[Tagging] SF Muni tram lines are layer=1? and general tagging levels considerations

[Martin Koppenhoefer]

2012/12/17 A.Pirard.Papou <A.Pirard.Papou at gmail.com>

>  A level is an altitude.  A layer is a drawing opacity.  Although OSM
> does not tag for the renderer, it uses the tag *layer=**. It defines *
> layer* as the relative "position" (is that "altitude"?)
>


no, it is not altitude (height over ground), it is the relative position
(relative to other objects at the same spot).



> . In fact, the only effect of assigning a layer is that upper layer
> objects hide lower layer ones (it's not a "mind your step" warning ;-))
>


it is a way to describe in the database which object is above which or
whether they are at the same level.



> It's interesting to keep all the rails in the same layer to avoid splits
> and layer =+1 may be needed for them to show at some places. My reaction
> would be that the person having cared to explicitly set the level might
> have had something on his mind.
>


The best way to know what he had in mind would be to write to this person
and ask. As tram rails are (usually) on the same layer/level as the road
itself I'd consider it wrong to have them on different layers.


I have traced lengths of streams
>
>    - stream as a constant layer=-2 way, uninterrupted end to end (even if
>    they "don't look so deep"),
>     - roads are at level 0
>     - and bridges and culverts at level -1, in the manner mentioned above.
>
>
very strange way of mapping IMHO, how did you come to this idea? Is there a
page in the wiki which encourages this style?

cheers,
Martin
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