Encounter Distance (PHB)

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Once your character or party has an encounter and it has been determined whether or not anyone was surprised, your DM will tell you the range of the encounter—the distance separating you from the other group. Many factors affect encounter distance. These include the openness of the terrain, the weather conditions, whether surprise occurred, and the time of day, to name a few. Although you do not know the exact distance until your DM tells you, surprise, darkness, or close terrain (woods, city streets, or narrow dungeons) usually results in shorter encounter distances, while open ground (deserts, plains, or moors), good light, or advance warning results in greater encounter distances (see Chapter 13: Vision and Light).