Differential Gps systems provide an accuracy arround 1 meter. They usualy use information send by a terrestrial FM radio broadcast transmitter which know its exactely position. Combining this info with the GPS info gives an very goog accuracy. Try a search on G00gle with these keywords (differential gps dgps).
In some public transportation like with BUS serivices in major USA cities operators of busses can call road intersections via GPS and have some service that tell "Next intersection is 4th Street and Pine Road" with a pre-recorded "sexy" lady voice. it is cool!
Hay
I'm a big fan of the palm os and the whole series of its licensees
there is one company that sell apalm computer with a gps inside that I read a couple of reports about If you like the idea that when you are not using the GPS tracker so just flip down town the back antenna and continue using your palm computer with hi-color hi-reselution 1.5 normal other palms, then you have to see one of those Garmin's iQue models at https://www.garmin.com/
best of luck with the acc. but I think if you are gonna use inside city then at least 1.5 meters is max tol. (@ half street min width) or combine with a trusted mapping system (I read there is one for us with the first Garmin's iQue)
This is not a commercial this is a peice of advice