Lay straight Edge along bottom corner of existing passage (Not sure how the existing passage was excavated, but nevertheless we'll move on)
Rough cut extension to the existing passage
Accurately cut lower corner of existing passage, then place straight edge into lower left corner while simultaneously congratulating himself on the accuracy of the lower corner which up until now has been inaccessible by the straight edge. (Primarily due to the fact that up until this stage the lower left corner didn't exist).
If by any chance there has been any deviation from axial center. (Undoubtedly brought about by the impossibility of using a straight edge to check and adjust a corner that at the time didn't exist)
Refill with solid bedrock and repeat the exercise.
A one degree deviation per foot over 180 feet is unacceptable as this may well lead to one ending up right back where one started.
Handy Carpenter? LoL
Even if R Avry Wilson can't appreciate the problems associated with this method, I'm sure most people will.
Edited 7 time(s). Last edit at 14-Feb-16 11:28 by Jon Ellison.