Backsight processing and missing backlength

[Wookey]

+++ Olly Betts [2015-03-03 22:10 +0000]:

> It looks like there's a bug with handling the case where the instrument
> and back instrument have different SDs, which is probably not common in
> reality (having different instruments for each sighting direction is
> reasonably, so the calibrations may well be different, but I doubt many
> people use a significantly less accurate instrument for sighting in one
> direction).

This came up on a recent houping china expo, with trad instruments in
one direction and distoXs in the other. 

I wasn't clear whether one-line readings/backreadings could have
separate sds (seems they can but it may well be buggy), or whether
they were treated differently from full loop closure (ie just
averaging the two readings, rather than making every leg a new loop).
Or whether there is any difference between entering single-line
fore/backsights vs entering fore on one line, back on another.

Is what actually happens documented somewhere?

I also found that we don't have 'backlength' which is very handy when
doing a backsighted distoX/distoX survey (which we did several of). I
know this is a tad peverse, but if you take distoXs on a project that
requires backsights that's the readings you get, and the distance
variation between front/back is usually a few cm so should be used to
improve dataquality/check for cock-ups.

I'll file a bug to remind about this and try to supply a patch for
backlength (probably not too hard).

Wookey
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