So I'm mostly copying a outline from another location discovery achievement but felt this achievement needed a better solution than the "just explore" solution and also I have never done a solution before so hopefully it's not too bad for everyone.
To view unlocked locations go to the map, and press the Y button to view from ORBIT, and then select the location and press LS (the left analog stick) to view progress
Locations should contain:
Sandblast Scar (4)
Devil's Razor (13)
The Great Vault (2)
Cathedral of the Twin Gods (5)
Destroyer's Rift (4)
Konrad's Hold (12)
The Splinterlands (6)
Guts of Carnivora (5)
Carnivora (5)
The Droughts (13)
Covenant Pass (3)
Ascension Bluff (5)
The Slaughter Shaft (1)
If you're like me and was 77/78 for a while you likely haven't found The Slaughter Shaft, an area accessible through Konrad's Hold and is NOT story related so can be missed very easily.
If you have more than one location missing and have the whole map uncovered likelihood is that an area or two didn't register as discovered despite you having been there. I have found a lot of people (including myself) having the same issue with The Splinterlands for example with Pitt's Stop being discovered but upon arriving there again being met with the 'Area Discovered' indicator and thus now registering.
Hope this helps and let me know if any other areas are similar to my problem with the Splinterlands, I'll try to update if so.
EDIT: Some people have said if you don't get the achievement while having explored all areas to explore them all once more in a single instance/session and it should unlock, others also said that loading the game without the DLC got them the achievement so be wary until a patch comes along to fix things