[Therion] Handling of tape and backtape in survey data

[Bruce Mutton]

> instead Toporobot just generates a new station for the backsite.

PocketTopo automatically detects backsights if they are taken immediately after a foresight.  So the software handles the station naming, no user intervention required.  While I processed the only survey I ever did this way with Therion, it never made it into the main cave dataset and so I didn't get as far as checking the survey length was correct (or doubled).  I'm pretty sure it would be correct though.  It was only done to find a reasonably precise gap in an 'almost looped' passage.  The gap was too big to dig so we lost interest and I haven't drawn the enhanced accuracy map yet.  It is at the bottom of my priority list.
The exercise was also helpful to gauge the frequency and size of foresight/backsight mismatch with 'three-shot' disto readings.  My conclusion: a few instances that may be of interest occur, but not enough to ever bother actually taking all those backsights.  Have never done a backsight since.
Our disto driven loop closures are typically 0.5% (I like to think) but a bad one will be 1.0%.  Compared to our pre-disto loops over a decade ago which were typically 2% and a good one 1%.  Although I prefer to think in Survex's standard deviation approach to loop closure, which tells a different story about which loop is good and which not...
Bruce