There are three levels of Fog of War available in Starcraft:
Fully Obscured - The area is completely hidden from the player.
Partially Obscured – The Player can see terrain but cannot see any troops or activity in the area.
Revealed – The Player can observe both terrain and units in the area.
To set the initial Fog of War settings for each player, you can use the masking tools in the Fog of War Layer. These tools allow you to set areas of the Fog of War to be either fully or partially obscured. From the Layer menu, select Fog of War. Click or click and drag in the main window to mark areas as partially obscured. Painting over these areas again will change them back from partially obscure to fully obscure. The preceding method will uncover the Fog of War layer for the current player selected. To alter the Fog of War layer for all players in a scenario, hold down the [SHIFT] key while clicking in the main window.
To reveal areas to players you will need to use map revealers.
Map revealers are treated as units, although players cannot see or control them in the game. Map revealers can be found in the Tool Tree or Brush Palette and are placed in the same way as units.
Placing map revealers in an area reveals that area of the map to the player at the start of the game. Since map revealers are treated as units, they belong to specific players. Only the map revealer’s owner benefits from the sight granted by that unit. For more information, see Unit Ownership.
Note: If you want to make an area visible to all players, you must create a separate map revealer for each player.
Areas can also be revealed on a temporary basis. By using triggers to add or remove revealers from the map, you can control when, where and how long areas are revealed during the course of a game.