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.
Colin
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<da...@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
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