Ø Has anybody suggestions how to use Therion to calculate the cave length based on this definition?

You would have to bear in mind the definition of cave length that you want to adhere to when surveying the cave, and choose stations accordingly.  The legs that do not contribute to length would be flagged as duplicate.

Alternatively or additionally I think a ‘third party’ add-on could be written to determine the true passage centerline (not therion’s survey centerline) based on the wall lines in the scraps.  If the effort is worth the small reduction in length I expect someone will do it.  Please not Stacho and Martin though, you have many more important features to work on.  

Good Luck

:) Bruce

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