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

Christopher Woods christof at infinitus.co.uk
Thu Jan 22 14:51:54 GMT 2009

> > ;-) I'm surprised you're not asking the obvious: How can I 
> tag a bit 
> > of road that is layer=0 at one end and layer=1 at the other!
> Yes, I know you need to split the way. But that begs the 
> question, which then becomes "should the way be split near 
> the bottom the the approach or the top?" possibly depending 
> on the different cases of how the approach is constructed.

>From a common sense POV, if the sections of road which 'ramp' up and down
(pun unintended) from the bridge itself are clearly obvious, include them as
part of the bridge by splitting the ways before the uppy bit and after the
end of the downy bit.

Sorry for the overly technical language employed there.

Purists will argue that until the bridge structure itself crosses over open
space, it is not a bridge. However, the underlying bridge structure is
likely to be incorporated into the subsurface of the road sections leading
on and off the bridge, so I'm fairly sure if you dug up the tarmac there'd
be supporting iron rods in the ground. Unless it's got dividing sections and
is free floating to allow for contraction and expansion of course ;)... 

For the sake of simplicity, I would separate the way and call the little
ramping sections of road either side as the bridge too. Why not imagine
where a stop line would be painted if the bridge was single carriageway, and
one direction of traffic had to stop to let the other pass across? Then
include all road areas which would theoretically be single carriageway with
the section of fully raised bridge as 'the bridge'.

However, I've managed to half convince myself that this may not be the best
way of doing it. Anybody care to further convince me otherwise?

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