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

Frank Little frankosm at xs4all.nl
Sat Mar 15 15:53:57 UTC 2014

Martin Koppenhoefer wrote:
>> Am 14/mar/2014 um 15:51 schrieb Fernando Trebien 
>> <fernando.trebien at gmail.com>:
>> Do you agree that the river can be tagged with layer=-1 as long as
>> this value is correct in relation to the layer of other
>> nearby/crossing ways?
> I would discourage you to do so. Layer tags should only be applied to ways 
> that actually cross other objects on different layers (ie without 
> intersecting them).
I agree totally with: "Layer tags should only be applied to ways that actually 
cross other objects....."

At its simplest, a layer tag is a hint to a renderer which of  two crossing 
ways should be rendered later (i.e. on top). If a renderer does not apply the 
real world knowledge that a bridge (by its definition) crosses over a way 
(road, water, whatever) underneath, then it can still take the hint to render 
it correctly. The renderers have no problem interpreting the situation 
correctly, with or without the layer tag, afaik.

A layer tag is not a way to define the relative height of different objects. 
Some of the discussion on the proposal's talk page is confused about that.

I would tag the structure (bridge or tunnel) with a layer tag*.
I would not tag a river or stream along its entire length.

Rivers, streams, canals, etc. are surface features (in most cases). The mere 
fact that the bed of a waterway is often  at a lower level than the 
surrounding ground level is not relevant for the layer tag since hinting for 
correct rendering is not necessary. (In the Netherlands and other polder 
areas, waterways are often above the surrounding area.)

*Actually, as I made clear on talk when we had this discussion very recently, 
I would prefer not to use the layer tag at all in most of these cases. The 
fact that somewhere between one quarter (taginfo) and one third (overpass 
turbo samples in the Netherlands) do not use a layer tag with bridges 
indicates to me that it is not as clear cut as people are suggesting. (Note: I 
realise that there are specific cases where explicit tagging for layer hinting 
is necessary (e.g. bridges or viaducts layered vertically). These are 
relatively rare.) 

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