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

Fernando Trebien fernando.trebien at gmail.com
Sat Mar 15 16:13:41 UTC 2014

Alright. I see that "applying layer to long ways" is bad for several
reasons. Surely this could be turned into a validation warning.

But what's the difference between tagging the bridge with layer=1 and
tagging the river underneath with layer=-1? Some people seem to think
that both are necessary, many think it's best to use layer=1 on the
bridge, I'm saying that layer=-1 on the river (let's say a short
section, not the entire length) is "equivalent". Is it not equivalent?
Is it wrong? If it is wrong, why is it wrong?

On Sat, Mar 15, 2014 at 12:53 PM, Frank Little <frankosm at xs4all.nl> wrote:
> 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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Fernando Trebien
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