-Fi base station MAC addresses and locations.

Answer 3

When using only GSM towers, it's +/- 500m (it varies greatly, sometimes it's more precise).

If it finds known Wi-Fi network, then it's down to +/- 50m.