One-Man Scouting Party

 is an Fallout 3 achievement on Xbox 360 and PC and a trophy on PlayStation 3.

It is granted for discovering 100 locations. You must literally go to the 100 locations, the explorer perk puts all locations on your map but it does not discover them for you, once you travel to the location, the symbol on your pip boy will invert color and it will be considered discovered.

Strategy
If you have a save around late level 19, one method is to level up and utilize the explorer perk, allowing you to have all locations revealed, once this is done, load up with weapons and ammo and head into the wastes going from location to nearest location until you get your achievement, condensed city centers are the best place for fast discovery as they are packed together and have many per instance. A good idea is to go to the city area, near The Washington Monument. There are many locations here, all very close together. Once you have the explorer perk, you can see how many there are.

The game requires you to find just below 66% of the map markers. You can find a good 20 or so of these in the downtown D.C. areas by wandering through the metro lines. Around the area of Arefu, there are more than 5 places within a close walking distance of each other. Canterbury commons is surrounded by 2 places.

In Playing Operation:Ancorage you will encounter a good amount locations. You do not need to go out of your way to find them.