seeker333

There are several free or low cost GPS Nav apps which utilize offline maps that you download beforehand in anticipation of need of specific map. Once you have the maps, all you need is a GPS signal, no cell signal necessary. Some that I have used are Osmand and maps.me (maps with me). There are others I have tried and cannot recall. Nearly all provide voice-prompted navigation for route, like motor vehicle GPS. Osmand allows you to use small maps (if your phone has limited memory) or put the maps on sdcard memory if your phone has this capability. It may takes hours to learn some of these GPS Nav apps, they are more complex than 90% of the apps out there, so prepare to put in more effort than usual for a new app.

You can extend your phone's battery life if you turn phone off whenever you know you are a long way (>10 miles/1 hour) from the next turn/waypoint, and use a Cateye or similar cyclocomputer to gauge your proximity to next turn, then turn phone back on when near waypoint. Phone will typically boot up and reacquire GPS signal in less than 90 seconds.

Although workable on a phone, I find GPS Nav apps are a bit easier to use with the larger display of a tablet, especially WRT route planning. Also a stylus/pen is very useful to minimize fat finger errors and display smudges/cleaning.