ATAK (Android Team Awareness Kit) is an open source geospatial mapping software developed by the U.S. Government for a variety of purposes. It is now available for free on the Play Store.
ATAK can use a wide variety of imagery sources (i.e. maps) and elevation data. It allows for easy route-planning, 3D View of an area, and many other cool features. Most importantly all of this stuff is free — in this guide we’ll set up an ATAK install for offline use. Note that ATAK is VERY feature-rich so we’re only going to cover some of the features here, please poke around in the app and see what works for you.