To get the Silent Cartographer achievement in State of Decay 2 do the following:
Billboards and radio towers are listed on your map with binocular icons. Go to these locations, scale them and press the aim button to look around at your area. Glance at each of the question mark symbols until they change into something more revealing. After 50 question marks are undone, this achievement will pop.
Note that you'll need to go to more than one scouting location, as seemingly nowhere can you view the full 50 from one location.