There is a BIG difference between a Goggle map, and a REAL GPS App that is completely self contained within the phone. I use an app called NAVIGON, that is owned by Garmin, and it works exactly like a real GPS and needs no cell service whatsoever. I also have it sync' with iCloud so any additions or changes I make with my waypoints will migrate to my iPad as well as keeping the waypoint file in a cloud backup in the backgroun
I carry a Garmin GPS in my flight bag for business travel and haven't pulled it out in at least three years since using NAVIGON. There are lots of GPS apps out there, but you get what you pay for.