Take a look at a hunting map app On-X. If you pre-download the maps it works without cell coverage and has altitude, GPS coordinates, trails, property ownership (not mining claims). $30/year for 1 state, $99 (often on sale for $70) for all states. You do have to manually download 5 or 10 square mile maps so you need to plan ahead. I got it for hunting, but find it dandy for prospecting also.