compute true north based on successive user location

I'm working on an app that should include a compass (it should work on iphone 3G also). however as there is no a compass on iphone 3G i'm looking for a way to know the true north orientation based on the CLLocation returned by the iPhone.

Note: the app should give as 开发者_如何学运维accurate as possible value even if the phone is using "cell towers" to know the actual position (not very accurate).

thanks in advance.

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You can approximate the users heading assuming they have been moving in a straight line between gps updates.

I would first set the desired accuracy to the highest. You will need it since a small change in position will result in a large change in angle.

Now call the coordinate of the first location L1, and the coordinate of the next location L2.

Your heading vector , h = L2 - L1

now if h = (lon, lat)

To get the heading you need use some trig:

x = arctan ( lon / lat )

I would consider taking averages / smoothing the result somehow.