[OSM-talk] zones for motorway/in town/outof town?

Cartinus cartinus at xs4all.nl
Thu May 21 08:42:40 BST 2009


On Thursday 21 May 2009 01:24:36 Radomir Cernoch wrote:
> Cartinus píše v Čt 21. 05. 2009 v 01:01 +0200:
> > It is completely possible for a village ringroad on a bridge
> > (highway=primary or secondary) to have a maxspeed of 80 km/h due to being
> > outside the build-up zone, not because there is a sign on it that says 80
> > km/h. It is also possible at the same time for the road under that bridge
> > to have a maxspeed of 50 km/h due to being inside the build-up zone. The
> > point of the bridge has one lat/lon, but the roads are vertically
> > separated. This will never fit in any 2D model.
>
> I do not think I am missing the point. Your situation would be handled
> like this:
>
> 1) There is 1 polygon with a tag "zone-50".
> 2) The road under the bridge has only 1 tag "highway=residental".
> 3) The ringroad over the "zone-50" has 2 tags:
>    "highway=primary" and "maxspeed=80".
>
> [There is _no_ polygon with "zone-80"!]
>
> If you ask for the limit on residental road, the answer is:
> "50", because it lies within the "zone-50" region.
>
> If you ask for the limit on the ringroad, the answer is:
> "80", because it has the tag "maxspeed=80", which has greater importance
> than the "zone-50".
>
> If you ask for the limit of the lat/lon, where the roads have
> intersection on the map, the answer is:
> "Ambiguous. You must specify the road, you are asking for!"
>
> Did I explain it clearly this time?

The ringroad does not have the tag maxspeed=80 !!!

It has a maxspeed of 80 km/h because that is the default maxspeed of 
roads "out of town" in that country. "Out of town" was one of the three zones 
in the initial idea. It is defined by the country polygon.


The whole idea behind tagging zones is that you wouldn't have to tag maxspeed 
on the road where it is not explicitly signed. By forcing the 3D reality into 
a 2D model you have to tag both the explicit signs and anyplace the model 
breaks down. Ergo: You need a better model.


You say (in an earlier mail) that the "point in network" model won't work, 
because missing points cause the zone to leak all over the whole world. In 
the next sentence you say polygons don't have that problem. The second is 
wrong. There are plenty of broken polygons in the database. They get fixed as 
soon as somebody notices them. Since traffic zones will not be rendered by 
the main renders, you will need some special purpose renderer/tool to 
highlight any breakage, before any widespread use of whatever model you 
choose will work. Even for the simple model of tagging maxspeed at all roads 
there are currently special renderers to find what is still missing.


-- 
m.v.g.,
Cartinus




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