The description for this trophy is slightly misleading as revealing the ocean is also required, however, focus first on revealing all of the land area. You will notice that at the start of the game, despite the whole map being available to explore, the vast majority of it is clouded over with its features indistinguishable. The best way to reveal the map this is to get a helicopter or a plane (the fighter jet which you can steal from Fort Zancudo is preferable since it's the fastest) and fly along the coast to define the land boundaries, then start in one corner and work your way north (or south, depending on your starting point) before turning round and going back the other way in a parallel line. Do not worry as much about the ocean as you will reveal the majority of it whilst searching for Nuclear Waste for Waste Management .
If after all this you are still without the trophy, set your waypoint on areas of the ocean still undefined and either fly or use a boat to uncover these areas. Note: The map "ends" where the water turns deep blue so if you carry on past this point, you are not uncovering any more of the map.
Luckily you do not have to reveal every last pixel as the trophy unlocks once around 95% of the map is revealed.