On 20/12/2008 16:05, Sven Rautenberg wrote: > The stream therefore is the natural surface wherever its water flows. > The bridge is above the water, so it is layer=1. And it is less > dangerous when just tagging the bridge compared to tagging the whole stream.
I find it is usually the other way round: I am making a stream in virgin territory and by setting layer=-1 ways that are added later and don't have bridges (more specifically layer=1) applied are still separated OK even without explicit bridges (which are sometimes OTT -see below). Someone who doesn't realise the stream is layer=-1 will still be OK if they put in a bridge at layer=1. A renderer that puts a stream at layer=-1 underneath a land area at layer=0 is rendering naively (Mapnik and Osmarender get this right, I'm guessing because they correctly renders all areas before any ways, but they're not so good on two areas e.g on natural=marsh being rendered on top of landuse=grass without explicit help, even though it is "obvious" to a human). Lots of major motorways and rail lines aren't properly divided up with bridges every few metres to cross minor streams (many of which are in practice just glorified pipes under the road) and footways. Yes, they should but in practice they don't. For minor waterways this entirely valid technique generally results in better rendering and is no specifically tagging for the renderer - it's a perfectly correct method. Anyway... - we're free to use whatever method suits us; what the original post was about was automatically and anonymously changing data in bulk based on an invalid assumption, not trying to justify the way it was tagged initially. - the layer tag is tagging for the renderer in 99% of cases with bridges. In nearly all cases a bridge/tunnel tag says "I am above/below" automatically and a layer tag should be unnecessary. There are a few exceptions, but they are very rare: primarily where a bridge crosses exactly over the top of another bridge, where there are more than two layers involved (e.g. - and this is the only one I have come across in 2.5 years mapping - http://www.openstreetmap.org/?lat=52.329684&lon=-0.192102&zoom=18&layers=B000FTFTTT where the A14 crosses the B1514 which in turn crosses the East Coast Main Line railway all at the same point). Had relations been invented earlier this case might have been better described using a relation. And ha! Mapnik gets it wrong anyway! It doesn't continue the bridge casement on the A14 where it should even though the topmost bridge is marked as layer 2. - I think I'd argue that layer is redundant in most other cases too. The renderer ought to be able to deduce that a lake in a wood is rendered on top and an island in a lake is on top, even if there is no multipolygon hole in the wood/lake. And the physical nature of the object concerned is more important in rendering than its layer - for example, a subway railway is rendered on top of a park even though it actually runs underneath it, otherwise it would be invisible. David _______________________________________________ talk mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk
