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 equivalent. 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 neither.

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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