On Thu, Aug 14, 2014 at 05:32:20PM +0200, Friedrich Volkmann wrote: > On 14.08.2014 13:18, Dan S wrote: > > >>> I think that it is an obvious idea, but wiki claimed that "At the moment > >>> there just a > >>> tag to map the entrance to a cave." despite fact that existing tags fit > >>> well. > >> > >> No, they do not fit. Caves are complex three-dimenional structures. In most > >> caves there are no paths. You go or climb or rope down whereever you feel > >> like. > > > > This is the same as with a pedestrian square - there's no specific > > route in the square and you go wherever you feel. However it's useful > > to make them part of the OSM database, both for showing their > > existence and to help with various routing applications. > > Pedestrian squares are 2-dimensional. Caves are 3-dimensional. Many cave > rooms overlap themselves a couple of times in the z-axis. > > Forget routing in caves. There's no GPS. And those who get lost without > routing apps will get lost in a cave anyway. > > > I'm afraid layer=-1 does not express that a feature is underground. It > > expresses that a feature is lower than all features at layer=0+, but > > there's no guaranteed relationship with ground level. > > In central Europe it is, but habits may vary around the word. Much chaos > these days... > > In my opinion, there is some misconception by people who are used to image > editing software such as Photoshop, Adobe illustrator, Gimp, Corel Draw, > Inkscape, etc., as well as CAD software. In all of these applications, > layers stand for rendering order. In OSM we need to think in physical layers.
key:layer is not a layer of something. It is a relation expressing a vertical relation between 2 or more objects. > Caves are just an example. There are many more underground objects which are > not tunnels. E.g. I used to go to school over a landfill for 8 years without > knowing, because it was covered with soil and grass. The only way for > renderers to know is by eveluating the layer tag. Of course you could set > some additional tag like underground=yes, but having two concurrent tags for > the same thing is just a mess. You'll soon get a lot of inconsistencies. location=underground is quite widespread. > > > There are quite > > a few objects with the implicit layer=0 but which are not at ground > > level (e.g. tunnel=culvert items: <http://overpass-turbo.eu/s/4zE>). > > Therefore we need to tag them all with layer<0. There was a proposal for > implicit default layer=1 for bridges and -1 for tunnels, but unfortunately > it was voted down, so we are damned to set it manually every time. actually, a large number of tunnel=culvert without an explicit layer are mapping mistakes. Most commonly someone splits a waterway and attaches the culvert tag to the wrong piece of the waterway. Those need to be fixed one by one. Richard _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
