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

Richard Z. ricoz.osm at gmail.com
Sat Apr 5 19:17:52 UTC 2014

On Fri, Apr 04, 2014 at 09:41:56PM +0200, André Pirard wrote:
> Hi,
> Regarding normalized layers.
> If I can believe my eyes, bridges/culverts are under (uninterrupted
> foil) roads
> <http://www.hdtimelapse.net/content/HDtimelapse.net_City/HDtimelapse.net_City_3290_hirez.jpg>: 
> bridge=road-1.
> Unless a renderer must assume that, which should be stated in the wiki,
> then bridge=road.
> If I can believe my eyes, 90° crossing rivers/roads run under bridges,
> that is river=bridge-1.
> Unless a renderer must assume that, which should be stated in the wiki,
> then river=bridge.
> A friend of mine says that we must start counting at 1 (like year 1) and
> hence that ground=1 (;-)).
> I'd like to know how many of you agree.  While waiting, let's assume
> ground=0.
> Hence, it depends very much on what the wiki states and we should settle
> that before discussing.
> Assuming assumption, we have road=0, bridge=0, river=0.
> If the renderer does not assume anything we have road=0, bridge=-1 and
> river=-2.

we don't have that. Perhaps you are concluding "uninterrupted level road" 
=> same layer all along the road?
There is no rule like this. You can split a completely level way into many 
parts and assign any of these levels a layer from -5 to 5 (you *should* use
layer<>0 only for segments which are bridge/tunnel etc but elevation is not

In addition, "key:layer" *is not* rendering layer/order.

One example, a road is going through a forest, both should have implicit 
"key:layer" ==0. 
Obviously they still have a defined rendering order, otherwise our roads 
would disappear where they go through forests.

If the road (for whichever reason, valid or not) has layer=-1 and the forest 
just the implicit layer==0, the road should still be drawn above the forest.


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