[Therion] Extended elevations from survex.data

[Jenny Black]

Hello again,

I've done some more experimenting with extended elevations and have
discovered some things that may be useful to other people and some
bugs (or maybe features I fail to understand!). All the files I refer
to below can by found on my ftp site:
ftp://ftp.gns.cri.nz/incoming/JennyB/therion/ along with the dataset
(107.3d)

(1) therion and survex can extend the same data differently. I don't
think this is a bug in either software, they just use a different
methodology. If you start off by using survex's extend command to
generate a extended 3d file to read in to therion and later switch to
reading in your un-extended 3d file directly into therion you risk
getting very confused, and having to break different loops. See:

 survex_extend.png shows 107_extend.3d, generated by running "extend
107.3d 107_extend.3d"
 therion_extend.pdf is when I used: "import 107.3d -surveys use" in therion

(2) therion can behave unexpectedly if the data has been extended
already in survex, I suspect this is due to the fact that some
stations now exist in two different locations after loops have been
broken.

 therion_107_extend_3d.pdf used: "import 107_extend.3d -surveys use"
in therion, having first run "extend 107.3d 107_extend.3d", I had
expected it to look the same as survex_extend.png

Due to this, I decided to extend the data in therion not survex for
now at least (I've no idea if this is the correct approach or not!).
So, I read in a not-extended .3d file (107.3d), the same file I would
use to make a plan survey in therion. I've then discovered a few more
things...

(3) choosing the correct station name is critical. Some stations
legitimately have two different names i.e. the final station on one
days survey may also be the first station on the following trip. In
this case, ropefree.18 is the same as envelope.1, these are marked in
survex with:
 *equate ropefree.18 envelope.1
When it comes to drawing up the plan I believe you can use either
station name. This isn't always the case for extended elevations,
often only one works, and the other really doesn't giving huge
distortions. I don't know why this happens, as they are not always at
junctions or involved in loops. I assume this is a bug? In this
example, if I use envelope.3 it looks rubbish (envelope_3.pdf) but if
I use ropefree.18 (ropefree_18.pdf) it looks fine (the only difference
in these two outputs in the station name, I have several example of
this for this dataset).

(4) the order matters: "extend ignore china.29 china.27" does what I
would expect, but "extend ignore china.27 china.29" does nothing, but
gives no error. Look at the bottom right on:
china_27_29.pdf
china_29_27.pdf
this seems wrong to me, and caused me quite a lot of confusion.

Sorry if my email has got a bit long and unclear, hopefully it makes
some kind of sense! Does anyone else have any helpful thoughts on
successfully generating extended elevations? Am I missing some obvious
things? Are some of my conclusions above wrong, or are there neat ways
around them?

Thanks,
Jenny