I second using GAIA.

You can download maps so that when out of cell range you can run the GPS with the phone on Airplane mode to save battery life. The phone isn't constantly searching for towers.

For hiking and driving in National Forest I like their downloadable maps that combine the USGS topo and the Forest Service User road maps. Much easier than trying to compare the tiny Metsker USFS maps with the larger USGS topos to find where you are on a remote dirt road without signs. You can also download satellite photos.