Question: 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...^^

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.
> 
> 
>
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.