[OSM-talk] Layer transitions

["Marc Schütz"]

> 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 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. 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.

Regards, Marc
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