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I want to create a tin surface as shown by the green area in the picture. However, the resulting tin-surface always includes the red area as well.
How can I delete the red trinangle from the surface?
I don't think there are any options to remove individual faces with BricsCAD TIN commands. However, in your example it should be possible with TINEDIT to create a boundary polyline, and use this to exclude portions of the TIN.
BricsCAD V20 Ultimate