Problem with のが in this sentence


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...^^




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.




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.

