[OSM-talk] Key:layer update

[Richard Z.]

On Wed, Mar 12, 2014 at 09:06:41PM -0400, Russ Nelson wrote:
> Pieren writes:
>  > On Wed, Mar 12, 2014 at 3:22 PM, Frank Little <frankosm at xs4all.nl> wrote:
>  > > Richard Z wrote
>  > 
>  > > As mapped, the waterway=stream (Way #138911739) runs underground (layer=-1),
>  > > probably through a culvert given the way the stream left and right are
>  > > separately outlined as waterway=riverbank (and without layer=*). The way
>  > > (stream) should be tagged as a culvert.
>  > >
>  > > Perhaps there is in reality a bridge not a culvert, in which case the road
>  > > needs splitting and the appropriate new road segment tagged as bridge=yes.
>  > >
>  > > In either case, a layer tag is not needed for rendering.
>  > +1
> 
> Nonetheless I add one out of habit. But I would be happy to stop,
> because as noted, the bridge or culvert carries an implicit layering.
 
With a new type of bridge we could do it. The current state is that if 
there is no layer tag the bridge has a layer=0 which is not what you 
want. The old definition can't be changed because it would affect many
existing crossings.

The implicit layering that you mention is a technical workaround that 
software does to avoid problems with OSM data. Those are not necessarily 
bugs in OSM data but very often it is missing information - someone was 
not sure is there a bridge/culvert or perhaps a ford.
Rendering and other software needs to make a guess in such cases. Although 
it might be more correct to paint a question mark there most renderers 
assume a culvert and implicit layering in such cases.

It isn't good to rely on that for technical reasons, many alternative 
renderers are much less fault tolerant than Mapnik and you will get 
strange results.

Also the validators need to improve their error checking to catch accidental
errors and this is nearly impossible until people agree on how to use layer
(and level and other similar tags).


Richard