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

Richard Z. ricoz.osm at gmail.com
Mon Mar 24 19:45:32 UTC 2014

On Mon, Mar 24, 2014 at 11:02:35AM -0300, Fernando Trebien wrote:
> On Mon, Mar 24, 2014 at 10:27 AM, Friedrich Volkmann <bsd at volki.at> wrote:
> > On 14.03.2014 15:51, Fernando Trebien wrote:
> >> This is a small issue that came up recently in Brazil. In my
> >> understanding, the layer tag has no specific meaning other than to
> >> specify a rendering order.
> >
> > That is a common misconception by people who worked with graphics editing
> > software such as Photoshop and Corel Draw. In these applications, layers are
> > only used for ordering and grouping of objects. As opposed to that abstract
> > layer model, we use a pysical layer model in OSM where layer=0 means
> > ground-level, layer<0 means underground, and layer>0 means above ground-level.
> This is not written anywhere in the wiki:
> http://wiki.openstreetmap.org/wiki/Key:layer

to clear up this misunderstnding, this has been a relatively recent change 
by me. If you look at it the wording has changed but the practical application
has not.

Earlier there was a longstanding mention of layer=0 as ground-level
along with a complicated definition of what "ground-level" is supposed to be.

I have changed that mainly because the way "ground-level" was defined was
prone to interpretation problems and difficult to define better. 

The new formulation of the kind "higher value" means "higher above" does 
not need the explicit definition of ground-level.
Also the definition of layer=0 as ground-level confused too many people to 
tag elevated roads and similar with layer=1.

However, the new formulation is so that in practice layer=0 will always be 
"ground-level" except perhaps in very complicated urban areas:

Ways passing above other ways on a bridge will have a higher layer value, ways passing in tunnels bellow other ways will have lower (negative) values. All ways without an explicit value are assumed to have layer 0.

> Again, no mention in the wiki
> (http://wiki.openstreetmap.org/wiki/Key:layer) to negative layer
> values being used to represent the idea of underground.

not an explicit mention, but if there is an object X1 with implicit layer=0,
with no level or location tags, and another object X2 with a layer=-1 than
there are not too many possibilities where to find X2. It could be underground
or it could be under a large overhanging rock. Both should have explicit
tags to clarify the situation.

> > No, except for underground rivers. They do exist in karst regions...

we need a way to tag underground rivers and lakes. layer=-1 itself is not
sufficient, we need additional tags. Perhaps tunnel=cave but this would
only describe part of those phenomena.

I am compiling a list of landforms and natural objects that would be worthwile
to map.


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