[OSM-talk] When is a bridge not a bridge?

Matthias Julius lists at julius-net.net
Fri Jan 23 20:19:13 GMT 2009

Marc Schütz <schuetzm at gmx.net> writes:

> (Forwarding to list)
> Am Friday 23 January 2009 17:28:17 schrieben Sie:
>> As I understood the text of the wiki when I first started, layer 0 is
>> the general level of the terrain, so levels 1, 2, 3, 4, 5 are above
>> the ground on successively higher bridges, and -1, -2, -3, -4, and -5
>> in successively deeper trenches.
> Indeed, that's what the wiki page says. Unfortunately I cannot find the place 
> where the discussion took place when the feature was introduced (probably 
> before today's "standardization process"); I'm pretty sure that this was not 
> intentional.
> One could argue that because the default is layer=0, the natural surface would 
> be in this layer. However, we do not model the natural surface, so this seems 
> to be a completely arbitrary limitation. Lacking a definition for "natural 
> surface", there isn't even an application where this information could be 
> utilized.

The layer tag is just the way to tell applications about the vertical
ordering of objects.  It is irrelevant for objects that don't
overlap.  And the absolute number doesn't matter as long as the higher
object has a higher number.

A somewhat special case are objects that are next to each other that
overlap in map renderings because they are rendered wider than
reality, like a road next to a railway.  Then it should be up to the
renderer to decide how to solve that.

Everything else is just a matter of convention.  I think it produces
the least amount of surprises if layer tags are applied to the smaller
feature (like the bridge instead of the river) to avoid effects on
more distant intersections.


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