Dave F. wrote: > http://wiki.openstreetmap.org/wiki/Relations/Proposed/Bridges_and_Tunnels [...] > This statement: > > "allows both the way(s) crossing the structure /and/ those passing under > it to be identified (river and most canals bridges only pass /over/ the > waterway, and using only the Way tag bridge=yes means that intersection > tests would have to be used to determine the bridges; now we can > associate the bridges with the waterway as well." > > is unclear (to me) as to why it's necessary. Why does an intersection > test need to be performed; doesn't the 'bridge=' tag define that the two > crossing ways are separate?
Let's say my application needs to answer, for whatever reason[1], the question "what ways are below this bridge". If there is no relation, then this is what the application would do: * gather at all the ways around the bridge * find ways that intersect the bridge way, ignoring the third dimension * check the layer on these ways -> all ways that intersect the bridge way (in 2D representation!) and have a lower layer are below the bridge. With a relation, these calculations would not be necessary. In my opinion, this is not a reason to use a bridge relation, though. (There /are/ reasons for using a bridge relation, but this is not one of them.) -- Tobias Knerr [1] most programs don't even need to know that. Routers can rely on the convention that there is no connection between ways without a common node, and 2D renderers only need the layer. A group of applications which actually need that information are advanced 3D renderers. _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
