Roughly Where Am I ?

A good number of location based services only really need to know roughly whereabouts you are located in order to provide a useful service. My suggestion is for a couple of new bits of information to be added to 'connected' devices in order to allow a device's location to be ascertained within a good enough degree of accuracy. It goes like this:

My wifi router is static (i.e. it sits on our living room floor), and I know it's geographical grid reference, it can therefore be an authoritative source for location data (and given that it's connected to the internet, it also knows the time too).
Now imagine that I walk past my WiFi router with my WiFi enabled PDA. My PDA now asks the router for it's location and the current time. The router is authoritative for both, so hands over the information.

Now I go out for a walk with my PDA, and 500m down the road (Charing Cross Road as it turns out), another Wifi PDA comes in to range of mine, they talk to each other and find out that
a) my PDA has 2nd hand knowledge of its location 2 minutes ago
b) the other PDA has only 4th hand knowledge of its location, from 2 hours ago (it's been switched off).
So.. the protocol works out that 2nd hand information 2 minutes ago is better than 4th hand two hours ago, and the other PDA walks off armed with a better idea of its location.


Now.. I know this only gives a rough idea of where a device is (but that's all I want), it offers a few advantages over existing more precise technologies
gps: no need for additional hardware therefore cheaper
gprs triangulation: no network charges from the mobile operator (also more private) and works for non phone devices.