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

Richard Z. ricoz.osm at gmail.com
Tue Apr 22 21:05:13 UTC 2014

On Tue, Apr 22, 2014 at 10:54:37PM +0300, Ilpo Järvinen wrote:
> On Tue, 22 Apr 2014, Richard Z. wrote:
> > > Layer tag is a *hint* to the renderer, nothing more.
> > 
> > the wiki page says 
> > <<
> > The layer=* tag is one of several methods used to describe vertical 
> > relationships between crossing or overlapping features.
> > >>
> > 
> > Not a single word of a "hint to the renderer" anywhere in the article.
> What would be the other "methods"?

>From wiki key:layer again:
Several other tags can establish or modify vertical relationships explicitly or 
by builtin assumptions. Some of those are level=*, location=*, covered=*, highway=steps, 
aerialway=*, power=line, waterway=dam

> > Now what is that key:layer anyway? Given that nobody knows or cares how it 
> > works does it do more good than harm? Do we need it at all?
> > Or should we make a fresh start with something well defined and deprecate
> > this mess?
> I personally think that only thing that would need to be added/defined, is 
> sensible defaults for bridges/tunnels (less weight than layer=*) and get 
> "validators" updated. For some yet to be explained reasons, some people 
> spread FUD about that being impossible but it's clearly incorrect claim
> if the implicit default is correctly defined.

that might actually work, but what is the advantage over a JOSM preset
with a layer=1 as default?

> The (supposed) problem with layer=-1 on rivers this thread is about would 
> be magically solved and all those could be eventually be removed (not in a 
> mass-removal but more carefully).

the problem has more causes. But mainly people got used to use the layer=-1 as
a cheap trick to hide all kinds of pesky keepright/JOSM warnings about missing 
bridges, incorrect crossings, waterway crossing waterway and a few more.

The problem is very rarely bridges with missing/incorrect layer but most of the
time the bridges are missing altogether.
The problem would go away if people would add those bridges but this is not possible 
unless you know for sure that those are actually bridges - and of course it is 
easier to add layer=-1 to a single river than adding several hundreds of bridges.
The problem would also go away if validators would detect this abuse.


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