> These could assist GPS location rather than replace it entirely.

Of course, that was the proposal. There's more datapoints if you're willing to get creative, wifi networks (already used for this), cooperative comparison with other mobile devices in a local meshnet, acoustic cues from the environment, machine analysis of captured images, etc. Obviously dead reckoning without gps is going to require a multi-pronged approach.

> cooperative comparison with other mobile devices in a local meshnet

Please explain how this will work?

I could imagine a secure location service that allows your phone to compare its current expected position with other nearby phones' expectations of their positions. If it's over bluetooth or wifi, the positions should be within meters of each other. This could provide an input to a kalmann filter type position estimator to help reduce drift as you (for example) walk down the street.

This doesn't make a lot of sense tbh.

What do you mean?

You don't think such a system is practical, or you don't think such a system is technologically feasible?

Or you just don't understand the system I'm describing?

I think there are both feasibility and practicality issues with the cooperative estimation scheme you are describing. Without already knowing where the phones are very accuratly there will be a lot of noise. Would need a lot more detail to really understand what you intend but first reaction is that it'd be very difficult to do well.

If you really care you could sketch out what exactly it is and how it'd work for yourself for a couple of devices (or more) and see what issues you uncover.