Obscurement encounter advice with size differences

Question

I would like to run a 5e encounter in a grassy plains, with tall grass about 3–4ft high. Based on the RAW in the PHB, I am taken to thinking that small creatures (goblins) are effectively under heavy obscurement. That is, creatures trying to see the goblins are looking through thick grass. Normal medium sized creatures, however, are standing about waist high, and would be lucky to be lightly obscured.

Concealment is explained in the RAW on page 183:

"A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight. A heavily obscured area-such as darkness, opaque fog, or dense foliage-blocks vision entirely. A creature in a heavily obscured area effectively suffers from the blinded condition (see appendix A)."

My problem is that the RAW designate an area as under concealment. In a case such as this, where there is a clear line between obscurement (grass) and no obscurement (no grass), there should be some deliberation here. It seems the lines of sight should differ for different size groups as a result of treating those groups as obscured or not. For example, these size groups in 3–4ft grass. (Read ‘→’ as “looking at”.)

• Small → Small (Small creatures would be heavily obscured. Is the grass interfering with the perception roll of the perceiver?)

• Small → Medium (Medium creatures, it would seem, should not be considered under the effects of obscurement, but would the small creatures still have trouble seeing over the grass? i.e. Are they under the effects of heavy obscurement by nature of being in the grass?)

• Medium → Small (Small creatures would be heavily obscured, so a medium creature should have trouble spotting them, but the perceiver itself isn't in heavy obscurement.)

• Medium → Medium (Neither creature should gain any obscurement, but maybe this case could be interesting if there were stealth checks involved or prone creatures.)

My actual question is if we can treat obscurement conditionally, with different sized creatures being a factor, instead of "always on" or "always off" in a given area. It would seem errata v1.0 says:

A heavily obscured area doesn't blind you, but you are effectively blinded when you try to see something obscured by it

Which is decidedly different, turning the condition to the area doing the obscuring to your target, rather than creatures being in a state of obscurement.

I am aware of a lengthy discussion about cloud of fog and how attacks work, so I'm pretty clear on that aspect, but I'm mostly asking about what the state of vision is like here.

The link:

How does concealment work?

Additionally, the only possible thing that was left unanswered in that discussion is about positions and whether sight is required to know a target's position, considering creatures making noise from attacks and movement. Of note, in RAW it says heavily obscured areas block vision but not any other senses. It would be interesting if someone had input here, but in this particular case, since they're goblins, they will be using bonus actions to hide.

Answer 1

If there's anything about noise detection range and accuracy of location in the rules, I must have missed it.
Personally, I prefer to interpret rules like this anyhow rather than wrestle meaning out of RAW and RAI. DnD is deliberately somewhat vague on a number of things. As I understand it, this is to encourage interpretation.

Anyhow, I know what grass is and I know about seeing stuff in it.
If you wanted actual range values then this is the stuff the real world military thinks about and numerous historical wargames rules interpret.
That includes the bresenham elevations interpretation I am implementing in the wargame I'm writing.

Something would have to be a fair distance away before you absolutely cannot see it just because only the head is poking out.
You're talking in the order of 100 yards for a static head sized "target" in good light.
Poor light will reduce this distance.
What the viewer is doing will alter this considerably.
If you're well rested and a responsible "guard" then you'll spot that head whilst someone tired, distracted or not particularly looking would see but not notice. It's just some more clutter in the sea of grass.

If the grass is not just the same plain green or there are patches of different colours or plants or the odd termite mound will reduce this as well.

Someone wishing to hide can crawl or stoop.
You won't see them until they're quite close.
They will likely beat down grass as they go and you're likely to see a suspicious sort of a gap before the goblin.

Anything breaking an outline up - like say grass attached to a hat or mask - will make it harder to spot someone. You can walk right up to someone in a ghillie suit and just not see them.

If you're low down, you won't see very far of course so a sneaker has to pop up and look round once in a while.

The higher you are the better an angle you're going to get and the more likely to see something. Which includes sitting on a horse. Plains riders would sometimes stand on the back of a horse to see further or get their horse to lay down and hide from pursuit.
Even seemingly flat land has undulations and the like which allow someone lying down to take cover or hide in. The army train to use these and you can often crawl right across a field hiding in low depressions.

Large groups of anything will be easier to see at longer range and fast movement is much more noticeable than slow or none. Humans ( and presumably elves etc ) are hard wired to watch out for danger. People who didn't notice movement went out the gene pool way back.

Lots of grass makes a fair bit of noise in any sort of breeze or wind and will suppress sound to some extent. Steel on steel is still going to travel a fair way.

Sound based location is notoriously difficult to be precise at all but short range.

I would expect tribes of goblins living in such an area would adapt hunting strategies to suit. Sneaky hit and runs and a slow wearing down of any enemy. A group travelling in "their" area are likely to find the occasional arrow is shot at them but not see who shot or where they are.
Shoot a couple of arrows from one "side" to get everyone looking that way.
Then the 30 goblins on the other side open up and shoot from behind.

Answer 2

You're on the right track but you just need to combine the concepts a bit.

RAW, Concealment is a property of an area, and can either be Lightly or Heavily Obscured.

The grass blocks line of sight, but only within a set distance from the ground.

Therefore, the grass constitutes a Heavily Obscured area that only exists close to the ground.

Anything fully within the grass is Heavily Obscured, and anything above the grass is visible. So a goblin is short enough that their entire body is Heavily Obscured, whereas a medium creature standing upright will have their legs Heavily Obscured but their torso completely visible. Whilst you could argue that being partially within a Heavily Obscured area might count as Lightly Obscured, I would contend that vision doesn't work that way, RAW, and the visibility would be dependant on relevance. Meaning, if somebody wanted to investigate your shoes, they would be unable to see them because your feet are within a Heavily Obscured area, but if someone wanted to attack you, they would receive no penalty (unless the DM rules that the grass also provides Cover against attacks, which is unrelated to Obscurement).

Of course, you could rule that the grass is thin enough to only count as a Lightly Obscured area, in which case the same rules apply but with Disadvantage on Perception checks instead of being completely unable to see. One might even decide that the grass is Heavily Obscured very close to the ground, and Lightly Obscured further up as it spreads out. Again the same logic applies except you need to keep track of what's in which area.

Answer 3

The area is light obscurement.

Goblins are in total cover (including other small creatures).

Medium sized creatures are in three-quarters cover.

Consider that the medium creatures actually have better sight over the area, looking down into the grass.

The grass is loud, if anyone is moving in it - the grass would reveal the presence of a creature in ear shot equally to the small and medium sized creatures.