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

[fly]

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

fly