DrWho,
Can you reproduce this?
I mean, can you try to recreate the problematic polygonal geometry and get the error again?
From the file you attached to the thread it seems to me that the geometry was actually not correctly created as we have indicated. This may be due to incorrect creation of the geometry.
So the question is: if you correctly create it as it whould be created, then do you still get the error?
By the way, as far as I know ArchiCAD stores numbers in floating point format which is a lot more than 12 digits.
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