[OSM-talk] Key:layer update

[Richard Z.]

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

* 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.

* in some cases "level" may be more appropriate than "layer"

https://wiki.openstreetmap.org/w/index.php?title=Key%3Alayer&diff=999107&oldid=935491


Richard