Value to who though? A Professional Jeweller knows his market, that's how he can estimate value. For the spell to do the same it would have to somehow read the minds of potential buyers.
I would think a cult like that could be very cool. Hermetic magic lends itself to understanding and manipulating the secrets of the natural and supernatural worlds very well and discovering the ways of the earth should have lots of interesting implications.
Ptolemy's cartography was actually based on astronomical observation to estimate longitude and latitude. But don't let that stop you from working with earth-based mapping systems, using magnetism, ley lines, or whatever.
Exactly.
Because the spell has no concept of market value? Because when a Jeweler looks at a gem, they estimate the quality and apply that to the current market to estimate a current value, but the two are not the same?
High quality can be confused with high value (you seem to be doing it now, RL) - but the two are not directly connected. Too many variables to know exactly or predict.
"Quality" is relative to other diamonds. Perfect, poor, flawed, very good, average. It's a physical, objective, inarguable aspect of the item. That combined with weight - and a knowledge of the current market - will give a good estimate of "value".
"Value" is "current price", the money/trade goods someone will actually offer for something. In a desperate famine, a diamond might only fetch a pittance, since you can't eat it. Otoh, a desperate mage might value that same diamond very highly if it's just the thing he needs to finish his talisman. The quality hasn't changed, but the value has.
And then that same diamond on a ring might be valued as "priceless" as a family heirloom - or, if from an unfaithful love, it might be valued as worthless to the betrayed. And all regardless of the diamond's actual quality, which has never changed.
Value fluctuates with human social pressure and individual whims. Quality remains stable and is objective to the class as a whole.
Well, the Central Rule can solve anything, true.
But I'm not sure that this isn't a circular tautology - that he knows where he is because he names a place that he knows because he knows where that is relative to himself... (but if he knows where that is, then he already knows where he is...) 
I'm not sure that you can set up an InTe effect to vary in the way you suggest just by saying the magical word "Paris" or thinking hard about Notre Dame Cathedral - but ysmv. 
Latitude is easy but longitude is much harder. Triangulation methods didn't leave Spain before 1300 and Jupiter's moon used as a clock was a few centuries later.
Of course, none of this is a problem for magic which offers instantaneous travel and communication. It all comes down on how "modern" is the mindset of magi.
Indeed, Ptolemy was famously bad with distances involving longitude. The contribution of al-Kwarizmi was to improve on this.
Does a jeweller who evaluates a gem have to read the mind of potential buyers?
I know that quality and value are not necessarily the same thing. However, the amount that a given gem will (probably) fetch in the market at Venice (say) is a fact about a gem that somebody with appropriate skill can tell just by looking at it. Why can't a spell do this, esp. when that is specifically what the guideline says it can do?
Depends on what you mean by Quality. That can be just as subjective as $ value.
That's not what I'm suggesting at all. A spell that told you specifically how far away something was from Paris, say, would require something like an Arcane Connection to Paris. The spell doesn't do that. The spell tells the caster where the touched location is. He receives this information in a way that is intelligible to him.
In real life, you know where your home is, I presume. That doesn't mean that you know specifically how distant it is from Paris, nor necessarily what its GPS co-ordinates are. You know its location in a way that makes sense to you. This is the kind of knowledge that the spell gives you.
Aren't you confusing price and value?
He can only tell the price of a given Gem in Venice if he know the Venice Market, or can read from afar the minds of potential buyers there.
For a modern exemple: With our global market, the price of a gem on markets is relatively constant. So, a Jeweler might be able to tell you for how much he could sell that gem of great quality. Now, send him to a "lost" tribe in the midst of the Amazon Forest (assuming there are still some). By our standards, the gem might cost more than their entire village. To them, it'll buy maybe a meal, because it's pretty.
A spell can't know the value of the gem in paris or the amazon. Not unless he can also tell you how much the people there value the gem (mindreading part) InTe will tell you "this is a perfect gem". Your skills (and, IMO, this is where area lore comes in) will tell you "I can sell that perfect gem xx in Paris, in Venice"...
You're arguing essentially that an Intellego Terram spell could give you the relative prices of the same good in any location possible: Instead of saying "this is a perfect gem", it'd say "this is a gem worth 1539$ in New York, 1325€ in Paris, 1006£ in London, 1423€ in Berlin"...
There were other ways to get longitude, but getting exact numbers or actually get it fast enough to navigate from, yeah thats mostly a later thing.
An experienced sailor or traveler pulling guesstimates could often be the fastest and most reliable way...
Could you name them? Was there anything better than days of travel? I couldn't find anything myself.
Iirc, it involved comparing star sightings. Very arcane (for the time) and very demanding, and very subject to errour.
(Warning - 2 longish replies to diff points, plus some misc. apologies.) 
I just said what I mean by "Quality" - you quoted it yourself. It's the item relative to the class as a whole. You need a dictionary definition so we can move past that?...
Mirriam Webster:
a : degree of excellence : grade
b : superiority in kind
So, if you made a bell curve of all the examples "in kind" of a thing, "quality" is where on that curve your example sits. Terrible quality, poor quality, average quality, good quality, excellent quality. Diamonds, steel, arguments, bar fights, manure - it all has "quality". While opinions might differ in some complex, social products (say, on "art" - altho' classic authorities might disagree), for most objects and substances, from a Hermetic, Platonic sense, either something is "of (high) quality", or it is not. I see nothing subjective there.
(And note that "location" has no "quality". It either is or it isn't. Just to remind people why that matters.)
Yes, he does.
You never been to a garage sale? Ever watch one of those "Pawn Broker" shows on TV? That's exactly what they're doing - looking at the item, evaluating that against the market they know, and then gauging the seller/buyer given what they know of human nature.
One is a skill relevant to the Target (determining quality), while the other is not, but is equally important in estimating "value".
Let's take an example we all, perhaps, can relate to.
Let's say you, Richard, have a copy of Ars Magica I, first-edition, first printing. It has been in storage, in the original store bag (which was fortunately acid-free paper) since the day it was bought, when it was thought lost and then replaced, and so has not been touched since. It has no stains, no tears or wear marks, no dog-ears, no scribbling in the margins, binding is solid - it even still has the original price tag on it (in a conveniently inconspicuous location) from a well-known store now long out of business.
I think, I'd hope, that you'd agree that the quality of that book is "excellent". Nothing could be improved; it is, for all intents and purposes, in "perfect" condition. Highest quality.
So... what's its actual value?
Well, it depends who you're talking to, and you just don't know that in advance. If you're selling to an avid Ars fan - quite a bit! If you're selling to an AM collector, and that's the last piece they need for their collection, even more! If you're selling to a gamestore bookseller, less, because they want to make a profit too. Otoh, if you're selling to someone who doesn't role play - probably nothing. Not a dime, not the pulp paper value, because they're just not interested in taking the time to recycle it for that little profit.
But the quality hasn't changed.
When you meet that buyer, how can you know which one he is? How can an InHe spell tell you what this buyer is willing to pay for it? How, without reading their mind, can you tell "value" in the current situation?
And don't confuse "current value" with "potential value". We can get back to the "diamond in a famine" model - value is what you can sell it for, not what you might, one day and in the right conditions, sell it for. One day, gold might be worth $100,000 an ounce - but that's not its value today.
Your interpretation of that phrase is far too broad.
Can an InTe spell tell that a suitor's girlfriend will love this stone? The suitor could, just by looking at it...
Can an InTe spell tell that this stone is just like the one that woman's mother wore? She can, just by looking at it...
Can an InTe spell tell that this stone is exactly the color of the morning sky at dawn? An artist can, just by looking at it...
Can an InTe spell tell that this stone is exactly what the Doge is looking for? An ambassador can, just by looking at it...
Can an InTe spell tell whether this stone would be an adequate bribe for a certain guard to release a prisoner? A rogue could, just by looking at it...
...because they have a skill to know their girlfriend's tastes. The InTe spell can't do that.
...because they have a skill to remember their mother's stone. The InTe spell can't do that.
...because they have a skill to know the color of the morning sky. The InTe spell can't do that.
...because they have a skill to know what the Doge is looking for. The InTe spell can't do that.
...because they have a skill to to judge that guard. The InTe spell can't do that.
Need I go on?
An InTe spell can't duplicate the "skill" to judge a potential buyer, and certainly not before that buyer has even seen the stone or the caster has met the theoretical buyer.
You're confusing "skill" with "personal knowledge". Your confusing "skill directly regarding the Target" with "skill related to social conventions and expectations that involve the Target". You're confusing skill to evaluate a stone with skill to evaluate a buyer. You are, despite your statements to the contrary, confusing value with quality.
A jeweler can tell quality. A merchant knows his potential market, and thus value - and if "knowing a market" is a "skill" rather than knowledge, so be it. But the two are not the same process, nor even close.*
(* The "skill" of Value comes into judging people and social,market pressures, not stones. The skill of Quality involves the stone. The "merchant" skill is combining the two.)
(You're presumption is correct, since it seems you're not sure about it.)
No, that's meaningless. "You know because you know" doesn't fly.
Richard, my home is right here - right here! I'm touching it! Now you know everything that I know from the spell you're describing. Nothing.
It's not homing instinct, like some pigeon (altho' that's a different idea that could work!)
An astronaut in a space station "knows where the space station is" - or do they? Without anything relative, it's meaningless to say "right here". The spell would tell the caster that they're "right here", but where is that compared to anywhere, anything else?
But without an AC to Paris (or a fixed equivalent built into the spell when designed, to somewhere), "in a way intelligible to him" is undefined and thus meaningless.
Ars Magica magic doesn't do "stuff", like some other RPG's. I'm just not seeing the explanation for this in traditional AM terms.
(And it's fine to go non-traditional - wave a hand, and it's done, all good. I'm just saying "by canon standards, guidelines and examples".)
Yup, pretty much. Repeated and/or prolonged observations of sun, moon, planets and stars more or less in combination worked to some degree. Depending on method, place and knowledge though, it can require up to literally years of observations to get a useful/"precise" enough position.
Which is why a sailor(usually) skilled in guesstimating longitude was so in demand up until 19th century when precision clocks allowed calculating.
And is quite similar (almost the reverse) to what a magus must do when he makes an daily (or nativity) horoscope for a target as a Sympathetic Connection (which depends on knowing the target's location and the time). So, it's certainly a concept that magi are familiar with, I think.
Let's imagine that in the course of a game, three player characters are teleported from Durenmar to somewhere beside a river.
Character One, a well-travelled grog, looks around. An Area Lore: Normandy roll is made for him against an Ease Factor. The roll is successful. The character proclaims that this is must be the lower reaches of the Seine river basin, "I spent my childhood in a village not two leagues downstream".
Character Two, a magus, mutters some words in latin, casting an InTe spell, and touches the soil. He says, "Now, I have lived my entire adult life within the confines of Durenmar and the Black Forest, however, I can sense that we have moved across the face of the earth about 400 miles northwest. Now, where would that put us?"
Character Three, a maga who has spent time in Paris, mutters some words in latin, and also casts an InTe spell, and she too touches the soil. She says, "Yes. I am sure that we are near Paris. This feels like French soil. I think that Paris must be a few leagues upstream from here."
Do you think that Character One, Two, and/or Three are doing possible things? To me, there seems to be nothing wrong with what Character One is doing. And if so, I fail to see, what is wrong about using InTe spells to discern what Characters Two and Three do; which is effectively replicating the mundane skill of the grog, Character One. Now, you could perhaps argue that the location of something is a mundane property (Base InTe 4) rather than a visible property (Base InTe 2). But it seems to be something entirely feasible.
Of course, the effect of what Character Two does could be replicated by GPS technology, which is fine. Equally, what Character Three does would seem largely inexplicable by GPS or any other modern or medieval technology. But it's Heremetic magic. It's allowed to be. This seems absolutely fine too.
Oh, so if there is a celestial event that is simultaneous everywhere on Earth, and time from sundown is measured at both places within 1 minute, you have an error of 30 kilometers or so. I think curvature was well enough known not to make it much worse. Now it is not clear which events would be simultaneous though.
It seems to me that as a spell on its own, Earther Awareness fits reasonably well with game play (it does not confer an unusual advantage to a magus compared to other spells) but fits poorly with Hermetic magic as is. It requires a broader structure of understanding to be possible.
For a magus to be able to cast this spell he should have to draw on a greater understanding of the Earth. This understanding would be based in Ptolomy's Geographia as suggested earlier, but would have been developed over time to include a magical/mystical understaning of the Earth as well. A magus versed in this subject could identify a location by reference to landmarks, but not to surface landmark, rather to aspects of the Earth at a location.
Consider that within this understanding, auras could be like islands, sticking up above the surface of the land where mortals can enter them but originating deep beneath the surface. The aura topography, sensed magically, would constitute a map which an experienced magus could interpret to determine location, direction, aura magnitude, the potential for vis sources, and a host of other things.
When I first posted this thread, when I first imagined this spell, it seemed a simple little cantrip, but now I think it only makes sense as part of a mystery cult. Ptolomeic Geography, it could be called. The study of the Earth through magical means. I can imagine a number of Te and Vi spells and devices for locating, enhancing and destroying auras and vis sources. Simply locating yourself on a map would be the least of the cult's spells.
I believe lunar eclipses were used by Ptolemy and by the Arabs for this purpose. Interestingly, the technique of using accurate time readings to calculate longitude was understood since Ptolemy's day, so you can't call it a modern attitude. There were just very limited tools available. Hermetic Magic can supply those tools.
I don't want to lose "medieval authenticity" but let's not confuse what passed for knowledge in, say, backcountry Wales with what was understood in the equally medieval Eastern Mediterranean. The OOH does extend through Byzantium and across the frontiers of the Islamic World, after all.
Lunar eclipses had been used since antiquity exactly for this purpose.
[Edit: Alas, Jabir posted before me! I fully with what he says.]
This is what an astrolabe did, or tried to do (or some of the more advanced ones, at least*). It had sites on it to take shootings of stars with, and you put in different plates for different (astrological?) months to account for the different stars that would be seen at different times of year. You'd adjust the dials to your star-sightings and read the result off the plate for that month - easy as calculus.
- Astrolabes have no clear single "date of invention", going back to before the current era in one form or another, but they became increasingly complex and accurate over time.
I think it's like trying to use Hermetic magic to change the (actual, spoken) words that one person is saying so someone else can understand them if no one else speaks that language. There are many sophistic and spurious arguments one can make, but in the end it's just not within the Technique/Form parameters. The fact that "someone with the skill" could do it doesn't always mean that a spell can (which is, btw, the large section of my reply that you ignored above).
I just don't think that InTe spells can communicate "near Paris", unless that spell is specifically designed with "Paris" as a reference point. They could possibly communicate "near a big urban area" (sensing the stone buildings), or "near a big population center" (via an InCo effect), but "Paris" is, simply and clearly, just not a concept that fits within the Hermetic Forms.
(Again - and I'll keep saying this for all readers - for ease of play, if you want to handwave that, please do! The Saga is more important than any Rule. Have fun first.)
See Sense the True Path
InTe 15
You can sense the direction of any location to which you have an Arcane Connection. You feel an instinctive pull towards its relative position to you while you concentrate, though you must make simple Perception rolls of 6+ to stay on track when traveling on foot through woods or brush, and if your destination moves, the Ease Factor of this roll increases depending on its speed. The spell is tethered, so that it may be controlled by another, and boosted so that it may be made to last longer if necessary.
(Base 2, +4 Arcane Connection, +1 Concentration; Boosted, Tethered)