Autogen and airport polygon

Hi guys, I am busy creating an airport using AFCAD 2.11 for the taxiways, aprons, and runway and gMax for the buildings. I have one problem though, the airport I create ends up having some trees and buildings appear through it as the "airport polygon" (I think thats what it is called) is not large enough. I would appreciate any help on what to do to move the trees and buildings back (autogen) or to make the airport polygon larger as this appears to remove them.Thanks in advance.Warren Baier