Hi Markus,
No, I'm indeed very unsure in aspects of the new schema. If the ways
don't have roles, the direction of the route will have to be
determined based alone on the sequence of ways in the route.
Yes, this is the only feasible way I can think of with PT routes if you
don't have roles. This is also the way the new PT schema does this. Once
the route uses the same OSM way more than once, you will find it very
difficult to figure out automatically how the route goes. Bus routes use
the same way multiple times all the time, and train routes do it as
well, albeit rarely (Munich's S3 and S7 at Ostbahnhof being an example).
A work-around would be to check the direction of the LineString right
after its creation. Something like this:
IF the first noderef of the LineString is identical to the first or
last noderef of the first referenced way of the underlying relation
THEN change the LineStrings direction
Right? Or am I wrong again? Please help...
I think you mean "IF the _last_ noderef ... " :)
Yes, that might work. It's still a heuristic though, taking advantage of
the fact that the probability that the first way member is correct in
terms of route directionality is higher than that overall relation
member order is correct.
Still, that assumes that you have exactly one LineString for a route in
the first place. And as far as I understand the GEOS LineMerger
documentation, its algorithm won't work for multiply-used ways (at least
there are some not-so-uncommon cases I am absolutely sure it won't
work). It won't crash but it'll return multiple geometries which you'll
need to sew together and orientate with some other algorithm. Which gets
us back to square one :(
I wouldn't even try to figure it out, I'd fail fast... I would just take
the relation order and try to assemble it way by way. If it doesn't work
(you can at least determine _that_ reliably :)), so bad luck, the
relation is broken, can't render that route. Well, you still can render
it (it's a bunch of LineStrings after all), but you can't draw arrows
that show which direction it goes. And you could put a marker on it a la
KeepRight so someone can go and fix it :)
Bye
Igor
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