[Therion] SDs for distoX surveys

[Footleg]

In my own experience, the most common blunder which the 'measure 3 times'
method with DistoX2 catches is mis-read distance. The Disto tends to be
used in the same orientation for all 3 shots, so the compass and clino tend
to be well aligned in all 3 (regardless of how well calibrated the DistoX
is, or is there are local magnetic fields throwing the compass). But the
laser does sometimes miss the target and overshoot, resulting in one of the
3 shots being out of tolerance for PocketTopo to count them as a valid leg.
Of course catching blunders so they never make it into the data (we would
reshoot when it happens) is not the same as the SD in the measured data.

The way we catch bad calibration problems or local magnetic interference is
to take backshots (3 of them). This has saved serious errors on multiple
occasions which would not have been detected taking shots in one direction
alone.

Footleg