All the map needs to be explored, even the coastline edges, until there are no fog patches visible. The easiest way by far is to use an airplane like the crop sprayer, and mentally separate the map into blocks, exploring each with the airplane, setting waypoints as guidelines. Unlike previous GTA games, the map is fully explorable from the start, just the pause menu map needs to be revealed, why I'm not sure, this achievement seems a bit silly. You need to explore the WHOLE map, to make it all visible. You'll notice when you first start the game that most of the map is greyed out with 'fog'. This achievement is obtained by 'unlocking' all the map.
