After determining surprise, the DM must determine the encounter distance. Determining encounter distance when outdoors is outlined on page 49 of the DMG. The base distance is 6″ to 24″ (6d4). If either party involved is surprised, the distance is modified by subtracting the value of the surprised parties’ rolls. For example, if party A is surprised and their roll was 2, the distance is 6d4 – 2. Surprise decreases the encounter distance.
Terrain will also modify the encounter distance:
Scrub— -1 per die on all 3’s and 4’s
Forest— -1 per die on all numbers
Marsh— -1 per die on all 2’s, 3’s, and 4’s
For example, suppose a roll for base distance results in: 1 + 2 + 3 + 4 + 3 = 13″. If the encounter takes places in a marsh, the rolls are modified as follows: 1 + (2 – 1) + (3 – 1) + (4 – 1) + (3 – 1) = 7″.
Determining encounter distance indoors is outlined on page 62 of the DMG. The base encounter distance indoors is 5″ to 10″ (1d6 + 4), subject to the following conditions:
- Modify the distance, if necessary, taking into account line of sight.
- If one of the parties is being noisy, the other has the opportunity to flee or conceal themselves. If concealment is chosen, the encounter distance is 1″ to 4″ before discovery.
- Next, the distance must be constrained by the actual size of the room.
- If the encounter is the result of an unplanned appearance, for example, teleporting in, the effect on distance is the same as when constrained by a room’s area.
- If either party is surprised, the encounter distance is 1″ to 3″.
- Lighting also affects encounter distance:
- If reliance on a light source is necessary, then the encounter distance is limited to two times the light source’s radius.
- The range limit of infravision and ultravision limit encounter distance as well.