From my understanding the phone apps use cell towers, which cause a few problems. First, cell towers accuracy, when compared to a dedicated GPS unit (satellite technology), is inferior. Second, cell towers reception is not as reliable as satellite technology. Lastly, use of cell phone apps will drain your phone battery and is not as efficient as a GPS unit.
I bought the Garmin eTrex 20, which allows the use of micro chip mapping. I used the OnxMaps (Hunt Maps) in this GPS unit and it worked perfectly.