Cell service would give you live maps, but you can download/cache them at home. And you don't really need detailed maps for basic topography orientation.
I used a broken (screen) phone with no sim card (no cell service) but turn my "mobile hotspot" to ON (on the regular cell phone with sim card) and then connect the no sim card phone to the wifi and all works well.
Not only is it not needed, I typically (unless I forget) turn on Airplane Mode when flying, just so I don't get disturbed by phone calls and notifications.