> Mercator useful for sailing+

It is also used by the military (or at least NATO). They use https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_...

While most of that system is a way to tag coordinates that are "simpler" than just latitude and longitude to a position, it's underlying system is also the Mercator projection. And some properties of Mercator, e.g. that it is angle-correct. So you can use UTM e.g. in Elint for direction finding.

It's not distance or area correct, but for normal infantry this is meaningless, since this only shows on large scales, but not the slow scales of infantry.