iPhone 4 background location service question

Question

I'm looking into the new background location service options in the iPhone 4 SDK. It allows an app to run in the background and receive location updates from the device.

There are two methods offered. One is a battery intensive mode that continuously gets location updates. The second recommended method sends the app location updates when there has been a "significant location change".

Does anyone know what a significant location change might be? Is a 30 foot walk considered significant, or is a 10 block walk considered significant? I imagine it also depends on the accuracy of the location mechanism being used at the time.

Answer 1

As Steve Jobs mentioned in the OS 4 introduction, the low power mode uses cell tower triangulation and does not activate GPS unit. Since the iPhone phone module needs to keep a connection to the cell network anyway, there should be no impact on battery life.

Since the precision of a location fix with cell tower triangulation is anywhere between a few dozen meters (in dense city locations) and a few miles, I think 30 ft is not a significant location change. I don't know the specifics, though (and as mentioned by the commenters, the Apple dev forums are the right place to talk about those).

Answer 2

I've recently done some field testing of the new background location service to get an idea of what constitutes a significant location update, what kind of accuracy to expect for the location hits and our general experiences using it.

The results are detailed in a fairly lengthy blog post:

iPhone Background GPS: Accurate to 500 meters, not enough for foot traffic