[Tagging] Bridges and layers

[Dave F.]

  On 26/07/2010 10:58, 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

I'm obviously a bit slow on the uptake - could you explain what you mean 
by 'gap-in-casing' & 'suppress end caps' please?
I see nothing wrong with the example you give


On my initial understanding I see layer_change as irrelevant.

Layer is used purely as an aid to the renderer to distinguish where two 
or more ways *cross* each other:

http://wiki.openstreetmap.org/wiki/Layer


Cheers
Dave F.