Vegepygmy said:
The DMG gives minimum Spot distances for various terrain, beginning on page 87. For example, in sparse forest, the farthest distance at which you can possibly Spot someone is [3d6 x 10] feet.
You know all that information was in a nice table instead of spread out among the paragraphs of text in the original 3.0 DMG (Page 60). Also gave the DC for spotting someone who wasn't hidding, based on size and activity. I'll post it below.
Spotting Distance
Smoke or Heavy Fog 2d4 x 5ft (avg 25ft)
Jungle or Dense Forest 2d4 x 10ft (avg 50ft)
Light Forest 3d6 x 10ft (avg 105ft)
Scrub, brush or bush 6d6 x 10ft (avg 210ft)
Grassland, little cover 6d6 x 20ft (420 ft)
Spotting Difficulty
Base DC 20
Size +/-4 per size category (from medium)
Contrast +/-5 or more
Stillness (not moving) +5
Six or more creatures -2
Moonlight* +5
Starlight** +10
Total darkness impossible***
*+5 bonus on Spot check if the spotter has low-light vision or darkvision that extends far enough.
** +5 bonus on Spot check if the spotter has low-light vision or +10 if he has darkvision that extends far enough.
*** Unless the spotter has darkvision that extends far enough.
Steps
1. Determine vision conditions and terrain. See table above.
2. If line of sight or illumination defines the distance at which the encounter occurs (as often happens indoors) start the encounter there. Otherwise roll for spotting distance.
3. All creatures make spot checks. Success means that the creature sees the other creature or group.
4. If neither group succeeds, all creatures spot each other at one-half the rolled range.
Hiding and spotting.
If you are attempting to move and hide as you move across terrain, you move at half speed, and suffer -2 penalty to your spot checks to notice other creatures as you are staying low and using cover. Instead of the Base DC 20 for other to spot you the DC is 25 + the hider's Hide skill modifier. If the hider's Hide skill modifier is -6 or lower treat them as if they weren't hiding. Also creatures do not automatically spot hiding creatures at one-half range 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 Hide checks.