[P:ES] Smell: military advice vs. RAW distance penalties

[vicky_molokh]

Originally Posted by Flyndaran

So true. Before my diabetes, I once smelled my cat's favorite outdoor "den" from over 100 feet. Now, I can smell cigarette smoke from that far, maybe.

Scents are unusual in that not all parts move at the same speed or last equally as long. They separate making identification much harder over time and disatance.

OK, so it seems that it should be possible for humans, especially those with a high Per/Acute Smell score, to detect these things at those distances. The question is, how would one best simulate this while sticking to GURPSy game mechanics?

[Daigoro]

Originally Posted by vicky_molokh

• Cigarette smoke should have modifiers of something like up to +400 to everyone's smell rolls, depending on circumstances. Other scents may have somewhat smaller, but still enormous bonuses.
• Humans should start with one level of Long-Range Smell for free, i.e. the default distance modifiers for smell should be as per SSRT, not -1/yard.
• Humans can trivially purchase the Long-Range Smell perk with minimal training, i.e. the first level should count as a non-exotic trait.

So, which one should it be?

Thanks in advance!

Realistically, I'd go for the second one, but "gamically" I'd go for the third. Perks are pretty well-established ways of bending particular rules, and it's less likely to break the system by changing rules around. Is that perk listed as supernatural in P:ES?

[Eukie]

In industrial chemistry and environmental science, a common model is the Gaussian Plume Model of odor dispersion, which looks something like this:

C(x,y,z,V,H) = Q/(2*pi*V*std_y*std_z)*exp(-0.5*y^2/std_y^2)*[exp(-0.5*(z-H)^2/std_z^2)+exp(-0.5*(z+H)^2/std_z^2)]

Where Q is the concentration of the odor at the source, x is the downwind distance, y is the crosswind distance, z is the height of the receptor, V is the wind speed, and H is the height of the odor source. Let's assume a ground-level receptor, for simplicity:

C(x,y,V,H) = Q/(pi*V*std_y*std_z)*exp(-0.5*y^2/std_y^2)*[exp(-0.5*H^2/std_z^2)]

The values of std_y and std_z are determined by power laws:

std_y = c*x^d
std_z = a*x^b

Where a,b,c, and d are constants determined by the "atmospheric stability", which depends on, among other things, time of day, cloud cover, and surface wind speed.

I propose, again, to simply by removing a dimension, and will assume that the emitter is also at ground level. This does not accurately model smelling birds or industrial chimneys, but is fairly accurate for smelling spills, animals, and cigarette smoke:

C(x,y,V) = Q/(pi*V*std_y*std_z)*exp(-0.5*y^2/std_y^2)

Humans notice odors when exposed to sufficient concentrations of them, which follows - depending a bit on the source - a logarithmic or power-law relationship. The probability of detection itself follows, as with most things biological, a Gaussian distribution.

P(odor detection) = integral of[ exp(-0.5*log(C/C_0)^2/std_c^2)/(std_c*sqrt(2*pi))]

All this math may seem overly complicated, but what it shows is that P(odor detection) has roughly the same form as that of visual spotting and the probability of hitting something with a gun. Hence, in GURPS, it would make sense to represent it using similar rules.

This means, among other things, the Size/Range table and rolling 3d6 against a skill or attribute. The "odor detection threshold" is defined as the concentration where an odor is detectable to 50% of a test panel. Hence, at the "odor detection threshold" concentration, detecting a smell should be a roll of 10 or less - this implies a roll against Per[1].

Since the Size/Range table follows a rough logarithm, it can be used to account for the wind-speed factor, the range factor, and the concentration factor, each as separate look-ups using appropriate factors. The "smellability" of each individual type of smell would be a bonus or penalty to skill.

Actually recognizing a smell requires a higher concentration. This can simply be modeled as having a high margin of success allowing recognition.

I haven't actually done the work on figuring out what the appropriate constants for using the Size/Range table to represent smelling yet should be like, but from what little research I've done, it doesn't appear to be too complicated or unknown to model GURPS-realistically.

[1] People who evaluate odor detectability tend to be trained professionals, so their roll is perhaps against 12 instead, but still has a 50% chance of success; give all smells -2 to compensate for this, or say that detecting odors is a Per/Hard skill.

[whswhs]

You know, I really debated this one. I thought that we needed to make a distinction between the human sense of smell (relatively poor) and the sense of smell of, say, a lot of carnivorous mammals (fairly good). And I could have gone either with human smell falling off rapidly, and carnivore smell falling off like human vision, or with human smell falling off like human vision, and carnivore smell having even longer range. I went with the former, and the playtesters went along with it; had there been an argument for the latter I likely would have gone with that.

But it seems to me that what's we're trying to define is the ability of a cat to say, in effect, "I think there's a squirrel over there" or "you've gotten something really interesting on your shoes" when a human won't notice anything. That is, we're talking about comparatively faint local smells. If the stove burner doesn't light, I'll smell the gas in a fraction of a minute when I'm standing right there; but if I were five yards away I probably wouldn't notice it till it was starting to reach a hazardous concentration.

Maybe the best answer is to use the shorter ranges for the faint smells where I would normally call for a Smell roll, and the longer ranges for, say, a cigar smoker. (I've joked for years about the first issue of Daredevil where the hero's superhuman sense of smell enabled him to follow the odor of the villain's cigar from several yards away. . . .)

[Flyndaran]

And I've joked about how the superhuman Discriminatory Smell advantage doesn't technically describe anything that I couldn't do.

[Anthony]

I'd probably distinguish between human and dog by giving the dog around +10 (or possibly the reverse -- having humans be considered incompetent at smell tests).

[evileeyore]

Originally Posted by Flyndaran

And I've joked about how the superhuman Discriminatory Smell advantage doesn't technically describe anything that I couldn't do.

You can recognize everyone by their smell alone?


That... I wouldn't want to be able to do that.

[Flyndaran]

Originally Posted by evileeyore

You can recognize everyone by their smell alone?


That... I wouldn't want to be able to do that.

Before diabetes knocked out much of my sense of smell, I could identify people, places, and most pets that way. Most purebreds smelled the same though.
My brother identified which cats in the house were having a "urine war" by scent of "remains".
Losing that ability made it easier for my anxiety as people aren't so "in my face and omnipresent".

[Bruno]

The smoke from someone actively smoking is, I suspect, particularly detectable for a big list of reasons that don't apply to, for example, stinky foot odor (a problem that anyone eg soldiers hiking in boots for a few days is going to struggle with).

• A burning cigarette is producing a big plume of smoke - it's actively disbursing light particles that can stay airborne for long distances.
• We get hooked on tobacco because of nicotine, of course, but we smoke it because it's particularly aromatic (ie has a strong smell that some people like) when burned.
• On top of general smellyness, burning smells are very out-of-place in a wilderness environment. It's like wearing hunter orange and trying to hide in a bush - it's just wrong-looking.
• I suspect we're hard wired to notice the smell of fire particularly well, but I don't have any citations on that.

So you've got this continuous source of very strongly scented hot particles that can go a long distance before settling - at which point they cling to things, as anyone who's ever encountered a smoker can attest, in an environment where the smell of burning in general doesn't belong. You're going to produce not just a pulse of "smellability", but a giant streamer or fan shaped blanket of it.

Perfumes, colognes, and perfumed products are specifically designed to be easily smelled by humans, and to be volatile and come off of whatever they're applied to in smellable quantities for extended periods of time. It's the whole point of perfume.

Same thing again with strongly scented foods - garlic being the usual example. We eat them because they have pungent scents. Garlic is the big example because it's a good source of not only sulfur compounds but also of selenium - next row up on the periodic table, and an order of magnitude more stinky. That entire column of the periodic table is filled with stinks, with the exception of oxygen; we've got a bit of a "dead spot" in our sense of smell for oxygen, and what we can smell of it we naturally find utterly inoffensive.

But these are like the spot-lights of scent! They're the fluorescent orange retro-reflective polka-dots of scent! They're designed and intended to be highly "visible" to the sense of smell, like the spot-light or retro-reflective polka-dots are for vision.
I wouldn't call "smelling a cigarette" the normal, unpenalized use of the sense of smell; "smelling a cigarette with favorable wind conditions" is even more outrageous. I'd be more comfortable with assigning it an absurd-looking bonus than calibrating scent such that a dog needs the absurd bonus to smell faint scents.

I don't know why radically reducing the range penalty for favorable wind conditions seems "unGURPSy" - it seems pretty natural to me. To make it very fourth-edition, I'd say the rule could be: "If atmospheric conditions are favorable or in an enclosed environment with the scent source, move from -1/yard to using the size/speed/range table. If both conditions are true, use the long distance modifiers table!" (or at least get a big bonus on SSR table).

In summary, I'm happy with giving everything on the nono list (cigarettes, perfume, garlic, etc) a bonus even up to +100, if someone can provide decent evidence for that number when the wind isn't blowing it right at you or away from you. I would combine a big bonus for stinky things with a very favorable shift in range penalties if the wind is right or if you're trapped in a relatively sealed space (building with closed windows and doors, vehicle, dungeon, whatever). I'd give a HUGELY favorable shift in range penalties (or a gigantic bonus) if you've got the air circulating in your direction in an enclosed space.

[vicky_molokh]

Originally Posted by Bruno

I don't know why radically reducing the range penalty for favorable wind conditions seems "unGURPSy" - it seems pretty natural to me. To make it very fourth-edition, I'd say the rule could be: "If atmospheric conditions are favorable or in an enclosed environment with the scent source, move from -1/yard to using the size/speed/range table. If both conditions are true, use the long distance modifiers table!" (or at least get a big bonus on SSR table).

It seems like the idea of multiplying range modifiers is pretty alien, coming up only in the odd case of Zen Archery/Marksmanship, and IIRC some obscure earthquake-related spell in Magic, both feeling like 3e'isms that survived from the Era of Myriad Exceptions.