I do not know any identification key for Geometridae, but I know very good books about the European Geometridae:
"The Geometrid Moths of Europe" from Apollo Books, Stenstrup (Denmark), Vol. 1-4.
It is continued by Brill, Leiden (Netherlands), untill now available Vol. 5 (Ennominae I).
With the help of this book you will be able to identify any European moth. I think it will be a big overlapping with turkish Geometridae.
As far as I know, there is no (binomial) key for Geometridae. Perhaps you can ask AXEL HAUSMANN, he is the leadig Author of the above-named books and he is here on Research Gate.
Try down this recently published book (DJVU file) . Should be well enough to ID dominant species because it is dealing with Central Europe fauna. Follow a link to download https://www.dropbox.com/s/p1y2k1t44h25y65/_%20Macek%20J.%2C%20Prochazka%20J.%2C%20Traxler%20L.%2C%202012_Motyli%20a%20housenky%20stredni%20Evropy..djvu?dl=0