[Tagging] SF Muni tram lines are layer=1? and general tagging levels considerations

Georg Feddern osm at bavarianmallet.de
Tue Dec 18 07:21:11 GMT 2012

Am 18.12.2012 07:31, schrieb A.Pirard.Papou:
> On 2012-12-17 22:16, Martin Koppenhoefer wrote :
>> 2012/12/17 A.Pirard.Papou <A.Pirard.Papou at gmail.com 
>> <mailto:A.Pirard.Papou at gmail.com>>
> We badly need precision.
>>     . In fact, the only effect of assigning a layer is that upper
>>     layer objects hide lower layer ones (it's not a "mind your step"
>>     warning ;-))
>> it is a way to describe in the database which object is above which 
>> or whether they are at the same level.
> Agreed. And this is why I said that the tag should be called level.
> Transforming that into layers is a renderer's matter that is strictly 
> forbidden to speak about. Yet...

possible - but its a historical evolution you have to 'correct' then.
And I (and possibly others too) see the key 'layer' differently, see below.

>>     I have traced lengths of streams
>>       * stream as a constant layer=-2 way, uninterrupted end to end
>>         (even if they "don't look so deep"),
>>       * roads are at level 0
>>       * and bridges and culverts at level -1, in the manner mentioned
>>         above.
>> very strange way of mapping IMHO, how did you come to this idea?
> Exactly as you say above.  They are the actual relative levels of 
> these objects.
> I have never seen a bridge sitting on a road (and hiding it, even just 
> as a hint).

Well, so you just understood the layer key as 'a level of the physical 
whereas in my opinion the layer key is defined as 'a level of a map 
feature (OSM object)'.

Physical 'level' is described best by ele (elevation, absolute) or may 
be by the key 'level' (as in buildings, relative).
The key 'layer' is just a relative hint for renderer - at least in my 

Bridge and the 'road element' may be seen as different physical objects.
But in OSM bridge and 'road element' are seen as one object (one OSM 
way) as long as no bridge relation is used.
So you are just not able to divide them in different OSM layers therefor 
- at least not until you map different 'layer' for bridge and road 
element then.

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