[Therion] Splays - Revisiting Breaking extended elevations

[Olly Betts]

On Wed, Jul 29, 2020 at 06:25:50PM +0100, Tarquin Wilton-Jones via Therion wrote:
> TopoDroid, Pocket Topo and Survex all extend only the legs. They all
> keep the splays in the "same" direction (relative to the leg) as they
> were before, which makes them normally perfect for drawing extended
> elevations. They can project forwards and backwards out of the page (so
> to speak), so they end up the right distance, visually, from the station.
> 
> There is always the issue when you are at a station where the extend
> direction changes from left to right or vertical, since at that point
> you don't know whether a splay should take its direction from the leg
> before or the leg after (or the "nearest" leg to it - perhaps the most
> useful answer). Survex gets this wrong sometimes, but is normally useful.

Examples of where Survex it gets it wrong are definitely of interest.

I added extending of splays just over 3 years ago, and the NEWS entry
(which also explains the algorithm used) explicitly invites feedback:

| + Splays are now carried over the extended survey.  The current
|   handling is simplistic, but should do a good enough job to be more
|   useful than discarding splays.  The splays at each station are all
|   rotated together based on the bearing between the stations either
|   side of the current one along the first path extended through that
|   station.  This nicely handles dead ends and the situation at the top
|   or bottom of a pitch, and should tend to pick an angle close to the
|   passage orientation along a traverse.  It's weakest at junctions.
|   Feedback (especially examples which could be handled better) most
|   welcome.

But so far the only thing I could possibly count as feedback on this
subject is what you wrote above (which is good to hear overall, but it'd
be helpful to have some details of where you've seen it go wrong).

Cheers,
    Olly