When an encounter between the PCs and an NPC or creature is imminent, follow these steps:
The circumstances that can affect the DC of a Spot check are as follows:
Size: Add +4 to the base DC of 20 for each size
category the creature being spotted is smaller than Medium-size or -4 for each
size category larger. You can make exceptions for creatures with unusual shapes,
such as a Large snake that's low to the ground and thus as hard to see as a Small
Contrast: How starkly the creature's coloring stands out against the surroundings. It's easy to spot a brightly colored couatl in a dark jungle and hard to see winter wolves in the snow.
Stillness: It's harder to see creatures that are not moving.
Six or More Creatures: Groups of creatures are easier to spot, even if the creatures are smaller than Medium-size.
Moonlight: Nighttime, but with moonlight (or similar light).
Starlight: Nighttime with no moon but a clear, starry sky (or similar light).
Total Darkness: Overcast at night, or otherwise lightless.
If creatures are trying not to be seen, it's usually harder to spot them, but creatures that are keeping low to avoid being spotted also are less likely to notice other creatures.
If creatures are hiding, they can only move at half their normal overland speed. They also suffer a -2 penalty on their Spot checks to notice other creatures because they are staying low.
Instead of a base DC of 20 for others to spot them at the standard spotting distance, the DC is 25 + the hider's Hide skill modifier. The modifiers from Table 3-2: Spotting Difficulty still apply, except for the size modifier (which is already part of the character's skill modifier). A character whose Hide ranks, Dexterity modifier, and armor check penalty total -6 or lower is actually has a lower DC than if he or she weren't hiding. In such cases, simply calculate the Spot DC as if the character weren't hiding (according to Table: Spotting Difficulty). If a creature gets a special bonus to Hide because of camouflage, special coloring, and so on, use that bonus rather than the contrast bonus from Table: Spotting Difficulty.
Additionally, the other creatures do not automatically spot hiding creatures at one-half the encounter distance. Instead, that is the distance at which the other creatures can make Spot checks to notice the hiding creatures. These are normal Spot checks opposed by the hiders' Hide checks.
|Table: Spotting Distance|
|Smoke or heavy fog||2d4 x 5 ft. (avg. 25 ft.)|
|Jungle or dense forest||2d4 x 10 ft. (50 ft.)|
|Light forest||3d6 x 10 ft. (105 ft.)|
|Scrub, brush or bush||6d6 x 10 ft. (210 ft.)|
|Grassland, little cover||6d6 x 20 ft. (420 ft.)|
|Total darkness||Limit of sight|
|Indoors (lit)||Limit of sight|
|Table: Spotting Difficulty|
|Size||+/–4 per size category|
|Contrast||+/–5 or more|
|Stillness (not moving)||+5|
|Six or more creatures||–2|
| * x25 if one side is hiding, and ignore size modifiers
** +5 bonus on Spot check if the spotter has low-light vision or if he or she has darkvision that extends far enough.
† x+5 bonus on Spot check if the spotter has low-light vision or +10 if he or she has darkvision that extends far enough.
†† Unless the spotter has darkvision that extends far enough
The rules for spotting creatures assume that both sides will eventually notice each other, and they simply establish the distance at which they do so. But sometimes you want to take into account the possibility that the two groups will miss each other entirely.
To handle these possibilities, simply let there be a 50% chance that the other creatures encountered and the PCs don't get any closer but rather pass by each other, such as when one group is moving north and the other east. (Creatures following the PCs' trail, of course, always close with them.)