Unfortunately, there’s not much you can do with the G.P.S. According to Apple, the iPhone’s G.P.S. antenna is much too small to emulate the turn-by-turn navigation of a G.P.S. unit for a vehicle, for example. Instead, all it can do at this point is track your position as you drive along [source]

huh? In fairness, I’ve seen more egregious butcherings of GPS related tech from bloggers in the mapping/gis domain, but still a 2 minute perusal of the wikipedia article on GPS by Apple could help to avoid further clouding of this already very misunderstood technology. If you can get 2 successive fixes, you can calculate direction and speed. assuming you have access to the underlying street network, you can then calculate turn guidance. software and data!