[OSM-talk] cadastre, ways, rendering

[Florian Loitsch]

On Tuesday 23 January 2007 00:42, Frederik Ramm wrote:
>  > could you elaborate on that? What in particular are non-contiguous
>  > ways?  (Apparently you can put roundabouts in between, and they are
>  > still contiguous). Why are they bad? ...
>
> In the OSM data model, a way is simply a number of segments thrown
> together. You can take one segment from Southern Spain and one from
> Norway and make a way from them.
>
> Of course this does not make sense. Some people, me included, maintain
> any OSM "way" should only contain contiguous segments (i.e. you must be
> able to visit all nodes in the way by only travelling on segments that
> are also in the way).
>
> Many people come to OSM with a semantic model in their heads that goes:
> "If it is the same road, then it should be the same way". But roads are
> not alway contiguous, for example you may have an intersection that
> looks like this:
>
>
> --------+
>
>          +------------
>
>
>
> If you want to travel along the horizontal road (assume it is the same
> road), you will actually have to travel a bit on the vertical road. If
> you put both parts of the vertical road in the same way in OSM then you
> have a non-contiguous way. The same is the case with a road when there's
> a roundabout in between.
>
> This, however, creates problems with rendering, because wen drawing a
> way on the map, many renderers will try to write the name of the way
> along its length and have trouble with "gaps". Also, considering
> potential later routing applications, it is a nice thing if ways are
> contiguous (traversable), therweise you will have to divide each way
> into traversable subparts.
I understand the rendering issue, but why is routing a problem? Unless I 
missed something routing should only look at the segments?
>
> So I (and others) strongly advise to make individual ways in such cases.
> This means that you may have five ways all having the same name because
> they are in reality the same road. No big deal; you have to do that
> anyway if attributes change. Since OSM cannot manage properties that
> only apply to a part of a way, you will have to split your way if the
> road suddenly changes from being a primary road to being a secondary
> road (even if it is still the same street with the same name). Or if it
> changes from being one-way to being both-ways.
Ok. So if there's a round-about you advise to split the road?

I currently put the oneway tag on the segments and not the ways. Do I need to 
change that? (by splitting the way and then readding the tags).

somehow related question: I marked the segments of roundabouts as 
being "junction:roundabout". Should this be moved to the "way"-level too? (IE 
make the roundabout a way, and put this label in the way level).
unrelated question: sometimes I selected the whole roundabout to add 
the "junction:roundabout" tag. Later I discovered, that this included the 
nodes too. Does this make any troubles? Do I need to remove this tag from the 
nodes and only leave it at the segment/way level?

thanks again,
// florian
>
> Bye
> Frederik