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

[Richard Z.]

On Mon, Mar 24, 2014 at 10:38:08PM +0100, fly wrote:
> On 24.03.2014 20:45, Richard Z. wrote:
> > 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:
> 
> As it might be even hard to define the ground level (we just have a
> discussion on talk-de@ about houses built on slops), I would never say
> that an negative layer value is an indicator/synonym for underground.
> 
> >> 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.
> 
> How about location=underground ?

seems good to me.

> covered=yes is another useful tag (eg your overhanging rock)

"covered" is already complicated enough and this might be too much of
stretching the original idea.

Richard