Encounter Distance (DMG)

Once an encounter occurs, it is necessary to know the range at which the creatures might first be noticed. This distance is dependent first on whether or not either group is surprised or, if no surprise occurs, on the type of terrain the encounter occurs in. Encounter distances for different conditions and terrains are listed on Table 58.
 Table 58: Encounter Distance
Situation or Terrain 	Range in Feet
Both groups surprised 	3d6
One group surprised 	4d6
No surprise: 	
Smoke or heavy fog 	6d6
Jungle or dense forest 	1d10 x 10
Light forest 	2d6 x 10
Scrub, brush or bush 	2d12 x 10
Grassland, little cover	5d10 x 10
Nighttime or dungeon 	Limit of sight
In situations where no cover is possible, encounters will occur at the limit of vision unless special circumstances dictate otherwise.
While it is possible to spot another group at quite a distance, the characters or creatures may not be able to identify them immediately. The observation ranges given in the Player's Handbook may require creatures to close in order to make a positive identification.