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...^^