The Google Maps iOS app finally has turn by turn navigation and it’s had good reviews, but one of the reasons I mostly use a stand-alone app (Navigon) is that it does not require a wifi or cellular connection. Google Maps does, at least to set up a route initially. The answer as to whether it needs one after that is both Yes and No.
I know it caches some map data for when it is out of wifi or cellular range, but my online searching wasn’t able to find out how much and to what level of resolution, so I tested this recently. I set up a route from my house to Eugene, OR, then turned off wifi and cellular data and started out.
Here is a screenshot of where I ran out of detailed map data. Google appears to cache about the next 2 miles worth of detailed map data along the route. This is in an area with few side roads. It may cache an even shorter distance in areas where there is more map detail to store.
However, the good news is that it also caches the entire route (at least my whole 175 mile route to Eugene). Sort of. What it caches is the main route, including turns and highway changes, but no other street and road data. It still gives turn by turn directions, so if you can stay on the route you will get to your destination. However, if you get off the route, it can’t get you back on it or recalculate a new route without a cellular or wifi connection. Even in this case, you can probably still work your way back to the route. The GPS in the iOS device will still show your position on the screen as well as the route line, so you know which general direction to go to get back on the route.
While you are driving you aren’t going to get wifi anyway, so Google Maps is dependent on having a cellular connection most of the time and bridging the gaps with the non-detail route information that is cached. Google Maps should work just fine for most trips in populated areas. Google Maps is less desirable if you are planning on getting lost in an area without cell coverage. For guys, who never ask for directions anyway, it doesn’t make any difference.