The Android Team Awareness Kit (ATAK), for civilian use, or Android Tactical Assault Kit (also ATAK) for military use – is a suite of software that provides geospatial information and allows user collaboration over geography.

ATAK was originally developed by the Air Force Research Laboratory (AFRL) and is now maintained by the TAK Product Center (TPC).

Overall, it’s a bit less user-friendly, has a steeper learning curve, than most of the rest of the sources on this page, but for the right person or task, is absolutely a god-send.  Here’s an introductory guide written for/on Medium.