So the lowest undisputed answer to the prior question seems to be BlackLiger with a CrTe 5 spell to create a temporary sand bridge (though the use of craft magic got a look in as well).
For the next question:
A Hoplite is searching for a Marched magus who is hiding invisibly amongst a crowd of other wizards. The Hoplite wants to expose the miscreant, but doesn't want to cast any spell that would affect any of the innocent mages. What is the lowest level spell she could cast to either see the invisible mage, remove his invisibility, or otherwise exactly pinpoint his location WITHOUT accidently casting a spell on any other wizard? Bonus points if your solution would be entirely unnoticed by the crowd of mages, DOUBLE bonus points if it would also be undetected by the criminal as well.
Depends on exactly how he's made himself invisible.
If he's using a perdo imaginem to eliminate all the species emanating from his body, there is an oft overlooked flaw with this method. Namely, it stops all SPECIES. Not all emissions. "Vision of heat's light", go to IR vision and spot the bugger
Other than the casting, which if you're powerful enough you can do still and silent, you can go completely undetected in using this, and technically not even violate the code, since it's not scrying. You are gaining no knowledge a mundane could not gain, though your methods of doing so vary. After all, you could just have walked into the invisible guy. He, on the other hand, IS scrying.
I wish the original poster could put in more details about the situation. In the case of the river crossing, for example, knowing the depth of the river and the height of the bridge would have been important: the "sand bridge" solution does not work (without additional magnitudes) if it's a bridge on a high gorge at the bottom of which runs a river. Similarly, not knowing whether the riverbed's rock or clay changes the magnitude of tunneling approaches. In this "invisibility" case, important details would have been how large and packed is the crowd, whether it is indoors or outdoors (and whether it's in an "enclosed" space), and even the weather.
Obviously, there exist solutions that work in more cases than others, but magic that exploits the situation at hand is typically more efficient. So, I'll just assume that we can make up whatever detail about the situation that has not been defined as a hard constraint and is reasonable at least in some situations.
I'll assume a meeting of several dozen, possibly a hundred magi moving freely in a largish but not too large space outdoors (no larger than a standard Boundary, 100 paces across) like the Durenmar meeting grounds. I'll assume that the typical weather for the area is a relatively cold, humid weather, but there's no fog on that particular day. One solution to spot the invisible magus would be to make a low patch of mist slowly raise from the ground over the course of maybe half an hour, up to a height of a few inches or so, with the caster watching for "holes" in the mist. Note that fogs naturally rise from the ground in cold, humid weather, so a) a spell that works at Touch range needs no extra magnitudes for the phenomenon not starting in the high air and b) if the spellcaster is deft enough, folks who are not really paying a lot of attention will not notice anything strange.
This is a level 3 Creo Auram spell: Base 1 (Create a minor weather phenomenon: a mist), +1 Touch, +1 Conc.
No. Perdo imaginem guidelines can destroy the visual, aural and scent, taste and touch based species. The heat based species are an ignem item. You're not altering the species, you're altering yourself to see them, so to hide from that, your theoretical opposing magus would need to have a perdo ignem upon themselves, specificly designed to destroy species that leave their body as heat. Otherwise, they take care of themselves for you, either freezing to death from a poorly designed perdo ignem, collapsing from heatstroke from a poorly designed rego ignem, or are visible to vision of heat's light.
You'd need a slightly modified variant that overlays itself on your regular vision rather than overriding it, technically, so you can differentiate the heat from the otherwise not invisible magi and servants. Though if I were this bastard, my get out clause would be to turn at least 2 servants invisible too.
In that case, one might opt for a R:Per, D:Conc variant of Eyes of the Cat that instead gives the nose of bloodhound to the caster - who then just walks around the hall sniffing for stuff that he can't see. This would be Muto Corpus level 3: Base 2 (change the target to give him a minor ability), +1 Conc.
Note that this may or may not make the caster conspicuous, depending on the event (certainly not at a masked ball), the magus' typical attire (a deep drawn hood that disguises his facial features), and how strange magi are in your sagas due to twilight scars and such. In any event, it just takes a level 3 MuIm illusion (Base 1, +2 Sun) can make the caster look "normal" - or, even better, like an entirely different magus so that the quarry does not get suspicious.
No, they do need to Penetrate. For Intellego spells there is no difference between direct sensing like InCo or having the senses to have a physical medium sense a target and tell about it.
ArM5 page 114, left hand column, "Magical Senses" (starts bottom page 113) says something along the lines of: "Magical Senses must Penetrate the Magic Resistance of Creatures sensed..."
Although I must admit that I don't understand the sentence found a little bit later: "The mundane sense through which the magical sense grants information does not need to penetrate Magic Resistance"
The spells mentioned here do not need to penetrate, they are not in any way targeting the magus. The first, Sense the Feet that Tread the Earth targets the species emitted by the footprints. Footprints do not have MR. The other one, Eyes of the Bat, it doesn't need to penetrate, either because it's interpreting all of the sounds bouncing around the room, off of everything to create an image. Now, the efficacy of these spells in a crowd is certainly debatable, but their need to penetrate isn't, because they don't target the invisible magus directly.
I should have declared Serf's Parma here. I was confusing Sense the Feet that Tread the Earth with Glowing Footprints of the Thief when I described the whole footprints thing. Sense the Feet that Tread the Earth tracks the vibrations of things stepping on the earth. Again, the effect doesn't require penetration of the magus's MR. There's still the whole efficacy issue of the spells.
Eyes of the Bat doesn't provide enough detail to know if someone is invisible directly, but one might be able to notice a void that is viewed is conflicting with an obstacle that the sense of hearing provided by the spell is indicating.
True, but that assumes that magical senses detect pre-existing species. And infra-red species don't exist, because we're in the medieval paradigm.
When you use a smell target InVi spell to detect magic are you detecting pre-existing "smelly magic species", which are separate from "visual magic species"?
When you use a hearing target InIg spell to detect heat are you detecting pre-existing "noisy hotness species"? And what about if you detect audible species through your sight, are the audible species giving off visual species?
Natural species will leave your MR and no longer be protected, but there's no reason to believe that sensory spells actually detect natural species.
My recommendation would be a level 10 InIm: "Vision of the body's scent" (has to be scent, because they won't be making much noise and taste+touch won't leave their magic resistance)
Range: Per, Duration: Diameter/Concentration (pick one) Target:Sight
See the olfactory species in the air. As humans have a sense of sight every bit as sensitive as a hound's nose this should give you enough information to recognise an individual person who you've seen before, or detect an invisible magus through their body odour.
You'r detecting species, and converting the information from whatever they were to a species you are able to interpret.
No, see above. Think synesthesia.
What? IIRC, there are only species, that is natural species. I'm going to claim serf's parma here. But this is the basis of illusions being able to fool magi. The species the illusions create are natural.
Above you seemed to say yes, that you were detecting pre-existing species.
If the species are pre-existing, but they're not subcategorised by "smelly hotness" "visual hotness" etc. then how come the magical sense you create determines how the (pre-existing) species move?
A) The Jerbiton section of HoH:S has some information on creating species directly which are therefore magical species. Imaginem covers both illusions (magical images that emit real species) and species (which can therefore be magical themselves). Much like you can CrIg a fire, which makes natural light, or CrIg some light, which is not natural.
B) You can have a sense that doesn't involve species. Species are involved in the natural senses. Magical senses, other than those from InIm abilities, may well not use species at all.
Heat species do. Medieval physics 101, in fact. Heat is a form of species, just as smell is. It's just more naturally associated with fire than sense. At least, according to the stuff I've read. If you have a superior source (since I admit, mine tend to be folk tales, and occasional snippets of text), please share with your fellow magi, before we have to have you fined for failure to uphold your oath