[Tagging] Tunnels and bridges

Martin Koppenhoefer dieterdreist at gmail.com
Fri Feb 1 12:30:44 GMT 2013

2013/2/1 Pieren <pieren3 at gmail.com>:
> If you keep the
> "bridge" tag on the multiple highways, it is duplicating the
> information.

yes, this is fundamental if you want to keep current renderers working
(it also ensures that filtering e.g. just the highway-network will
still retain the bridge information).

> And you don't fix the rendering issue because Mapnik will
> continue to draw one bridge per highway.

this will entirely depend on your rendering style, it is not a mapnik issue.

> 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.

also, you would have to do more complex analysis this way because
areas are commonly rendered with their real width while highways are
rendered with augmented width (the lower the zoom level the more). So
for a bridge in (e.g.) zoom 15 you would have to take care that the
bridge outline isn't hidden underneath the rendered road.

> 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).

+1, yes, if you want to evaluate the bridge polygons you will have to
do spatial analysis, but if you don't want to you can simply continue
like you did (noone would be forced to do that analysis)


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