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 <frank...@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 _______________________________________________ talk mailing list talk@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk
