Hi! I have amended the bridge and tunnel pages with the reason for those rules: Rendering breaks if you have different layers on junction nodes. But whats more important, in real life bridges don't start in the *middle* of junctions so there *is* a little bit of non-bridge roads between the junction and the bridge.
For very tight features this is a problem, I agree. But I haven't seen a good solution for this yet. The rules on the wiki work in most cases and give good guidance to newbies. But they should never be read as gospel, so if you have a solution that works better for your case, just use it. For canals in Dutch cities, I'd consider not tagging the roads special but giving the canals a layer=-1 and maybe use tunnels where they cross under the roads. That might be a bit of a hack, but sometimes its easier to tag something as a tunnel even if it might technically not be one. Jochen On Wed, Aug 12, 2009 at 10:27:17AM +0200, Lambert Carsten wrote: > From: Lambert Carsten <lhc....@solcon.nl> > To: talk@openstreetmap.org > Subject: Re: [OSM-talk] Layer transitions > Date: Wed, 12 Aug 2009 10:27:17 +0200 > Cc: Jochen Topf <joc...@remote.org> > > Hi Jochen, > > Could you please comment on this thread: > http://lists.openstreetmap.org/pipermail/talk/2009-July/039217.html > http://lists.openstreetmap.org/pipermail/talk/2009-July/039259.html > http://lists.openstreetmap.org/pipermail/talk/2009-August/039847.html > > since you created the 'T-junction' rule in the first place back in feb 2007: > > http://wiki.openstreetmap.org/index.php?title=Key:tunnel&diff=next&oldid=23441 > > As you are the co-author on a book on tagging I feel I should ask your input > on this :). > > On Tuesday 11 August 2009 15:31:24 Lambert Carsten wrote: > > On Saturday 08 August 2009 14:11:21 Marc Schütz wrote: > > > > > no, it's not, it's about relative order in the db. > > > > > > > > Fair enough. In other words, at any node which is a junction of way > > > > segments with different layers (whether the segments are part of the > > > > same way or different ways), the physical implication is that the > > > > slope of the way changes in the close vicinity of that node. > > > > > > Not even that: it only says something about the relative ordering, not > > > about slope. > > > > > > Regards, Marc > > > > Here is just one of the many examples in Amsterdam concerning bridges: > > > > http://maps.google.com/maps?f=q&source=s_q&hl=de&geocode=&q=bayreuth&sll=37 > >.0625,-95.677068&sspn=59.467068,107.138672&ie=UTF8&t=k&ll=52.371356,4.897274 > >&spn=0.001132,0.003447&z=19 > > > > All the connecting roads are more or less on the same physical 'layer'. The > > roads that do not intersect with anything on another level don't have a > > layer tag and are assumed layer=0. (A layer tag only has meaning relative > > to something that is intersected). > > Clearly the bridge starts at the (in this case 'X') junction. All the > > connecting roads are (implied) layer=0. > > The T-junction rule makes it necessary to add a non existing 'segment' OR > > add three (for this bridge actually 6) layer=1 tags. Of course the other > > end of those ways will connect to other roads. So the T-junction eastwards > > will also need layer=1 tags which in turn will force the X intersection > > northwards to have layer=1 tags. The result is of course that most or all > > of the roads will need a layer tag. And where we finally connect Amsterdam > > to the rest of Holland we have to hope that there are enough I connections > > so we don't endup forcing the whole world to sit on layer=1. > > The 'segment adding solution' does not only not represent the reality. > > Those extra bits have hardly any room because it has to fit between the > > junction and the 'natural=water' to work. > > > > All the layer tag does is make it clear that a crossing of ways do NOT > > connect/intersect and the numbering helps the renderer to make a decent > > representation on a 2D drawing. It is not about slope or height and > > shouldn't be. > > > > >I think we should insert a "in most cases" into this rule. > > > > What is the point of this? > > > > O.K. if I delete the 'offending' :) bits in the wiki? > > http://wiki.openstreetmap.org/wiki/Key:tunnel#How_to_Map > > http://wiki.openstreetmap.org/wiki/Key:bridge#How_to_Map > Also here (since july 2009): > http://wiki.openstreetmap.org/wiki/Editing_Standards_and_Conventions#Bridges > > Lambert Carsten > > > -- Jochen Topf joc...@remote.org http://www.remote.org/jochen/ +49-721-388298 _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk
