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

André Pirard A.Pirard.Papou at gmail.com
Tue Apr 22 08:58:35 UTC 2014

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.

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.

Second, because, even if physical like the often debated case of a
forest, where there is a road there is no forest, no plants, it is
interrupted, it does not hide anything and there is no question of
levels where something does not exist.  The same applies for river beds,
railway lines and buildings. In the eyes of a renderer, they are
exclusive with a forest and other "something else".

A different case is a railway line in a tunnel, which is effectively
under ground. In that case, the line must be dotted (in hope that the
source of the data used the same idea and you know where it is.  Never
tag source=GPS in a tunnel ;-))
Don't laugh:  
https://taginfo.openstreetmap.org/tags/source=GPS#combinations  and
combine with "tunnel" ;-)

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).



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