I'm looking for a bit of help understanding what とした means in this sentence describing the UN flag:
この旗は、真ん中に北極を中心とした世界地図が[描]{か}いてあります。
While I can get the gist of the meaning, can someone help explain exactly what とした means here? I googled around a bit, but wasn't really able to figure it out. Especially, it would be great to get an understanding of this grammatical construction so I can use it / recognize it again.
Thanks in advance!