Hi Tarquin
> I don't quite understand why asking it to ignore a leg then changing your
> mind and asking it to draw it anyway, would cause it to then split it at the
> point you want. I wonder if I will be able to make sense out of that in
> future.
I think of extended generation as a trial and error sequential process.
Tell it where to start, then what direction. See what happens.
At the point where the generation deviates from what you would like, add an
instruction, in the correct sequence. See what happens.
Once that part works OK, look further along the centreline to find the next
deviation, and add another instruction.
So it is not changing mind or contradicting previous statements. See sequence
below.
>What is makes me wonder is; why doesn't this exist?
>extend break 6
>This could cause a centreline "weakness", and prefer (but not mandate)
>breaking at that point. It would massively simplify this control, and be much
>more predictable.
This is pretty much exactly what ‘extend ignore 6 <station at other end of
leg>’ does. It causes the centreline to break at station 6, and continue
automatic extended centreline generation elsewhere.
Here is the plan view, and the solution I came up with. There are probably
others.
survey extendedloop
centreline
data normal from to length compass clino
1 2 1 0 5
2 3 1 0 -5
3 4 1 45 -10
4 5 1 90 5
5 6 1.71 225 45
6 2 1.71 225 -45
5 7 1 90 0
extend start 1
extend right 1
extend ignore 6 5
extend right 6 5
endcentreline
endsurvey
Stepping through the process I take;
extend start 1
extend right 1
This produces an extended centreline that propagates 1 2 6 5 4 3 2, and now
that the loop is closed (and broken at 2), propagation continues for the rest
of the cave from 5 7.
Could I have predicted this sequence? No. Usually I think Therion chooses the
surveyed sequence, but not always. This is why I use a trial and error
approach.
>From the diagram above, I think that I want the map-connection line (the
>break) at station 6, and the extended generation direction for the leg to
>ignore is 6 5 (despite the survey data enumerating this leg as 5 6). So now I
>try;
extend start 1
extend right 1
extend ignore 6 5
This is much better, but the leg 5 6 is extending right and not left.
The apparent generation sequence is now 1 2 6, 2 3 4 5 6, 5 7
So I could try;
extend right 6 5
or
extend left 5 6
I chose the former because it involved less typing, but the latter makes more
sense perhaps if you are following the apparent sequence of generation. It
turns out that they both work, and produce this (same apparent generation
sequence, just the extend direction differs for 5 6);
Don’t worry, extend is a mystery for me as well.
This example turns out to fairly straight forward, but it can be very
complicated for a real cave, especially if you don’t start thinking about the
extended elevation until you have 10 km of passage in your project.
Any further insights greatly appreciated.
Bruce
PS: I just noticed on page 25 of the Therion Book, that extend directions can
be explicitly influenced from within a scrap drawing.
I’d be interested in finding out about usage scenarios.
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