[Tagging] Mapping cave tunnels passable by human

[Friedrich Volkmann]

On 14.08.2014 13:18, Dan S wrote:

>>> I think that it is an obvious idea, but wiki claimed that "At the moment
>>> there just a
>>> tag to map the entrance to a cave." despite fact that existing tags fit well.
>>
>> No, they do not fit. Caves are complex three-dimenional structures. In most
>> caves there are no paths. You go or climb or rope down whereever you feel like.
> 
> This is the same as with a pedestrian square - there's no specific
> route in the square and you go wherever you feel. However it's useful
> to make them part of the OSM database, both for showing their
> existence and to help with various routing applications.

Pedestrian squares are 2-dimensional. Caves are 3-dimensional. Many cave
rooms overlap themselves a couple of times in the z-axis.

Forget routing in caves. There's no GPS. And those who get lost without
routing apps will get lost in a cave anyway.

> I'm afraid layer=-1 does not express that a feature is underground. It
> expresses that a feature is lower than all features at layer=0+, but
> there's no guaranteed relationship with ground level.

In central Europe it is, but habits may vary around the word. Much chaos
these days...

In my opinion, there is some misconception by people who are used to image
editing software such as Photoshop, Adobe illustrator, Gimp, Corel Draw,
Inkscape, etc., as well as CAD software. In all of these applications,
layers stand for rendering order. In OSM we need to think in physical layers.

Caves are just an example. There are many more underground objects which are
not tunnels. E.g. I used to go to school over a landfill for 8 years without
knowing, because it was covered with soil and grass. The only way for
renderers to know is by eveluating the layer tag. Of course you could set
some additional tag like underground=yes, but having two concurrent tags for
the same thing is just a mess. You'll soon get a lot of inconsistencies.

> There are quite
> a few objects with the implicit layer=0 but which are not at ground
> level (e.g. tunnel=culvert items: <http://overpass-turbo.eu/s/4zE>).

Therefore we need to tag them all with layer<0. There was a proposal for
implicit default layer=1 for bridges and -1 for tunnels, but unfortunately
it was voted down, so we are damned to set it manually every time.

-- 
Friedrich K. Volkmann       http://www.volki.at/
Adr.: Davidgasse 76-80/14/10, 1100 Wien, Austria