[Therion] Difference between Therion and Survex when calibrating tape

[Tarquin Wilton-Jones]

> sorry, but what is the reason to use two calibrate commands in two consecutive lines in your example?

In reality I had a load of legs and splays in between the two commands.
Several were shot with a Disto through water, to allow me to accurately
render a sump. Water exaggerates the lengths, but that can be corrected
with a scaling factor (refraction ignored) so my actual data looks like
this:

calibrate tape 0 0.92 # shot through water
4 - 2.23 97 -45
...loads more splays...
4 5 6.52 123 -87
calibrate tape 0 # back to regular stuff
3 6 8.78 74 12

It got all the lengths wrong from that 3-6 leg onwards.