[Therion] Cartesian STD for individual legs

[Tarquin Wilton-Jones]

> I believe that a typical speleological topographer thing: being a little
> bit ingenious and a lot insane...

Well indeed. It reminded me of the original UK OS triangulation pillars,
but they used angles instead of lengths. Most of us cannot even dream of
getting the accuracy they did.

> Nevertheless I will
> prefer to use cartesian data and dX, dY and dZ notations. It is because
> transversal and longitudinal expected errors are very different in
> trilateration methodology and I have been able to estimate it in
> cartesian coordinates.

No problem. If you are using position fixing, you can of course use the
inline standard deviation statement.

fix 1 at surfacelegs 12345.123 23456.234 102.304 0.17 0.24 0.29

And if you are doing it for individual station position:
https://survex.com/docs/manual/datafile.htm#AEN1286
sd dx 0.123 metres

(Therion uses Survex processing for the statement, so you can see the
finer details of its meaning within the Survex manual.)

> I am a little worried if it will also work in "leap-frogging" legs, but
> I will check it, for sure. 

There are actually options relating to that. Such as:
data normal from to backlength ... etc.
now you can set the "sd backlength" differently from the "length".

But I think that since "sd dx" relates to the station position, it might
only refer to the station you are "leaving" or both stations at once
(since it is perhaps better described as "how well you held the tape
measure onto the station itself, or whether you allowed the station
position to move because it wasn't a fixed object"). Perhaps Wookey can
say which stations it really relates to when leap-frogging, or whether
it just adds to the overall "this leg has a large error margin" for use
when deciding where to distribute errors, rather than being a specific
error in each dimension. The latter is certainly a possibility.

When I tested, I found that I had to supply sd for more than just the
single thing that I was trying to affect. When I tried to specify sd for
just compass, it ignored me. When I specified it for length, compass,
clino, it worked correctly. Experiment, and see if you need to add sd
for the other dimensions.