I use Gaia GPS for my backcountry hikes which is available for iOS/Android. You can download a variety of topo maps and aerial photos ahead of time in a particular area and load them when out of cell service.

http://blog.gaiagps.com/

https://itunes.apple.com/us/app/gaia-gps-topo-maps-trails/id...

https://play.google.com/store/apps/details?id=com.trailbehin...

The only issue I'm still trying to figure out when using my phone is how to disable the cell antenna with the GPS antenna still activated to conserve battery.

There is also Backcountry Navigator which has similar features but is only available for Android devices: http://backcountrynavigator.com/

As far as I know, mobac is only for raster. The app mentioned in a sibling comment supports the tile maps:

https://galileo-app.com/manual.html#offline_maps

I guess it would at least be fine for visualization.