Likewise, I use an app called Gaia. It's $20, but everything after that is free. There's lots of different map layers you can choose from - USGS, USFS, OpenStreetMap, Google Terrain, Satellite, Shaded Terrain, etc. You can download these maps ahead of time, create and upload tracks/routes/waypoints. I use it all the time and it's saved my butt in whiteout conditions.
