One problem is "map matching". That is, given your lat/lng (or, more realistically, a time-series of lat/lngs) and a map, figure out which road you're on.
The thing is that most consumer devices with GPS (like phones and even standalone GPS navigators) have a lot of internal "magic" to hide the raw noise in location fixes from the GPS signal.
For example, certain Garmin devices (intended to be used off road) will snap your location to an Interstate if you are going more than 55 mph and heading roughly in the direction of the road (which is evident when you run into traffic and your location keeps switching between the frontage road and expressway--the error distribution of the noise doesn't have mean=0!).
The thing is that most consumer devices with GPS (like phones and even standalone GPS navigators) have a lot of internal "magic" to hide the raw noise in location fixes from the GPS signal.
For example, certain Garmin devices (intended to be used off road) will snap your location to an Interstate if you are going more than 55 mph and heading roughly in the direction of the road (which is evident when you run into traffic and your location keeps switching between the frontage road and expressway--the error distribution of the noise doesn't have mean=0!).