[Therion] Breaking a loop at a specific station on extended elevation

[Tarquin Wilton-Jones]

On 20/12/2019 20:58, Tarquin Wilton-Jones via Therion wrote:
>> If extend algorithm follows stations in order 1 - 2 - 3 - 4 - 5 - 6 - 7
>> - 8 - 9 and you specify
>> extend vertical 3
>> extend right 8
>> Then stations 3,4,5,6,7 should be placed vertical and 8,9 on the right.
>> I have tested it and in my example, it works like this.
> 
> In the case of what you said, I would have expected the following legs
> to be default (right):
> 
> 1-2
> 2-3
> 
> the following legs to be vertical:
> 
> 3-4
> 4-5
> 5-6
> 6-7
> 7-8
> 
> then the following legs to be right:
> 
> 8-9
> 
> Instead, leg 2-3 will be vertical, and leg 7-8 will be right.
> 
> At least in my head, "extend vertical 3" feels like it should mean
> "everything from station 3 onwards". I think Bruce also expected this,
> given what he wrote in the wiki "extend" article:
> https://therion.speleo.sk/wiki/extend
> "Usually this option will not work as described, unless it is preceded
> by one or two extend right <leg> statements."
> 
> So perhaps this is either a generic misunderstanding; Therion assumes
> that by <station> it means <the leg that goes *to* the station>. We
> assume that by <station> it should be <the legs that come *after* this
> station>.
> 
> I don't know which of those is right. I have also never used this with
> Survex, so I don't know if Survex agrees with Therion - Perhaps Wookey
> can say?

Survex agrees with me. Therion does not.

    1 2 20 0 -80
    2 3 5 62 -10
    3 4 5 12 -15
    4 5 5 132 17
    5 6 5 18 -15
    6 7 5 201 -7
    7 8 5 63 -10
    8 9 20 0 -80
    9 10 5 15 -45
    10 11 5 357 -30
    11 12 5 27 -25
    12 13 5 290 -18
    13 14 5 323 -22
    14 15 5 300 -7
    15 16 5 15 -35
    16 17 5 337 -42
    17 18 5 281 -38
    18 19 5 225 -20
    19 20 20 0 -80

Therion:
extend start 1
extend left 3
extend right 8
export map -proj extended -o "somepitches.pdf"

Survex specfile:
*start somepitches.1
*eleft somepitches.3
*eright somepitches.8
extend.exe --specfile=extend.svx somepitches.3d somepitches_extended.3d

I have attached the two outputs. You can see that they are not the same.
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