How can I mask points of a freeform surface?

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Hi, I measure a very small freeform surface and need to exclude a certain circular area from evaluation. I use scanned point sets in my strategy and it would be very difficult to exclude the area during scanning, because the surface is so small and the tip radius is rather large in comparison. I fear the stylus might touch that area anyway and produce faulty points. It'd be better to mask the area afterwards, but I have no idea how to do that.