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

Richard Z. ricoz.osm at gmail.com
Tue Apr 22 14:55:18 UTC 2014

On Tue, Apr 22, 2014 at 10:58:35AM +0200, André Pirard wrote:
> On 2014-04-21 22:20, Martin Koppenhoefer wrote :
> >
> > 2014-04-21 20:48 GMT+02:00 Richard Z. <ricoz.osm at gmail.com
> > <mailto:ricoz.osm at gmail.com>>:
> >
> >     > Without any additional tags like "tunnel=*" or "covered=*", a
> >     > "layer=-1" river shouldn't be rendered differently than a
> >     "layer=1" or
> >     > even in the absence of any "layer" tag. This is a bug in OsmAnd. You
> >
> >     except for the the very frequent case when the river with a layer=-1
> >     goes through a landuse=* area with a layer=0.
> >
> >
> >
> > +1, as soon as there is any other object on a different layer, be it
> > landuse, a place area or something else, with the lower layer tag you
> > are excluding the river from this feature and putting it below.
> -2
> First because generally a landuse (think of residential) is not a
> physical object but a boundary.  Just like highlighting a municipality
> area by coloring it does not hide what is inside it, coloring a landuse
> must not either.

take "natural" instead of "landuse". How many more exceptions do you
want to make? Is a natural=stone not physical enough for you?

What good are those exceptions anyway?

There is no reason and no justification to tag anything wiht "layer" unless 
it is a bridge, tunnel or similar. So why complicate things?

> The Osmand's (or its renderer's) bug looks much like this.
> To say it more precisely than "it looks bad", It uses dotted lines for
> -1, -2 and probably below. There is no reason why.
> It should be corrected and not be worked around by changing all levels
> all over OSM (and discovering that doing so raises another bug in Osmxor).

Correcting bugs like this in OSM data will raise many hundreds of warnings 
because someone deliberately tagged the river with layer=-1 to obscure all 
those warnings in the first place.

In 99% of cases that I have seen every river randomly tagged with layer=-1
obscures some other 10-100 errors which become visible as soon as you remove 
the improper layer=-1 tag.

Those bugs will remain lurking forever if nobody cares about applying correct
layer tags. The validators do not appear to be capable doing any kind of checking
as long as layer(A)!=layer(B) 


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