[Tagging] Tunnels and bridges

Steve Bennett stevagewp at gmail.com
Sat Feb 2 05:33:43 GMT 2013

On Fri, Feb 1, 2013 at 11:21 PM, Pieren <pieren3 at gmail.com> wrote:
> By "consumer", we all think about "renderer" (which is in my knowledge
> the only consumer looking for bridges in OSM atm). If you keep the
> "bridge" tag on the multiple highways, it is duplicating the
> information. And you don't fix the rendering issue because Mapnik will
> continue to draw one bridge per highway.

I think any renderer that supports bridge relations will be smart
enough to not also draw bridges on the highways. That is, if you have:


and a relation

and a bridge structure:

Then any renderer that supports the relation and man_made=bridge will
also not draw a bridge casing on the highway. (This is another example
of why relations are much easier to process than using geospatial


> To avoid the duplicate rendering, we need the "bridge" tag only on the
> polygon. But then, the rendering software will have to decide to draw
> the polygon itself. Or like today, draw some symbols along the line.
> In both cases, if you want a correct rendering (draw only one bridge)
> wihtout simply drawing the bridge polygon, the software will need some
> spatial requests anyway (to determin the group of highway segments
> that belong to the same bridge).
> Pieren
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