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

Martin Koppenhoefer dieterdreist at gmail.com
Mon Dec 17 15:24:41 GMT 2012

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.


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