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

Fernando Trebien fernando.trebien at gmail.com
Fri Mar 14 15:35:53 UTC 2014


"Layer= tag clearly (logically) implies that some data is above or
below some other data. At least to my logic."

>From this logic, layer=-1 means the object is >rendered< beneath
anything that has layer=0 (or, conversely, that anything with layer=0
is rendered on top of anything with layer=-1). It does not mean that
it >is< in fact below it (though it almost always is). That's the
situation of a river passing under a bridge. Likewise, layer=0 means
the object is rendered below anything that has layer=1. Also the same
situation for the situation of a bridge over a river (or of a river
under a bridge). Both approaches should be possible.

"At least OsmAnd renders all waterways with layer=-1 with dashed
casing, as if they were underground, which to me makes sense"

This makes absolutely no sense to me. OsmAnd has confused the concept
of "rendering layer order" with the concept of "level" (underground,
on the ground, above the ground). It may also have confused it with
the concept of a "tunnel", which is rendered using dashed casing in
OSM-Carto (the default map style of the main website).

"I don't understand why anyone would do this. That's it. Why?"

Reason: to avoid having to add a layer tag to every bridge. If a
single river is crossed by 100 bridges, it is easier to add a layer=-1
tag to the river than to add a layer=1 to each of the 100 bridges.

Another reason: to work around the limitations of validators in order
to avoid warnings about "missing bridges". But IMHO, this is the wrong
way to handle the situation, since a workaround is easy AND is
exploited in practice. See here:
http://wiki.openstreetmap.org/wiki/Talk:Key:layer#layer.3D-1_erroneously_used_for_many_italian_rivers

"So, again : why tag things with with layer= tag when they are
essentially on ground level = not above or below other data (or at
least natural objects that might be mapped one day *) ? Am I missing
something here?"

I believe you are missing this: layer=0 does not mean "ground level".
The closest to that idea is level=0, even though it is not always
"ground level" but nearly so almost always. Layer=0 means: "render
this to layer 0". Layer=-1 means: "render this to layer -1", which is
rendered before layer 0, therefore, anything on layer 0 is drawn on
top of things on layer -1.

On Fri, Mar 14, 2014 at 12:15 PM, Jaakko Helleranta.com
<jaakko at helleranta.com> wrote:
> Re; tagging a (complete or longer segment of a) river with layer=-1
>
> I don't understand why anyone would do this. That's it. Why?
>
> Layer= tag clearly (logically) implies that some data is above or below some
> other data. At least to my logic. And I don't seem to be the only one with
> this thinking. At least OsmAnd renders all waterways with layer=-1 with
> dashed casing, as if they were underground, which to me makes sense, but
> what makes all rivers with this unnecessary tag render oddly. There are
> quite a number of such problematic waterways (also) in Nicaragua and Haiti.
>
> So, again : why tag things with with layer= tag when they are essentially on
> ground level = not above or below other data (or at least natural objects
> that might be mapped one day *) ? Am I missing something here?
>
> Cheers,
> -Jaakko
>
> *) I do add layer=1 to all bridges I map even if there wouldn't be any data
> under then at the moment and think that this kind of layer tag use makes
> sense
>
> --
> Sent from my Android device. * +505-8845-3391 * http://about.me/jaakkoh
>
> El mar 14, 2014 8:52 a.m., "Fernando Trebien" <fernando.trebien at gmail.com>
> escribió:
>>
>> Hello everyone,
>>
>> This is a small issue that came up recently in Brazil. In my
>> understanding, the layer tag has no specific meaning other than to
>> specify a rendering order. The wiki, however, states that it is wrong
>> to tag a whole river with layer=-1. The reason for that, as far as I
>> could figure, is because current validators (such as JOSM's or
>> KeepRight's) will not issue a warning on a waterway x highway crossing
>> when their layers are different, leading some users into tagging the
>> river with layer=-1 in order to get rid of warnings about missing
>> bridges and tunnels. So, I think that the validation rule is
>> inadequate: it should warn when a waterway crosses a highway that has
>> no bridge or tunnel tag, regardless of the value in the layer tag.
>> (Fords are the only exception, AFAIK.)
>>
>> 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?
>>
>> --
>> Fernando Trebien
>> +55 (51) 9962-5409
>>
>> "The speed of computer chips doubles every 18 months." (Moore's law)
>> "The speed of software halves every 18 months." (Gates' law)
>>
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>
>
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-- 
Fernando Trebien
+55 (51) 9962-5409

"The speed of computer chips doubles every 18 months." (Moore's law)
"The speed of software halves every 18 months." (Gates' law)



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