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

"Marc Schütz" schuetzm at gmx.net
Fri Jan 23 13:51:47 GMT 2009

> > 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:

Comments are welcome.

Regards, Marc

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