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

A.Pirard.Papou A.Pirard.Papou at gmail.com
Tue Dec 18 06:31:26 GMT 2012


On 2012-12-17 22:16, Martin Koppenhoefer wrote :
> 2012/12/17 A.Pirard.Papou <A.Pirard.Papou at gmail.com 
> <mailto: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).
Altitude is not height over ground but above sea level.
I am of course speaking of relative altitude.
Position is an improper term as it applied to all directions.
We badly need precision.
>
>     . 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.
Agreed. And this is why I said that the tag should be called level.
Transforming that into layers is a renderer's matter that is strictly 
forbidden to speak about. Yet...
>
>     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?
Exactly as you say above.  They are the actual relative levels of these 
objects.
I have never seen a bridge sitting on a road (and hiding it, even just 
as a hint).
> Is there a page in the wiki which encourages this style?
No but their should according to what both of us say about levels.

Respectfully, I have only tagged streams that way because it doesn't 
hurt anything and it's superb.

When I don't agree with some way of tagging, I just don't tag.
It's well enough having been accused of badly tagging boundaries when I 
only continued tagging the same way they were being tagged and it's done 
all over the world I investigated.
Strange thanks.  I simply stopped tagging boundaries.

Cheers,

André.


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