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

Chris Hill osm at raggedred.net
Mon Apr 21 20:53:21 UTC 2014

On 21/04/14 21:20, Martin Koppenhoefer wrote:
> 2014-04-21 20:48 GMT+02:00 Richard Z. <ricoz.osm at gmail.com 
> <mailto:ricoz.osm at gmail.com>>:
>     > Without any additional tags like "tunnel=*" or "covered=*", a
>     > "layer=-1" river shouldn't be rendered differently than a
>     "layer=1" or
>     > even in the absence of any "layer" tag. This is a bug in OsmAnd. You
>     except for the the very frequent case when the river with a layer=-1
>     goes through a landuse=* area with a layer=0.
> +1, as soon as there is any other object on a different layer, be it 
> landuse, a place area or something else, with the lower layer tag you 
> are excluding the river from this feature and putting it below.

Layer is a hint to the renderer. The item with the lowest layer value is 
drawn first, then the next highest and so on. In the example of where a 
road crosses a railway on a bridge, the railway (often default layer=0) 
is drawn, then the road (often layer=1) is drawn,casing first then inner 
and any part that crosses the railway obscures that part of the railway.

What you are saying is that the river (layer=-1) is drawn then the 
landuse (layer=0) is drawn over it, obscuring the river. This is clearly 
not intended and not what actually happens in Mapnik renders, which is 
much smarter than that.

Layer tag is a *hint* to the renderer, nothing more. In the case of a 
river (or road) passing through a landuse area the renderer doesn't need 
a hint and to say the river is somehow below the landuse does not make 
sense. The landuse is an arbitary, geopolitical invention, which we find 
useful and does not actually exist, therefore it cannot be above or 
below a river, nor is it an object that somehow needs separating from a 
river. Indeed the river can be part of a landuse area, such as an 
industrial wharf in the same way as a road can be part of a residential 

Cheers, Chris
user: chillly

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