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I'm running an adventure for my two most creative players. One of them currently has Tremorsense. This tremorsense is the same as the one given by a pair of Tremor Boots.

The players are in a (magical) completely dark area, so he's using Tremorsense to perceive nearby enemies.

However, this area has a huge chasm. It's not really wide, but it is really deep (exactly 563.12 feet; don't ask why). It is just 10 feet wide however. Consider it infinitely long.

If the character comes really close to the chasm, the area of his tremorsense would reach the other side. Would he be able to detect enemies on that side? If the chasm was bottomless, would that make a difference?

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4 Answers 4

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Some notes for tremorsense: While the rules are written for easy use, I prefer to have a logical background so that the players cannot corner me with inconsistencies. Sorry if this is a bit explicit.

The background in physics: Tremors is simply sound waves transmitted through the earth. It works best with hard and elastic material and worst with soft and plastic ones. Therefore it will work good on rock and granite, but will have problems e.g. in swampy or sandy areas. Adjust precision and range for the location.

Now for your problem at hand: Could a gap be detected and/or things on the other side of the gap ? Every vibration will move through the earth and it hits a zone where the density/sonic speed changes, it will be reflected. The higher the difference in density and sonic speed, the stronger the reflection.
So your gap will be strongly perceived by the character, but he is unable to perceive anything on the other side if there is no connection. The gap does not need to be wide.
If you lower the frequency of the vibration gaps can be penetrated, but it will give a very, very small signal. And the other problem is that the things to be perceived must be have at least the dimension of the gap. If the gap is one inch, but very deep, the character may perceive if someone on the other side is stomping madly on the ground. If the gap is several meters wide, human-sized adversaries cannot be located.

Some other problems you may use to your advantage as master:
- If someone is standing still, he cannot be sensed by tremorsense (he is identical to a rock).
- The tremorsense can be fooled if the ground is very inhomogenous. This will cause too much reflections, disturbing any signal.
- If you need a surprise attack: If zones of different densities exist, they may cause "silent zones" where the tremors of people in vicinity cannot reach the party.
- You can incapacitate a character with tremorsense with a shock wave moving through the ground. No ability without drawback.

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Trying to apply real-world physics to D&D (or its derivatives) tends to be a bad idea. –  Matthew Najmon May 6 at 22:30
    
I actually really liked this idea! I will run a couple of sims to see how this feel on the table! –  Thales Sarczuk May 6 at 22:36
    
@MatthewNajmon I know that you can get crazy if you try to get something logical out of magic, but magic is something which is explicitly handled with. Your spells have this and this restrictions and do not work under this circumstances because players have found loopholes and authors of spells have closed it (or counterbalanced with other spells). But it seems that tremorsense has no good explanation from Pathfinder so you are open to expand on it. And in this case I find it acceptable to use physics because it also coincides with reasonable assumptions from the players. –  Thorsten S. May 6 at 22:58
    
While the other answers are pretty good, this one is the one I liked the most and that solved my issues. I upvoted the others (they are pretty good, too), but since I can't mark more than one... –  Thales Sarczuk May 8 at 13:08

So far as I can tell, you've quoted the entirety of the rules for Tremorsense in Pathfinder.

Logically speaking, Tremorsense functions by feeling vibrations in the ground. Those don't travel through the air (which is why Tremorsense is useless to anything not touching the ground, even if it's next to you). It's also usually an Extraordinary (Ex) ability, so it's not considered magical.

Given that, I'd assume the way it works is that if you don't have a direct path through the ground, you'd follow an indirect path. So a chasm would block it if you don't have enough range to go around or underneath the chasm and back up to the target. At no point does Tremorsense work if the only way to see the target with it is to go through the air.

Based on your description, if there's enough distance in the Tremorsense to go around the chasm and to where the target is through solid ground (or water), it would work.

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OpaCitiZen and Tridus have some good points, and while my answer includes theirs in some ways I feel that the semantics change the meaning entirely, so here goes:

You almost need to think of distance for tremorsense like a character governing (otherwise mundane) walking distance with Spider Climb. Sometimes when people measure distance they'll say "as a crow flies" or something meaning if you look at a map and draw a straight line then that's the distance. However, 20 miles as a crow flies can easily be 50 miles driving distance - for example if you're navigating a mountain and thus have to traverse the winding paths going around the peaks you'll easily go a long distance than a direct flight would bring you. So where vision is almost always a direct line of sight (like flight), tremorsense should be calculated by the distance the vibration needs to travel to reach the character.

Usually people ignore ceilings and the like because in a cubic room it's X feet to a wall, Y feet up the wall, and Z feet to the actual source of vibration across the ceiling and with something limited like 20ft that gets used up fairly quickly. Therefore, most pits/chasms/gaps will absorb your tremorsense because a 10' deep pit is 20' without factoring width. Thus to reach the opposite side, the depth on both sides and the breadth of the gap could not be more than 20'. This small a range is best served in buildings because just like our intestines they wind about to create segments and paths in a confined space. So while the walking distance gets immense, and visual distance becomes quite limited, connected walls and ground lets you feel a lot farther than other sense can clearly detect.

In your specific example, if a character is standing on the edge of a chasm (on ground connected to the chasm), their tremorsense is wasted unless they hop the gorge, in which case it is cut off from their prior side but get twenty new feet of this extra sensory ability. At ten feet across and any arbitrary distance greater than 20' wider than the character's position at the ledge on either side, the chasm can be no deeper than 5' on either side just to see the other lip let alone anything else.

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"So where vision is almost always a direct line of sight (like flight), tremorsense should be calculated by the distance the vibration needs to travel to reach the character." That's a really interesting concept. I will give some thought to it. –  Thales Sarczuk May 6 at 18:07

I'd go for the following (even though it's not RAW):

Tremorsense, in my interpretation, is not a 2D effect. If you have 60 feet tremorsense, you can feel vibrations in the ground, as long as that ground is continuous, in a half-globe below your feet, supposing you're standing on flat ground, without any interruptions.

If the ground is interrupted, that is, part of the (half) globe is totally separated from the very ground you're staning on, the separated part is out of reach for tremorsense, as there's nothing to transmit the vibrations...

...unless the DM feels generous, and thinks adding a bit of physics to the game won't hurt, in which case they may allow air to transmit the vibrations (probably requiring an additional or a more difficult perception check.) But that's really not RAW, I think.

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