Can I assume the absolute GPS positioning error doesn't change much within a certain timeframe?

There's an idea for a small tool going round in my head. It would, amongst other stuff, require measuring the distance the users device has moved. This is going to be an iPhone App, however the question applies to all sorts of GPS enabled devices I guess.

I don't need the direction, speed or exact positions, just the scalar distance. The thing is, the distances to be measured are rather small in terms of GPS precision - propably like 20 to 30 meters. With an average iPhone GPS CEP of 10 meters, this would mean the measured distance could in the worst case end up anywhere between 0 and 40 meters.

Now I'm wondering - since I don't need absolute positions, is it safe to assume the offset imposed by the position error will not change significantly if the timeframe is sufficiently small?

Say the user moves the device 30 meters in 30 seconds, going straight from position A to position B - can I assume that the absol开发者_如何学JAVAute error has shifted only slightly and thus my relative precision to be better than the absolute precision as reported by the device?

My experience doing lots of gecaching with a dedicated gps device is that you can't make that assumption. The position will bounce all around within 10 meters or so all the time. So you can see lots of movement even when the person is standing still.

I don't have an iPhone but I doubt it is better than a purpose built GPS receiver.

I would say the best way to get the answer would be to do some tests on your own (assuming you have an iPhone).

Also, you can integrate over the output of the accelerometers and end up with delta position. That will accumulate error, but I'm not sure how quickly.