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

Matthias Julius lists at julius-net.net
Fri Jan 23 21:26:47 GMT 2009

sergio sevillano <sergiosevillano.mail at gmail.com> writes:

> i did not meant to tag ramps with layers
> just that normal layer usage, gives a hint if there is a ramp
> i dont see important to tag ramps but if someone wants to do so
> is better to tag the ramp alone "bridge_ramp" instead of the 80n proposal
> of tagging all parts as bridge and then each part.

The point is that you can not reliably deduce the presence of ramps
from the layer tag because many people (myself included) tag bridges
with layer=1 regardless whether the bridge is at the same level, above
or below the surrounding area.

> as for the layer issue is something to discuss in an thread of its own 
> or the layer talk page.
> but my opinion is that layers are only a convention to distribute what 
> is over what
> so it relates to each other, thus the number is not that important as 
> long as they are in the correct order.
> also i consider layer=0 or the absence of that tag (the default) as 
> ground level.

Right, the ground is just not level everywhere.

For me layer=0 is where you hit the dirt when you jump from an
airplane, no matter whether that is on a flat field, a mountain, a
manmade embankment or trench or a natural valley.

> i dont agree with "take "layer=1" as a default".
> for me a bridge that has layer=1 over a way without layer means that the 
> undenneath way is at ground level and the way avobe rises to layer=1 to 
> pass over the other way.
> a bridge with no layer tag passes over a way that should be tagged 
> layer=-1 this means that that last way is digged or has ramps to go to 
> level=-1 but is not a tunnel, if that second way is a tunnel layer=-1 
> then of course there is no bridge.

For me, a bridge with layer=0 is useless because there would be no
space under it ;-)

But of course, all this is not that important as long as the bridge is
on a higher layer than whatever is underneeth it.  Until that street
at layer=-1 crosses a tunnel.


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