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そこで、マッカーサーが自分で作って見た **のが** 地図Aである。
<https://www.docdroid.net/847v2dg/img-20170413-0001-new.pdf.html> The sentence
is from Line 5 in the above linked text.

the problem lies with のが. I don't really understand why it is there. I usually
would expect the sentence to be like this: そこで、マッカーサーが自分で作って見た地図Aである =>
"Therefore, the map A is there, which McArthur tried to craft by himself." or
"Therefore the McArthur tried to craft by himself map A is there" in a more
literal way.

In this very chapter, my textbook taught me that a sentence element further
determining another sentence element can be marked through both が and の, like
here: 留学生 が/の 描いた絵を見ました。

But nowhere has it been said that both can be used at the same time xD So,
apart from the fact that I really don't see why が or の have to be there at
all, I understand even less why both of them are there...^^
CC BY-SA 3.0
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question
grammar
Problem with のが in this sentence
This の is an "empty" noun - a noun in form and function but with no
content/meaning. It is always modified by a sentence (of theoretically any
length, from a single word up) and takes on the meaning of that modifying word
or sentence. To put it another way it turns what modifies it into a noun,
which can then serve as a subject, an object etc. Here, the sentence
マッカーサーが自分で作って見た, "MacArthur tried making one himself" or "MacArthur made it
himself as an experiment", becomes a noun, which can be rendered into English
as "The thing MacArthur tried making himself", or "What MacArthur tried making
himself". This noun phrase is marked with が as the subject of the verb である.
The whole therefore means, in an over-literal translation, "What MacArthur
tried making himself is Map A". In this sentence it is the first part that the
writer regards as new information for the reader. The reader clearly knows
there is a Map A because it is (presumably) there on the page, and the writer
is supplying the new information that what this map represents is MacArthur's
own effort at making a map of whatever-it-is. I would render this
straightforwardly as "Map A represents MacArthur's own draft" or "MacArthur's
own draft is shown as Map A".

Consider:

> バスに乗る [Somebody] gets on a bus
>
> バスに乗るのを見た I saw [somebody] get on a bus
>
> きのう、駅の前で田中さんがバスに乗るのを見た I saw Tanaka-San get on a bus in front of the station
> yesterday
>
> きのう、駅の前でむかし英語を習った田中先生がバスに乗るのを見た I saw my old English teacher Tanaka-San get
> on a bus in front of the station yesterday

And so on.
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answer
This type of sentence is called a [_cleft
sentence_](https://japanese.stackexchange.com/q/19204/5010). This の works as a
placeholder just like "it" in English cleft sentences. This is a very common
pattern. See the link for details and lots of similar examples (linked under
"Related" section).

> マッカーサーは地図Aを作ってみた。  
>  McArthur tried making Map A.
>
> マッカーサーが作ってみた **のは** 地図Aである。  
>  **It is** Map A **that** McArthur tried making.
>
> マッカーサーが作ってみたの **が** 地図Aである。 (exhaustive-listing _ga_ )  
>  It is Map A (among others) that McArthur tried making.
CC BY-SA 3.0
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answer