Theodolite is an excellent app. In the US, the ScenicMap apps are very functional offline, and Topo Maps has the best set of USGS maps that I have found so far. Galileo is good for custom offline map production.
How do you feel about being on the same internet as them? Much less secure than the uni-directional multicast network that is proposed here, which makes your concern the equivalent of worrying that criminals might be watching the same TV station that you are.