[OSM-talk] When is a bridge not a bridge?

sergio sevillano sergiosevillano.mail at gmail.com
Fri Jan 23 20:40:59 GMT 2009

Marc Schütz escribió:
>>> if the way is "layer=0" and the bridge is "layer=1" there is a ramp.

>> A bridge crossing any other way should be always tagged with layer=1 (or
>> higher). In fact, the renderers could easily take "layer=1" as a default
>> for bridges, as they are usually on top of something.

>>> if the way is "layer=0" and the bridge is "layer=0" too
>>> (the crossing way under is "layer=-1", digged)
>>> then the bridge has no ramps.
>> Something with negative layer has to be a tunnel.
> I think you're both overinterpreting the layer tag. As the concerning ways do not cross each other, their relative layers don't matter. And I don't see a reason either why a negative layer should imply a tunnel.

>> Bridges are useless if
>> there is a tunnel. Do not tag bridges as "layer=0" just because there
>> are no ramps. The layer tag is to help a renderer put the ways in the
>> right visual order - not to indicate whether or not a way is elevated or
>> not. Do not tag ways as "layer=-1" just because they are below the
>> general earth surface (such as rivers).
> This may be a good idea, because it keeps the effects of the layer constrained to the short bridge instead of the long river, thus helping to avoid mistakes. But I would not see it as wrong.
> Because there are widely varying interpretations of layer, I wrote down how I understand it:
> http://wiki.openstreetmap.org/wiki/Talk:Key:layer
> Comments are welcome.
> Regards, Marc
i did not meant to tag ramps with layers
just that normal layer usage, gives a hint if there is a ramp
i dont see important to tag ramps but if someone wants to do so
is better to tag the ramp alone "bridge_ramp" instead of the 80n proposal
of tagging all parts as bridge and then each part.

as for the layer issue is something to discuss in an thread of its own 
or the layer talk page.
but my opinion is that layers are only a convention to distribute what 
is over what
so it relates to each other, thus the number is not that important as 
long as they are in the correct order.
also i consider layer=0 or the absence of that tag (the default) as 
ground level.

i dont agree with "take "layer=1" as a default".

for me a bridge that has layer=1 over a way without layer means that the 
undenneath way is at ground level and the way avobe rises to layer=1 to 
pass over the other way.
a bridge with no layer tag passes over a way that should be tagged 
layer=-1 this means that that last way is digged or has ramps to go to 
level=-1 but is not a tunnel, if that second way is a tunnel layer=-1 
then of course there is no bridge.

so layer=-1 does not imply tunnel.

just my oppinion


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