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

Sven Rautenberg sven at rtbg.de
Fri Jan 23 12:30:08 GMT 2009

Hi there!

> im with Mattias here.
> if there is air under the way i tag it as bridge, leaving the ramps out.

That's what I would do, too.

> the layer of the way gives you the hint of the existance or not of ramps.

That's not what I'd suggest for the usage of the layer tag!

> 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. 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).

The effects of the layer tag should be kept as local as possible.

> so if we want to give more detail as to describe the ramp,
> i would just tag the ramp as "bridge_ramp=yes"
> and the airborn part of the way leave it as "bridge=yes"

I doubt such details are very useful, and personally I wouldn't tag them.


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