How does a perception check while flying work? Is there a min/max height to fly when scouting?

Question

As a flying character I'm trying to do some aerial recon/scouting and wanted to know how a perception check works (passive and active) while flying.

Aside from lightly/heavily obscured (PHB p. 183), what other penalties/factors are there?

For example:

• Does height play a factor? (See note 2)

• Does speed play a factor? (See note 1. If yes, assume moving at base fly speed (50ft)).

I'm planning on doing daytime scouting for mobs (aka targets/enemy units), both hidden and wandering (seeing if the group's path is clear or not), and anything out of the ordinary (campsites, lairs, caves, huts, anything that might require further investigation).

At night I plan to sky patrol (using ground level light spell, dancing lights spell). Also looking for other light sources (campfires, torches) that may be in the area/on approach.

If height makes a difference/you need a number, assume aerial scouting at just over 600 ft (say 625 ft). This is the magical number where long bows (all normal ranged weapons) cannot hit during the day.
For night patrols, assume just over 120ft (say 125ft), which is outside most night vision sight.

If possible please include any RAW that relates to this question.

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Note 1: On PHB p. 182 there are penalties to passive perception checks for traveling at a "fast" pace (see table below). However this is for ground not flying (and I believe only passive checks not active checks).

Travel Pace

Pace      Distance Traveled per...      Effect
          Minute   Hour      Day        

Fast      400 ft   4 miles   30 miles   -5 penalty to passive Wisdom (Perception) 

Normal    300 ft   3 miles   24 miles   —

Slow      200 ft   2 miles   18 miles   Able to use stealth

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Note 2: My understanding (and would like input/confirmation of this) of RAW is that only lightly obscured (disadvantage on perception checks), heavily obscured (blocks vision), and possibly travel pace are the only factors in perception checks (both ground and air). Only LOS matters, thus the higher up I go the farther I can see/get perception (passive or active) checks on any items of interest (mobs, structures, etc.) not heavily obscured. Any limitation on this would be a DM house rule.

Answer 1

Ability Checks

The DM calls for an ability check when a character or monster attempts an action that has a chance of failure. When the outcome is uncertain, the dice determine the results. (PHB p.174, "Ability Checks," emphasis mine.)

 

When deciding whether to use a roll, ask yourself two questions:

• Is a task so easy and so free of conflict and stress that there should be no chance of failure?
• Is a task so inappropriate or impossible--such as hitting the moon with an arrow--that it can't work? (DMG p.237, "Using Ability Scores," emphases mine.)

So the rules say that the GM should only allow your perception checks in situations where it would even be possible to perceive something. On top of this they might be disadvantaged, or penalized, or have DC-adjustments. In other words, it's up to your GM to decide "\$Y\$ is the altitude above which I do't even have you make checks, \$Z\$ is the altitude above which those checks are disadvantaged" or however they'd like to handle it.

Do they apply in your hypotheticals? Impossible to say without considering, as DaleM does in his answer, the sizes of objects and your flying height and speed. Really, this is one of those situations where you and you GM might spend an afternoon at a coffee shop trolling Wikipedia for information on visual acuity.

I do know one thing, though: If you're putting a lot more mechanical granularity into your character's vision than your table-mates are into their characters' vision, be careful. In that situation you're actually playing with a different quality of playing-piece than the rest of your table-mates, which can lead to tension down the road.

Answer 2

RAW: as far as you want.

If you want to introduce real world physics, the most distant thing that most people can see with the naked eye is the Andromeda Galaxy - at 2.25 million light years that's probably further than your fly spell will take you.

What you can see is a function of how far away it is, how big it is, how bright it is compared to its background and how much ambient light falls on your eye. While Andromeda is a long, long way away, it's bloody huge, puts out the light of 100 billion stars, sits against a black background and is only seen at night a long way from urban areas.

A normal human when tested using the Snellen eye chart has a visual acuity of 20/20 (see here). Which means that in perfect conditions, you can distinguish letters that are about half an inch high at a distance of 20 feet. What is important to note here is that as the distance increases this is a trigonometric ratio, that is there thing you are trying to see must subtend the same angle in your field of view.

Except, it's more complicated than that. If we are dealing with near perfect contrast (like an eye chart) the light received from the bright bits falls off as the square of the distance which complicates things. Also, we can see greens and yellows much more easily than reds or blues. Oh, and moving things attract our attention - this is not so much an eye thing as it is a brain thing - moving things might be moving to eat you. Plus things that are self illuminated are usually brighter than things that reflect light. But specular reflectors are much easier to see than diffuse reflectors provided you are on a line that reflects a light source, otherwise they are much harder to see. Do you see?

With all this in mind, remember that you are playing D&D, not looking for ISIS missile emplacements in Syria, so it really doesn't matter if you mess it up. Darkness gives disadvantage by RAW, after that just keep piling on negative modifiers for distance and roll the dice.