[Therion] Embedded image does not appear in some layouts

[Tarquin Wilton-Jones]

> Tarquin, I had confused the meaning of "extended" and "elevation" 
> scraps.  What I really want is "elevation" where there is no projection 
> on a specific bearing.


OK, this is only slightly different. Much of the process is the same 
though. If you have a very simple cave, and you have already drawn a 
perfect extended elevation while you were in the cave, great, you can 
skip a bunch of steps. However, I find that in the cave, I have no idea 
how the extended elevation will end up being drawn, so I don't have 
anything useful, and need to redraw it all when I get out.

Firstly, you need to decide whether you start on the left or right of 
the survey. You probably decided this while you were in the cave (or 
maybe not), but you will need to re-decide now that you have enough cave 
data to work out where nasty overlaps happen, and what direction makes 
most sense. You add the "extend start" and "extend left" or "right" 
commands for your survey.

Then decide where to fold left and right to make the cave most 
understandable - this is an art form. Decide if some passages need to be 
shortened or lengthened. Add the commands to change the extend direction 
as needed, or to scale legs as needed. Keep exporting it as a PDF so you 
can see how your design looks. It really helps to do this before you 
have drawn scraps. If you have already drawn scraps ... well ... I have 
a map like this:

map skeletonMEE
   thecave # that is the survey *not* a map
endmap

Then this thconfig:
layout local
   code metapost
     def l_survey_cave (expr P) =
       T:=identity;
       pickup PenC;
       if ATTR__shotflag_splay:
         thdraw P withcolor (1,0.5,0.5);
       else:
         thdraw P;
       fi;
     enddef;
   endcode
endlayout
source "thecavearea.th"
select skeletonMEE at thecavearea
export map -proj extended -layout local -o "thecave-skeleton.pdf"

That gives me a PDF to see my design.

Now, once you are done, export a new XVI, just like I said for the 
projected elevation, but this time use "extended" as your -projection. 
Add a .th2 and add it to your .th in the same way (I call mine 
"surveyeecustom" to show it is one I have customised), create scraps in 
it as you would for a plan (I name mine somescrapSEE for Scrap Extended 
Elevation, and use MEE for the maps). From there on, the instructions 
are basically the same. However, I find that all too often, the 
extending algorithm points a leg in completely the wrong direction. It 
is much better than it used to be, but just randomly, it is nonsense. Or 
it just scales the splays weird. So I often end up looking at it in 
Survex Aven, seeing "that one looks wrong", then measuring from station 
to splay in elevation or plan, to work out where it should have pointed, 
and how far away. (It really depends how much you care.)

You will need to refer to the instructions for extended elevations, to 
see what the "extend" options are. Some really useful stuff got added 
relatively recently for scaling legs, so you can shrink a passage to 50% 
of its natural length. And a tonne of other useful options.

If you want more than one variation of the extended elevation, eg. with 
the leg folded left in one, and right in the other ... you are in for a 
small nightmare, and I will let Bruce help you out, because he has a way 
to do it. It is not straightforward, because Therion is really designed 
to have just one extended elevation.