What are the usages of のだから?

Question

I'm reading a book and I came across a usage of のだから I couldn't really understand.
The whole sentence is:

その上に[無慮]{むりょ}百に及ぶ階層が積み重なっているというのだから、[茫漠]{ぼうばく}とした広大さは想像を絶する。

My translation to it was : "It has been said that on top of that, about 100 levels are accumulating (on top of each other), and therefore, its vast size is unimaginable". Obviously this is not a literal translation.

What I don't get is the role that のだから plays in this case.
I'd say it has a nuance of cause/result, but then why ので・から were not used in this case?

According to the "best" answer in this question, のだから has 2 rules:

1. Used when both the speaker and listener know some fact, but expresses a strong feeling on the part of the speaker that the listener, although conscious of said fact, does not fully appreciate its implications.
2. The clause following ～のだから often expresses the speaker's judgment, intent, wish, or request.

I know for sure that rule (1) is not true in this case, since the fact given in the first part of the sentence was never previously mentioned in the book.

As for (2), I'm still not sure whether the part that comes after のだから can be counted as a "judgment"...

Hopefully someone can help me grasp the idea of のだから!
Thanks in advance!

I'd also appreciate even a list of all of the variations の・ん＋です・だ, so I could at least research them myself as I always tend to get confused with them... a link or a short summary of them would be even better but it's really too much of me to ask!

Also, if you want to know, the sentence above was taken from the book ソードアート・オンライン１－アインクラッド。

Answer 1

の at the end of a statement expresses emphasis. I believe that the のだ here is simply used that way.

For example:

あの山は大きいの。

You could add on to this sentence:

あの山は大きいのだから、登るのが大変でしょう。

When using から in to express causation you need to precede it with a verb or です or だ. You simply can't grammatically say のから. So the の was added for emphasis and the だ was added to make the sentence grammatically correct.

ので would probably be ok in here I think but ので is more colloquial and expresses less emphasis than のだから. I generally see it used more in spoken or informal language to express thoughts and feelings rather than facts, so I don't generally see it in written Japanese like your example.