[Tagging] Bridges and layers

Colin Smale colin.smale at xs4all.nl
Mon Jul 26 14:22:28 BST 2010

  Shouldn't the layer_change be on the common point, not a way? A way 
(usually) has two ends, so putting the tag on a way will not indicate at 
which end of the way the layer change takes place. But then it 
degenerates to two (or more) connected ways with a different layer=* 
value, so the layer change can also easily be inferred without 
introducing a new tag.

Am I right in assuming that "bridge=yes" refers more to the construction 
(with parapets etc) to determine the rendering style, whereas "layer=*" 
is more a hint to the renderer for handling the case where unconnected 
objects overlap? This is sounding a bit like "tagging for the renderer" 
which is AFAIK officially Frowned Upon. Maybe the real problem is the 
fact that mapnik is not layer-aware.


On 26/07/2010 11:58, Richard Mann wrote:
> One of the things Dave Earl mentioned in his talk about rendering was
> the gaps-in-casings you sometimes get at bridges.
> What I've ended up doing is
> 1) rendering casings in layer (not underneath everything like Mapnik)
> 2) putting in a flag to identify ways that join ways of different
> layer, with a view to suppressing end caps
> layer_change=yes (suppress end caps on this way; taken to be implied
> by bridge=yes; use on ramps down to layer=-1 ways)
> layer_change=no (enforce end caps on this way; use on bridges that
> have ramps separately tagged)
> The traditional layer=0 / layer=1+bridge=yes / layer=0 tagging is
> unaltered (the renderer sees the bridge=yes and suppresses end caps).
> The main place you end up changing the tags is for grade-separated
> junctions, for example:
> http://www.openstreetmap.org/?lat=51.7557415366173&lon=-1.29630088806152&zoom=18
> Here the underpass gets separated into three sections, with
> layer=0+layer_change=yes ways bracketing a layer=-1 way. The renderer
> draws the end caps on the layer=-1 way, but suppresses them on the
> layer_change ways. Also notice that the roundabout is all at layer=0,
> including the bridges. You can't make the bridges longer at lower
> zooms, but at least the layer=0 casings stay on top as the underpass
> gets wider.
> Good idea, or just a local fix?
> Richard
> On Sun, Jul 25, 2010 at 7:56 PM, David Earl<david at frankieandshadow.com>  wrote:
>> My two talks for State of the Map
>>   * Tag Central - a schema for OpenStreetMap
>>   * What I learned making a real map on real paper for real people and real
>> money
>> are now available online at http://www.frankieandshadow.com/sotm10/
>> David
>> _______________________________________________
>> talk mailing list
>> talk at openstreetmap.org
>> http://lists.openstreetmap.org/listinfo/talk
> _______________________________________________
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> Tagging at openstreetmap.org
> http://lists.openstreetmap.org/listinfo/tagging

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