There are 72 unique points required for this achievement. You'll want to explore every area thoroughly as it seems some buildings in an area may count as a location by themselves, or what appears to be one area may be separate into a few (such as the interior of the hotel). To see if you're on track, you should have 18 at the end of Act 1, 45 at the end of Act 2 and then 57 at the end of Act 3. Make sure you have 57/72 locations before the "point of no return" (the game warns you when this occurs) as there are 15 locations to find in the prison before the game ends.
Note: If you have 71/72 locations before the patch released on September 14th, you will need to use this trick to reset your achievement data, because the patch upped everyone's count to 72/72 but did NOT unlock the achievement. Be warned that ALL your achievement data will reset, not just this one! I would suggest getting every other achievement in the game first. Your save files will not be affected by the trick. If you had less than 71/72 before the patch, you won't need to do anything and it should work fine when you reach the last location in the prison. If you started the game with the patch in place, you're also good there.
Commonly missed locations:
- Southwestern Moresby: There is a fenced in warehouse, as well as the beach directly west of it.
- Southern Moresby: The boat on the shore along the bottom of the map.
- Northern Jungle: There are two small villages up there.