On 14.03.2014 15:51, Fernando Trebien wrote: > This is a small issue that came up recently in Brazil. In my > understanding, the layer tag has no specific meaning other than to > specify a rendering order.
That is a common misconception by people who worked with graphics editing software such as Photoshop and Corel Draw. In these applications, layers are only used for ordering and grouping of objects. As opposed to that abstract layer model, we use a pysical layer model in OSM where layer=0 means ground-level, layer<0 means underground, and layer>0 means above ground-level. Renderers typically use a different rendering style for underground features, e.g. dashed or greyed-out lines, or these features may even be omitted. The only way how renderers can determine whether a feature is underground is by looking at the layer tag. tunnel=* and bridge=* do not necessarily mean that a feature is underground / above ground. There was even a negative voting on this (a proposal for implying layer=-1 or 1 respectively). And the other way round, not all underground objects are tunnels or culverts. A tunnel is a way with an entrance on each end. It is not a tunnel if it has a dead end. And what about POIs and areas? It would be stupid to tag them as tunnels. The tunnel=* and bridge=* tags do help the renderers when it comes to the curved bridge and tunnel signatures, but the dashing can only depend on the layer tag. > The wiki, however, states that it is wrong > to tag a whole river with layer=-1. The reason for that, as far as I > could figure, is because current validators (such as JOSM's or > KeepRight's) will not issue a warning on a waterway x highway crossing > when their layers are different, leading some users into tagging the > river with layer=-1 in order to get rid of warnings about missing > bridges and tunnels. The warning is ok, but the big problem with validators is that mappers mess around with the data without local knowledge, just to shut up the validator. They connect ways which are not really connected, they insert bridges and culverts that do not really exist, and so on. Among these so-called "fixes", the layer=-1 on waterways are relatively harmless, because it is so obvious that they are wrong. A culvert that has been made up by a sofa mapper is much more difficult to correct, because you only know when you go there. > Do you agree that the river can be tagged with layer=-1 as long as > this value is correct in relation to the layer of other > nearby/crossing ways? No, except for underground rivers. They do exist in karst regions... -- Friedrich K. Volkmann http://www.volki.at/ Adr.: Davidgasse 76-80/14/10, 1100 Wien, Austria _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
