[OSM-talk] cadastre, ways, rendering

[Jochen Topf]

On Tue, Jan 23, 2007 at 09:56:01AM +0800, Mike Collinson wrote:
> At 07:42 AM 23/01/2007, Frederik Ramm wrote:
> >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.
> 
> Well, that's not strictly true. OSM supports segments. They manage 
> properties that only apply to a part of a way. I chip in because I 
> think I'm in a distinct minority in thinking that, render compute 
> power drain aside, they have great potential for recording local 
> ephemeral changes to a highway: oneway sections, abutters, bridges, 
> unsealed sections, width ... etc and there needs to be balanced 
> debate.   From that perspective I'd agree that splitting a way when 
> it goes primary to secondary makes sense, a major change and doesn't 
> change back.  I'd also agree that making ways only from contiguous 
> segments also makes good practise but I can't see that this should be 
> hard and fast.  I would though certainly record changes to oneway at 
> a segment rather than way level where a road does things like split 
> as it approaches a major junction - a local temporary change.

Putting some information on ways and some information on segments is
inconsistent. You might have an opinion on what constitutes a major
change which warrents a new way and what is a minor change which you
only put on the segments. Somebody else has a different opinion. This
gets messy very fast. So IMHO all tagging should be on ways.

Now consider the rendering issue: If you have, say, oneways on the segments
and other tags on the way, you'll not see the oneway arrows because of
the way Osmarender works (I don't know about Mapnik). To correct this
in the renderer, it would first have to find all ways and all segments
sharing a certain stretch of road, "add" all the tags together and then
render. It makes everything much more complicated.

Also it leads to another problem: Lets say you have a road with a tram
line running down the middle. This should probably be modeled as several
segments which have two ways on top, one for the road and one for the
tram line. But to which way does the oneway=true on the segment apply?
The road or the tram line?

Or from another perspective: "oneway" is an attribute of a road, so it
should stay together with the other attributes of the road.

So for now, the rule is quite easy and consistent: Don't tag segments at
all. Tag everything on ways. Split up ways if you have to. Be it for oneways,
speed limit changes, bridges, tunnels, abutters, etc. I see the problems
with this and we had this discussion here already several times. At some
point we need a better solution, but currently tagging only ways seems
to be best practice.

Jochen
-- 
Jochen Topf  jochen at remote.org  http://www.remote.org/jochen/  +49-721-388298