On Mon, Mar 24, 2014 at 10:27 AM, Friedrich Volkmann <b...@volki.at> wrote: > 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.
This is not written anywhere in the wiki: http://wiki.openstreetmap.org/wiki/Key:layer > 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 concept of "underground" seems more closely related to the tags level and location: http://wiki.openstreetmap.org/wiki/Key:level http://wiki.openstreetmap.org/wiki/Key:location Again, no mention in the wiki (http://wiki.openstreetmap.org/wiki/Key:layer) to negative layer values being used to represent the idea of underground. >> 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. I understand this issue, but don't you think validators could be a little bit more clever, so that users couldn't trick them? In some previous message I described some very simple logic that would make these tricks (in this situation) impossible to work around, and this would discourage people from using layer=-1 on the rivers. >> 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 -- Fernando Trebien +55 (51) 9962-5409 "The speed of computer chips doubles every 18 months." (Moore's law) "The speed of software halves every 18 months." (Gates' law) _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
