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I am confused to the interaction between the monster special ability tremorsense and the skills of perception and stealth.

Specifically what my issue is that tremorsense gets to "automatically pinpoint the location of anything that is in contact with the ground".

When reading the perception rules you have "Creatures with the tremorsense special quality have a +8 bonus on Perception checks against creatures touching the ground and automatically make any such checks within their range".

The action for perception is "Most Perception checks are reactive, made in response to observable stimulus. Intentionally searching for stimulus is a move action." This could also be worded as "Most perception checks are automatic". Now this doesn't include pinpoint, which does matter?

So the automatically part is talking about the action to make the check, they don't have to focus or take time, it just happens. This is further supported by perception getting a bonus of +8 as long as both are touching the ground, otherwise, why bother giving a bonus to notice things if you are always successful in noticing things touching the ground within your range?

So, could you use stealth to sneak up on something only using tremorsense?

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A creature that possesses the extraordinary ability termorsense "can automatically pinpoint the location of anything that is in contact with the ground," but the creature's tremorsense "range is specified in the creature’s descriptive text." Thus it's normally impossible while walking on the ground to use Stealth to sneak up on a creature that possesses tremorsense while the sneak is within the range of the creature's tremorsense. The creature with tremorsense pinpoints the sneak's location within the range of the creature's tremorsense automatically, no action necessary and no roll needed.

The skill Perception adds this detail: "Creatures with the tremorsense special quality have a +8 bonus on Perception checks against creatures touching the ground and automatically make any such checks within their range." This expands the mandate of tremorsense ability rather than clarifying or replacing it.1

With this addition, the creature that possesses the extraordinary ability termorsense still automatically pinpoints the location of creatures that are within range of its tremorsense like normal. However, in addition, the creature also automatically succeeds on any Perception skill checks it makes against creatures that're touching the ground if those creatures are within range of its tremorsense (e.g. the Perception skill check that opposes many Sleight of Hand skill uses—and, as can be seen, this can get a little goofy if applied indiscriminately). Further, the creature receives a +8 bonus on all Perception skill checks it makes against creatures that're touching the ground if the creatures are outside the range of its tremorsense.2

Thus it's possible while walking on the ground to use Stealth to sneak around a creature that possesses tremorsense while staying outside the range of the creature's tremorsense. The creature with tremorsense can't automatically pinpoint such a sneak's location, but the creature will receive a +8 bonus on its Perception skill checks against the sneak. Again—unless the creature's tremorsense makes an exception—, making such Perception checks usually don't take any actions on the creature's part, but such a check could take an action if the creature is using its tremorsense to, for example, deliberately search an area rather than just passively receiving and judging sensory input.


1 This reader wouldn't worry about the game using make for succeed on in that quoted description. That's a really common misuse that permeates the Pathfinder corpus, and the writers almost always mean succeed on when they use make in this kind of context. Such is the assumption made here.
2 The Perception skill's additional rules for the tremorsense ability were—so far as I can tell therefore like the Perception skill itself—, added by Pathfinder, a change from the ability's original appearances in the D&D 3.5 rules (Monster Manual 316 and Dungeon Master's Guide 219 or here and here, respectively) for reasons I know not.

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  • \$\begingroup\$ Im not happy about this answer, but it appears well supported, and also fits with the common accepted application of the two. \$\endgroup\$
    – Fering
    Feb 25 at 20:40
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Under the Stealth skill it says:

"If people are observing you using any of their senses (but typically sight), you can't use Stealth." (PF 1st ed, Pg 106). So I suppose you'd need to decide if someone with Tremorsense counts as observing you, as Tremorsense states that the owner always makes perception checks if the target is in contact with the ground.

The Appendix entry in the core rules and the GM guide don't even specifically mention Tremorsense; only the Bestiary finally defines it:

Tremorsense (Ex) A creature with tremorsense is sensitive to vibrations in the ground and can automatically pinpoint the location of anything that is in contact with the ground. Aquatic creatures with tremorsense can also sense the location of creatures moving through water. The abilities range is specified in the creature's descriptive text

At first I thought, essentially you've got a variation of Blindsense (PF 1st Ed, PG 560):

The creature with blindsense usually does not need to make perception checks to notice and locate creatures within range of its blindsense ability - you could extrapolate that for Tremorsense, as above, but only for things in contact with the ground?

But then when I looked up the difference between Blindsense and Blindsight, Blindsight specifically references using "nonvisual senses" such as sensitivity to vibration, echolocation, et al. The big difference seeming to be that Blindsense still suffers from the 50% miss chance from concealment, whereas Blindsight doesnt?

It goes on to say "Invisibility, darkness, and most kinds of concealment are irrelevant, though the creature must have line of effect to a creature of object to discern that creature or object" (Beastiary, Pg 298) -- which, as far as Tremorsense RAI goes, I'd suggest would equate to "the creature must be in contact with the ground".

I suppose if you don't want to make Tremorsense unbeatable (which it seems is the intention?) you could play it such that subject with Tremorsense, 60Ft gets to make a perception roll to detect you as soon as you're within that 60Ft range (opposed by your stealth roll), rather than having to be actively searching for hidden figures; but if you try to activate your stealth while in the proximity of something with Tremorsense it automatically makes its perception check to detect you.

One related rule - the Cave Druid from the Advanced Player's Guide gets the ability Lightfoot (Ex) which states: At 3rd level, a cave druid cannot be detected with tremorsense. This ability replaces trackless step. -- so while it is clearly possible, I'd ask if trivializing a class ability with a skill is fair?

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  • \$\begingroup\$ The situation I was thinking of is If I am stealthing around in a cave, and happen to approach something with tremorsense, does it just get the check to notice me, or does it automatically succeed at noticing me. Big different between the two. Tremorsense isnt direct observation, which is why you are able to gain precision damage against them. \$\endgroup\$
    – Fering
    Feb 22 at 13:53
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Don't hesitate to slag this if it doesn't count.

Looking around in Spheres of Might (which is 3PP), I found this talent:

Lurker

Creatures using unusual forms of sensory perception such as blindsight, greensight, or tremorsense cannot automatically foil your use of Stealth; such creatures must make a Perception check as normal to detect you when you make use of the Stealth skill.

Which implies, with what you've quoted, that the default assumption (of the SOM writers, so nothing official) is that Tremorsense automatically picks out creatures being stealthy, unless they have Lurker or something similar, whereupon they instead get a +8.

But this is based entirely on a 3PP abilityan speculation, so it might not be worth anything.

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    \$\begingroup\$ You get the same sort of language on the Vigilante talent "Blind Spot": "Blind Spot (Ex): The vigilante is so skilled at stealth, he can use the Stealth skill to hide from creatures with unusual senses that normally automatically detect creatures. This includes senses such as blindsense, blindsight, lifesense, scent, or tremorsense." So that rules text implies that, at least for vigilantes, tremorsense creatures automatically detect you. \$\endgroup\$ Feb 22 at 0:20

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