My last question on this topic has gone off on a tangent to a proto flame war on the history of calculus.
It sounded like the majority opinion was that the aegis was not a cylinder that reached to the Lunar sphere.
So what/howmuch is protected?
eg can a griffon overfly the aegis?
If a tree planted inside the aegis has a branch extending beyond the boundary, how much of it is protected by the aegis?
Other questions like this.
I broke down how my group plays it (our HR) and pretty much that is the best I can provide you. Somethings in AM are just not very well defined and that leaves it up to your individual group to define.
Things like this are actually why every game I have played in ends up with a rolling house rules document. Things come up and to keep the groups decision on it fairly consistent we write it down.
The best you can get is advice on how to "make a decision in your troupe" is a bit of advice to inform said decision making.
here are my two cents.
You can either pick a geometry and go with that. For example decide that the aegis curves inwards in such a manner that it would form a (hemi)sphere if it was perfectly circular and as such things that poke out of the "bubble" are not protected even if they are part of a greater thing that is mostly inside the bubble. This would exclude the hypothetical branch.
you can go with an interpretation based on methaphysics/feelings and say that if something feels like or seems like it should be protected then it is. For me it feels like the branch should be protected because the tree is inside the aegis, but others will feel differently.
However unless you decide that the aegis extends very far up into the sky then a griffon will most likely be able to overfly the aegis. but not do anything to approach from above.
None of these answers are particularly satisfying but its what we have.
As others have mentioned there is no canonical answer to this.
The way I would rule is that the Aegis protects everything within the Boundary that it targets. It extends far enough upwards to include all buildings, tree-tops and the like that is inside it, and far enough downwards to include any caves and the like that may be part of the covenant as well as far enough downwards to make it difficult to undermine the protected area with PeTe spells.
In the case of a tree growing right inside the border of the Aegis, with branches extending outwards it seems equally sensible to rule that the entire tree (including all branches) is protected by the Aegis or to rule that the branches extending too far outwards aren't protected. You decide which would work best for you.
In short, the Aegis protects everything inside its target boundary, for whatever defintion of "inside" seems most sensible and intuitive to you and your troupe.
Or, you can decide it has a precise geometric shape with a specific size (I would suggest a sphere) and anything extending outside of that isn't protected - in which case you may well need to increase the Size of the target (and thus the level of the spell) in cases of really large covenants.
I would like to add that Aegis spells with Size +1 should actually be a fairly common spell available for trade. The default 100 paces size is really only useful for the smallest Covenants. For people who live in the USA, that is only about the area 6 houses and their yards take up in the suburbs (0.25 acre lots are the most common) if you squeezed them in tightly.
Most Covenants would fall into the 101~1,000 paces size (Size +1 or Size +2). That is over 160 acres (about 0.25 square miles or 0.65 square kilometers depending on where you are in the world) at the upper end. Heck my parents house is on 20 acres and would need a Size +2 (I learned my lesson cutting that sucker, 0.25 acres for me).
Now Size +3 (or even more) you are getting into the unusual range. Not many covenants need 1,001+ paces to cover their claimed area. Size +3 could handle over 1,600 acres. Size +4 could handle over 16,000 acres or 5x5 miles. That would be enough to cover a fair amount of medieval cities.
EDIT: I was tired when I answered this and another question and its seems I made easy mistakes on both. I fixed my error thanks to Callens catch.
I agree with size +1 probably being common, especially since multiple canonical covenants have an AotH that is bigger than a base Boundary.
However, I question why you mention 101-1,000 paces and then later 1,001+. 1000 paces in diameter is size +2. Extra magnitudes of size multiply the maximum area by 10, so they multiply the maximum diameter by a little over 3. Between 317 paces and 1000 paces is size +2.
I am hoping to get some group consensus, with the pertinant arguments, to present to my (inexperienced wih Ars) group.
It is the airspace question that is really bugging me. An AotH is supposed to block supernatural creatures trying to cross it, which poses no difficulties when a Giant walks up and tries to cross the boundary. But in the case of a high flying creature (eg griffon or dragon), where does it get stopped?
Or, importantly, can you levitate a boulder directly above the AotH, then let go and have it fall on the Covenant?
After the last couple of days, I am leaning towards the idea that an AotH acts something like the dustcloth pulled over the bench with the model train set, or the cling wrap over a food container, except slightly inflated so there is some walking room on top of the Wizard's Tower that is protected. Even for a mage of size +2.
Still feeling out if there are any flaws in that interpretation.
Suppose you have a typical non-ostentatious covenant with lots of cottages and longhouses and maybe the odd two-storey structure for council hall and library. One day, a covenant member decides he needs something better for his sanctum, and decides to Conjure the Mystic Tower in the middle of the Aegis. Will the tower grow out of the Aegis? Or not?
(It is certainly playable either way, so it is not necessarily a flaw.)
That is, indeed, the most important question to settle. It seems to boil down to two alternatives. Either you can overfly the aegis, because it only protects the covenant/farm/castle/whatever it is cast on, or you cannot, because the aegis extends to the lunar sphere. If you cannot agree, I suggest you toss a coin. (If you cannot even agree on how to toss a coin, you probably should not be playing together.)
The exact shape of the Aegis, in the case that it does not extend to the lunar sphere, is highly unlikely to matter materially to the story, so I do not see the point in debating it.
This question is interesting if the answer is that it is not protected. It could be a story hook if such a branch extendig outside could be exploited in one way or another. If you want to tell that story, you had better houserule that it is not covered, and have fun with that decision.
Otherwise it is a fringe detail. Why on Earth would you have a tree reaching its branches out of the Aegus? The only reason to mention that it does, is if it actually is not covered, and this non-coverage has potential story impact.