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

[Fernando Trebien]

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

> Renderers typically use a different rendering style for underground
> features, e.g. dashed  or greyed-out lines, or these features may even be
> omitted. The only way how renderers can determine whether a feature is
> underground is by looking at the layer tag. tunnel=* and bridge=* do not
> necessarily mean that a feature is underground / above ground. There was
> even a negative voting on this (a proposal for implying layer=-1 or 1
> respectively). And the other way round, not all underground objects are
> tunnels or culverts. A tunnel is a way with an entrance on each end. It is
> not a tunnel if it has a dead end. And what about POIs and areas? It would
> be stupid to tag them as tunnels. The tunnel=* and bridge=* tags do help the
> renderers when it comes to the curved bridge and tunnel signatures, but the
> dashing can only depend on the layer tag.

The concept of "underground" seems more closely related to the tags
level and location:
http://wiki.openstreetmap.org/wiki/Key:level
http://wiki.openstreetmap.org/wiki/Key:location

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.

>> The wiki, however, states that it is wrong
>> to tag a whole river with layer=-1. The reason for that, as far as I
>> could figure, is because current validators (such as JOSM's or
>> KeepRight's) will not issue a warning on a waterway x highway crossing
>> when their layers are different, leading some users into tagging the
>> river with layer=-1 in order to get rid of warnings about missing
>> bridges and tunnels.
>
> The warning is ok, but the big problem with validators is that mappers mess
> around with the data without local knowledge, just to shut up the validator.
> They connect ways which are not really connected, they insert bridges and
> culverts that do not really exist, and so on. Among these so-called "fixes",
> the layer=-1 on waterways are relatively harmless, because it is so obvious
> that they are wrong. A culvert that has been made up by a sofa mapper is
> much more difficult to correct, because you only know when you go there.

I understand this issue, but don't you think validators could be a
little bit more clever, so that users couldn't trick them? In some
previous message I described some very simple logic that would make
these tricks (in this situation) impossible to work around, and this
would discourage people from using layer=-1 on the rivers.

>> Do you agree that the river can be tagged with layer=-1 as long as
>> this value is correct in relation to the layer of other
>> nearby/crossing ways?
>
> No, except for underground rivers. They do exist in karst regions...
> --
> Friedrich K. Volkmann       http://www.volki.at/
> Adr.: Davidgasse 76-80/14/10, 1100 Wien, Austria
>
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-- 
Fernando Trebien
+55 (51) 9962-5409

"The speed of computer chips doubles every 18 months." (Moore's law)
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