[Tagging] public_transport=platform rendering on osm-carto

Paul Allen pla16021 at gmail.com
Wed Jun 20 18:20:38 UTC 2018


On Wed, Jun 20, 2018 at 6:53 PM, <osm.tagging at thorsten.engler.id.au> wrote:


> So, valid minimal tagging under PTv2 is very simple:
>
>
>
> You have one node (if there is no clear platform) or a way (along the
> platform edge) or area (the whole platform), which is tagged as
> public_transport=platform (plus whatever mode of transport is served at the
>
>
platform, so bus=yes or tram=yes, or …)
>

Which all sounds fine, until I try to make sense of the relation.
Something (or somebody) seems to like
shoving all platforms at the start of the relation, then the ways in order.
   Which would work (just) for a routeing
algorithm, if you throw enough CPU at it, but is inefficient.  It also gets
very confusing when you have a route which is
circuitous and doubles back on part of its earlier route in the same
direction, causing some stops to be in the
relation twice.  The route I'm thinking of has a stop on that revisited
section which is NOT used on the first
traversal but IS used on the second.  It's hard to figure out what's going
on unless the stops appear in the
relation next to their ways rather than all lumped together at the
beginning.

And it gets worse.  Suppose I have a simple route, from X to Z with a stop
at Y.

  X --- bat street --- o --- cat street --- o --- dog street --- Z

X is at the start of bat street, Z is at the end of dog street.  Y is in
the middle of cat street, not at either of its
junctions with the other two streets.

The choices I have for relation ordering (I'm still learning/battling JOSM
to do it) are X, Y, Z, bat street, cat street,
dog street; or X (which appears to be how it's ordered by default), bat
street, Y, cat street, dog street, Z;
or X, bat street, cat street, Y, dog street.  None of which make it clear
to a mapper or consumer what the reality is
without also looking at the map (simple inspection of the data is not
enough).  Or should cat street be split at Y
so I can have X, bat street, cat street 1, Y, cat street 2, dog street, Z?

Or does it simply not matter where the stops appear in the relation?  If
not, does it even matter what order they're
in?  I would have thought that for efficiency of routeing you'd NEED stop
positions on highways too, otherwise
there's an extra search outwards from the platform until it finds the way.
Oh, and the nearest portion of the
way may not actually be reachable from the platform because of obstacles.

I'm confused.  The wiki doesn't seem to make it clear and nor do the
tools.  Am I entirely missing the point?
Probably.

I would appreciate somebody with a deep understanding of this stuff
clarifying matters, here or on the wiki.

-- 
Paul
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