[Tagging] layer=-1, rivers, bridges and tunnels

Frank Little frankosm at xs4all.nl
Sat Mar 15 16:39:12 UTC 2014

Fernando Trebien wrote:
> Alright. I see that "applying layer to long ways" is bad for several
> reasons. Surely this could be turned into a validation warning.
> But what's the difference between tagging the bridge with layer=1 and
> tagging the river underneath with layer=-1? Some people seem to think
> that both are necessary, many think it's best to use layer=1 on the
> bridge, I'm saying that layer=-1 on the river (let's say a short
> section, not the entire length) is "equivalent". Is it not equivalent?
> Is it wrong? If it is wrong, why is it wrong?
I don't think 'wrong' is the way to approach this; afaik, they are indeed 
There are four alternatives which mappers follow, none of which are 'wrong': 
tag the bridge segment, tag the water segment under the bridge, tag both, tag 

I've run waterway=stream or =canal (or =ditch, I think) through a few of the 
small rivers and streams here in the Netherlands. Roads need splitting to make 
bridges, so it makes sense to do all the relevant tagging on the road segment 
with the bridge tag when you are working on it.

I don't have any reason to split the waterway=*, so I just draw on without 
stopping. Since I put the name on the waterway and not on the riverbank, it 
leaves it to the renderer to find a good place to fit in the 
river/stream/canal name. In principle, that should mean a cleaner map if the 
renderer can work out the proper placement (difficult job, though).

So I would be against splitting the waterway at a bridge and tagging it 
layer=-1 on practical grounds. And you can be sure that it would cause 
confusion with other mappers who would imagine that you are trying to model an 
inverted siphon with the piece of waterway tagged layer=-1, or that you had 
simply made a mistake. (The 'level' confusion again.) 

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