[Tagging] SF Muni tram lines are layer=1?

Paul Johnson baloo at ursamundi.org
Mon Dec 17 15:29:23 GMT 2012

Well, I say this because I've yet to come across any renderer that doesn't
automatically assume layer=-1 for tunnel or layer=1 for bridge when it's

On Mon, Dec 17, 2012 at 9:24 AM, Martin Koppenhoefer <dieterdreist at gmail.com
> wrote:

> 2012/12/17 Paul Johnson <baloo at ursamundi.org>:
> > On Monday, December 17, 2012, Clay Smalley wrote:
> >>
> >> (Of course, some of these lines run through tunnels where they are
> tagged
> >> layer=-1, and on bridges where they are tagged layer=1 correctly. The
> layer
> >> tag on these bits of track would remain untouched.)
> >>
> >
> > Though this layer tag is unnecessary if there isn't more than one tunnel
> or
> > more than one bridge crossing a spot.
> From a data point of view layer-tags are always necessary when 2 or
> more objects cross each other at different levels. For bridges and
> tunnels it is good practise to always set a layer tag (and mostly
> there will be other objects crossing, because that's the reason why
> there is a bridge). In the standard mapnik style there is no need to
> set an explicit layer tag though, so for 2 crossing objects one of
> which a tunnel or a bridge the rendering will be correct also if the
> data isn't complete.
> cheers,
> Martin
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