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

Fernando Trebien fernando.trebien at gmail.com
Sat Mar 15 18:19:36 UTC 2014

Here are a few arguable reasons to split the waterway and tag it with layer=-1:
1. Bridges may come in pairs for dual carriageways. In this case, it's
a single layer tag for the waterway versus 2 layer tags for the
bridges. This may happen many times in a row. In this case, it makes
sense to split the waterway at 1 point (dividing into "urban" and "not
urban" parts) and tag the whole urban part with layer=-1. That's the
case in my hometown (54 tags, one for every bridge vs 1 tag only + 1
split waterway), see here towards the East:
2. If you split only near the bridges, the name of the waterway will
be rendered between the bridges, which is the optimal position. (But
this "could" be considered mapping for the renderer.)

Situation 1 happens in many other cities across the world, and if you
tag the bridge as layer=1, you may end up inverting the rendering
order of highways, leading to this:

"(The 'level' confusion again.)"

If this is a common mistake, let's write the distinction at the very
top of the respective article in the wiki.

On Sat, Mar 15, 2014 at 1:39 PM, Frank Little <frankosm at xs4all.nl> wrote:
> 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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Fernando Trebien
+55 (51) 9962-5409

"The speed of computer chips doubles every 18 months." (Moore's law)
"The speed of software halves every 18 months." (Gates' law)

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