TopoDroid is an advanced Android application for cave surveying. It will run on your phone or a tablet, and is best used in conjunction with a modified Leica Disto X310 (making it a DistoX2 to cavers). TopoDroid is free, but you must buy your own disto, upgrade kit and computing device. Sets new standards for DistoX2 interaction, however complicated to use, and not well posed for use with Therion, despite adopting Therion's drawing paradigm.
Download from Google Play Store
Comparison of TopoDroid with similar applications. Therion In Cave Mapping wiki page.
DistoX, TopoDroid and Therion wiki page.
Some TopoDroid and DistoX2 ‘How to’ videos by Derek Bristol
King Country, Awakino projects.