[Therion] Handling of tape and backtape in survey data

[Olly Betts]

On Sun, Dec 26, 2021 at 08:00:49AM +0000, Ben Cooper wrote:
> What does Survex actually do for all these loops?

It deal with any backsights up front and just adds a single link to the
network for each survey leg.

It's harder to properly account for partial backsights if we created a
separate link for the backsight (e.g. backcompass and backclino but no
backtape is common).

But even just from a network solving perspective, if we generated a
trivial loop for each backsight the network optimisations would
later rediscover each trivial loop and collapse it to a single leg,
but that would be less efficient than collapsing it when we have the
knowledge it's a trivial loop as we read in that leg.

Cheers,
    Olly