[Tagging] Carriageway divider

[Markus Lindholm]

On 20 August 2012 10:55, Gregory Williams <gregory at gregorywilliams.me.uk> wrote:
>> -----Original Message-----
>> From: Markus Lindholm [mailto:markus.lindholm at gmail.com]
>> Sent: 19 August 2012 19:26
>> To: Tag discussion, strategy and related tools
>> Subject: Re: [Tagging] Carriageway divider
>>
>> On 19 August 2012 18:23, Tobias Knerr <osm at tobias-knerr.de> wrote:
>> > On 19.08.2012 15:09, Markus Lindholm wrote:
>> >> On 19 August 2012 14:49, Fabrizio Carrai <fabrizio.carrai at gmail.com>
>> wrote:
>> >>> This could be a solution but it is against the reality: this kind of
>> >>> road are indeed a single entity. The "legal" division, i.e. the
>> >>> "solid_line" is just an attribute.
>> >>
>> >> There's a multitude of cases where a single entity is represented by
>> >> multiple objects in the database, e.g. when the road changes speed
>> >> limit it has to be split into two highway objects. The same with bus
>> >> routes, to accommodate then the road was to be split into many parts.
>> >
>> > A major difference is that it is comparatively easy to re-assemble a
>> > way that has been split (because they have common nodes).
>> >
>> > It's not so easy with two parallel ways that somehow "belong together"
>> > - the connection could only be established by rather complex
>> > heuristics based on proximity among other things. In practice, it
>> > would simply result in gaps or overlaps appearing randomly depending how
>> "parallel"
>> > the mapper has actually drawn the ways, and on the width assumed (or
>> > tagged) for the ways.
>>
>> For which purpose would the two highways be "reassembled"?
>
> Split highways may be reassembled when you're not interested in the
> attributes that do change between them. For example when you want to
> reassemble the portions of the same road with the same class and name
> together but aren't interested in the fact that the speed limit changes
> partway down, the lighting changes, the surface changes, or that a small
> portion of it has a cycle or bus route which crosses it for a few tens of
> metres. If building a routing graph from the data you'd want to keep the
> graph as simple as possible by ignoring the tags not relevant to your
> routing and reassembling the adjacent otherwise identical segments.

Yes, I understand why one would reassemble highway segments on a route
that only differ on the maxspeed tag or other such minor issue. But
why would one want to reassemble two highways going in opposite
direction and from which there is no direct legal route to the other?

/Markus