Does the 5e DMG discuss encounter distance?

Question

As a DM, I am frequently unsure exactly where to start an encounter. Especially since most of my players have ranged characters and can often seriously harm an enemy before they get close to them. The only reference I have right now is the table on the official DM screen, but those distances seem so big (like 200 feet in grassland) compared to what I usually see on streams.

I know the easy answer is "You can just set the distance as a DM" but it is sometimes really hard to make it seem believable for myself and the players that they have not spotted something from far away. I'm not considering creatures with special abilities like invisibility, shapeshifting, and other stealth options since I am looking for general advice that applies to all encounters and some general mechanic on how to determine it.

A good answer would cover the above and also give specific sources from the DMG, PHB, or other (official or not) sources.

Answer 1

In 5e, with few exceptions, this is left to the DM

Combat Step by Step on page 189, PHB states:

1. Determine surprise. The DM determines whether anyone involved in the combat encounter is surprised.
2. Establish positions. The DM decides where all the characters and monsters are located. Given the adventurers’ marching order or their stated positions in the room or other location, the DM figures out where the adversaries are—how far away and in what direction.

So this is explicitly left to the DM, with little further guidance. For Underwater Encounters, the DMG provides an an encounter distance table on p. 116:

Creature Size [sic]	Encounter Distance
Clear water, bright light	60 ft.
Clear water, dim light	30 ft.
Murky water or no light	10 ft.

For sea travel, it offers the following (p. 119)

A relatively calm sea offers great visibility. From a crow's nest, a lookout can spot another ship or a coastline up to 10 miles away, assuming clear skies. Overcast skies reduce that distance by half. Rain and fog reduce visibility just as they do on land.

On the Ethereal plane, visibility is limited (p. 48 DMG):

Visibility in the Border Ethereal is limited to 60 feet. The plane's depths comprise a region of swirling mist and fog called the Deep Ethereal, where visibility is limited to 30 feet.

In closed environments or darkness it may be relatively easy -- line of sight or darkvision limit how far you can see, at least, and can form a natural baseline. The DMG covers Noticing Other Creatures on page 243, and bases this on skill checks, but does not provide distances by terrain:

While exploring, characters might encounter other creatures. An important question in such a situation is who notices whom.
Indoors, whether the sides can see one another usually depends on the configuration of rooms and passageways. Vision might also be limited by light sources. Outdoor visibility can be hampered by terrain, weather, and time of day. Creatures can be more likely to hear one another before they see anything.
If neither side is being stealthy, creatures automatically notice each other once they are within sight or hearing range of one another. Otherwise, compare the Dexterity (Stealth) check results of the creatures in the group that is hiding with the passive Wisdom (Perception) scores of the other group, as explained in the Player's Handbook.

Environmental Conditions

In Tomb of Annihilation, you get visibility in heavy rain (p.11):

Visibility in heavy rain is limited to 50 yards. Beyond that distance, only Huge or larger objects can be distinguished. Missile weapon ranges are halved during rain.

In Wild Beyond the Witchlight you get visibility in thick fog (p. 74):

Thick fog hangs over all outdoor areas, limiting visibility to 20 feet.

In Icewind Dale: Rime of the Frostmaiden, you get blizzards (p. 10) [Thank you to Pepijn]:

A blizzard's howling wind limits hearing to a range of 100 feet (...) It also imposes disadvantage on Wisdom (Perception) checks that rely on hearing. Visibility in a blizzard is reduced to 30 feet.

Real world limits to visibility

Lastly, 200 feet in grassland is not a large distance to see someone at. The human eye can see things that are miles away if there are no obstructions, and you can see the light of stars that are thousands and galaxies that are even millions of light years away. On a planet comparable in size to the earth (such as Toril of the Forgotten Realms), you would be limited primarily by the surfaces curvature, and be able to see about 3 miles on flat terrain, and much further if you are higher up on a tower, mast or mountain.

What is mostly limiting seeing distance in the open in addition to this is the atmosphere (air scattering light, fog, rain etc.), things in the way (trees, hills, mountains, houses etc.), light conditions (twilight, darkness), and how big or small the things you aim to see are (small detail being lost in distance). The many combinations of realistically combining these is likely why the DMG does not aim to codify it as a rule.

The relatively short sighting distance on the DMG screen for terrain of only 6d6 x 10 (average 210) feet in terrain like desert or grassland helps playability: they allow ranged combat for just a few rounds and in a range where longer-ranged weapons have reach. But realistically, you would see another group at a larger distance than that in the open, if they were not hiding or lying in ambush.

Answer 2

3.5e has terrain specific rules for this.

The guidance in 5th Edition materials for this sort of thing is unfortunately sparse and spread out across multiple sourcebooks and published adventures; Groody documents them in his answer. You mentioned the official DM screen, and the table there isn't even printed in any of the sourcebooks. Fortunately, 3.5e has some pretty robust guidance for this in several different wilderness environments. The full chapter from the SRD can be found here, and it is freely available, so I'll reproduce the relevant bits for us below.

I've used several of these before, they integrate rather seamlessly into 5th Edition, substituting 5e's Wisdom (Perception) check for 3.5's Spot (Wis) check. They give good estimates for distance at which two different groups of creatures could conceivably detect one another, which puts a ceiling on the distance encounters can begin at.

If you're on flat plains with no cover, your scout characters are going to be able to begin encounters at long range, and there's really no way around that. And that's okay - you should let your characters who have invested in their eagle eyes be good at that. But if you're walking through a dense forest, it doesn't really matter how high you can roll Perception - if you can't see, you can't see. They can rely on ears and nose, but the range at which the characters can trigger initiative rolls via directed hostile action will still be greatly reduced. After all, you can't shoot at a smell (except possibly if they smell like menthol).

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Stealth and Detection in a Forest

In a sparse forest, the maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 3d6×10 feet. In a medium forest, this distance is 2d8×10 feet, and in a dense forest it is 2d6×10 feet.

Because any square with undergrowth provides concealment, it’s usually easy for a creature to use the Hide skill in the forest. Logs and massive trees provide cover, which also makes hiding possible.

Stealth and Detection in a Marsh

In a moor, the maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 6d6×10 feet. In a swamp, this distance is 2d8×10 feet.

Undergrowth and deep bogs provide plentiful concealment, so it’s easy to hide in a marsh.

Stealth and Detection in Hills

In gentle hills, the maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 2d10×10 feet. In rugged hills, this distance is 2d6×10 feet.

Hiding in hills terrain can be difficult if there isn’t undergrowth around. A hilltop or ridge provides enough cover to hide from anyone below the hilltop or ridge.

Stealth and Detection in Mountains

As a guideline, the maximum distance in mountain terrain at which a Spot check for detecting the nearby presence of others can succeed is 4d10×10 feet. Certain peaks and ridgelines afford much better vantage points, of course, and twisting valleys and canyons have much shorter spotting distances. Because there’s little vegetation to obstruct line of sight, the specifics on your map are your best guide for the range at which an encounter could begin. As in hills terrain, a ridge or peak provides enough cover to hide from anyone below the high point.

Stealth and Detection in the Desert

In general, the maximum distance in desert terrain at which a Spot check for detecting the nearby presence of others can succeed is 6d6×20 feet; beyond this distance, elevation changes and heat distortion in warm deserts makes spotting impossible. The presence of dunes in sandy deserts limits spotting distance to 6d6×10 feet.

The desert imposes neither bonuses nor penalties on Listen or Spot checks. The scarcity of undergrowth or other elements that offer concealment or cover makes hiding more difficult.

Stealth and Detection in Plains

In plains terrain, the maximum distance at which a Spot check for detecting the nearby presence of others can succeed is 6d6×40 feet, although the specifics of your map may restrict line of sight. Plains terrain provides no bonuses or penalties on Listen and Spot checks. Cover and concealment are not uncommon, so a good place of refuge is often nearby, if not right at hand.

Stealth and Detection Underwater

How far you can see underwater depends on the water’s clarity. As a guideline, creatures can see 4d8×10 feet if the water is clear, and 1d8×10 feet if it’s murky. Moving water is always murky, unless it’s in a particularly large, slow-moving river.

It’s hard to find cover or concealment to hide underwater (except along the seafloor). Listen and Move Silently checks function normally underwater.