On 09/03/2014 10:30, Richard Z. wrote:
Hi,
for some time now I have been working on the wiki page to state the rules
as clearly as possible.. hope that most of the improvements are fairly
uncontroversial. Some of the changes:
* the vertical ordering established by the layer values is valid exactly only
in the point where the ways cross or objects overlap
If you take that literally users will join rivers flowing under a bridge
with a node & add the layer tag to it, which is incorrect: those ways
should not join.
* define layer as higher value means above, lower value means bellow. Avoid
the complicated layer=0 definition as "the natural ground level as it
would be shown by contour lines on a topographic map". Explicit layer=0
seems to be deprecated now.
* layer on ways should be used only in combination with one of tunnel=*,
bridge=*,
highway=steps, highway=elevator, covered=* or indoor=yes. For areas, it could
be used in combination with tags such as man_made=bridge, building=* and
similar.
The motivation for this is to make it easy for validators to spot errors such
as when the wrong segment is accidentaly tagged, bridge/tunnel forgotten, or
someone tags excessively long ways for no good reason - common problem with
waterways and elevated roads/railroads.
I have validated this rule for ways in large parts of the world, there are
exceptions which currently I do not know hot to tag better but those are
rare.
This is not my understanding of the layer tag. It is a tool to help
renderers place objects on top of each other & has no real world
implication in differences of height.
For instance if you had an area tagged 'park' & another area within it
tagged 'lake' you could add a 'layer' tag to 'lake' to ensure the render
displayed it. Using multi-polygons is not the solution as it would take
the lake /outside/ of the park, so if the renderer didn't want to render
it's internal details (playground, wood buildings etc) it would end up
with more holes in it than Swiss cheese.
Dave F.
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