[OSM-talk] Indiscrimate layering

"Marc Schütz" schuetzm at gmx.net
Sat Dec 20 20:32:50 GMT 2008

> I also tag many waterways around here as layer=-1, because that's what
> they
> are... thank god they aren't layer=0 or the place would be under water!

I know that many people assume that layer=0 means ground level, but this assumption is unnecessary. Indeed, the wiki (AFAICS) doesn't state it either, merely that data consumers should assume layer=0 if no layer tag is present.

This of course also means it's perfectly okay to tag a river with layer=-1, so as to not require a layer tag on bridges.

Regarding the claim about layer on bridges being tagging for the renderer:

I disagree. The layer tag is a "workaround" for deficiencies in our data model (which only provides for two dimensions), the same way as area=yes is a workaround for there not being a polygon primitive. And being redundant (i.e. derivable from another tag, which it unquestionably is in many cases) does not make it tagging for the renderer. Contrarily it keeps the information of vertical order in one tag instead of spreading it over several (layer, bridge, tunnel, power=line etc.) And noone would call highway=track tagging for the renderer because it is redundant if there already is a tracktype=grade2.

Regards, Marc

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