[Therion] Extended elevation

[Bruce]

Hi Andrew
I have not really used extend yet, but have it in mind for some future
projects. I am hoping to use 'revise' to allow multiple extended elevation
projections of the same piece of cave and to move the 'extend' out of the
survey data and into the map definition where it more logically sits if one
is keeping map structure independent of survey structure.  I digress.
 
I had a play with your hazelnut file with v5.3.8 to see what would happen.
I got exactly the same outputs as you did.

I read the Therion book on extend, and figured that although I agree, there
is quite a bit of unexpected behaviour as you described, the extend normal
or reverse before the first station did not make sense as there is no
previous station. See description on pg 21.  So that aberration at least
could be excused.  The examples near the end of the Therion Book are also
quite helpful.

Maybe Therion needs a bit more specification to help it along with the
hazelnut survey?
For example...

extend start 2.0
extend normal

...before the first data line makes it do what you might expect, although
possibly not for any logical reason.

Similarly...

extend right 2.0 2.1

... before the first data line makes all the legs head right correctly.

So there are some aberrations, but some lateral thinking seems to make it
work out quite efficiently.

And I agree, PocketTopo does seem to make a better stab at where the splays
should be drawn.  Maybe the algorithm could be borrowed from Beat?

Bruce