Problem is (as I understand it) that phone based GPS'es usually don't have the maps in their memory. A 3G/4G connection is necessary to access maps beyond a small area. Germin's (for the most part) have them internally.
Yes some apps have the ability to download maps of limited area, but it would take a shitload of them to cover something like the TAT.
Anyone know of a phone app which has all the maps internally ?