[OSM-talk] Layer transitions

Lambert Carsten lhc.osm at solcon.nl
Fri Aug 7 15:49:46 BST 2009


On Friday 07 August 2009 12:29:09 Marc Schütz wrote:
> > On Friday 31 July 2009 17:25:19 Richard Mann wrote:
> > > I saw some strange rendering effects when a side road was straight onto
> >
> > a
> >
> > > bridge. The bridge was layer=1, so the side road was rendered on top of
> >
> > the
> >
> > > main road. That's why all the ways approaching a junction should be on
> >
> > the
> >
> > > same layer. You can either achieve this by inserting a short way
> > > between the bridge and the junction, or by altering the layer of the
> > > thing that
> >
> > is
> >
> > > bridged (ie making the stream layer=-1)
> >
> > Recently we have been discussing this problem on the Dutch talk list
> > regarding
> > bridges. Keepright doesn't like T-juntions  with different layers and
> > tags these as 'not so obvious' errors.
> >
> > The reasoning that we map the centre of the road is faulty. That
> > reasoning implies that we need to split up the road because the sidewalk
> > does not either continue to the centre of  the crossing road.
>
> +1
>
> It was always my view that the way is simply an abstraction of the entire
> road, not only its centre.
>
> > Adding a little piece of road so the junction can be on one layer just
> > does
> > not make sense. In Amsterdam there are lots of bridges and canals.
> > The canals there are physically not on the same layer as the road and
> > bridges.
> > But for practical reasons we only add layer tags where ways cross without
> > connecting (bridge over water). This 'T-junction rule' is causing just
> > about
> > every bridge to have small extra bits added. Or have roads that do not
> > cross
> > anything tagged as layer=1.
> > On the discussion list the argument is made that we don't need a layer
I meant discussion "page" by the way:
http://wiki.openstreetmap.org/wiki/Talk:Key:tunnel#Layer
> > tag for
> > bridges and tunnel (except of course when there are more than two ways
> > above/under each other) and I agree with that. So simply removing the
> > layer
> > tag on most tunnels and bridges would resolve the layer issue. However I
> > am
> > not sure if it is generally accepted that it is wrong to tag a bridge or
> > tunnel with a layer attribute.
>
> Please don't do that. 
O.k. I won't. I have no problem with explicitly tagging the layer and have 
always done so by the way.

> On the wiki (and the mailing lists) there are also 
> strong arguments against implied layer. Layer should be kept as simple as
> it can be, and this also means keeping it as an independent feature that
> doesn't change its default value depending on other tags. 

Also I am also not sure that removing the layer tags would actually 'resolve' 
the 'T-junction layer issue' since Keepright or whatever system will probably 
regard the implied layer no differently from a separate layer tag.

Lambert Carsten





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