On 31.01.2013 12:06, Martin Vonwald wrote:
> I'm looking for some alternatives to map tunnels and bridges that
> contain several ways. I'm not really happy with the proposed relation
> [1]. Is there any other approach for this? I'm asking myself why don't
> we simply map the outline of the bridge/tunnel (the latter may be more
> difficult to obtain), tag it with something like structure=bridge (or
> similar, maybe even building=bridge), bridge=<type> (if necessary) and
> layer=x. Connect the ways running over the bridge to this structure,
> use the same layer tag and you're set.
For starters, the relation means that you do not have to rely on layer
tags to find out which elements are on the bridge/tunnel (or even on
which bridge/tunnel in the case of intersecting bridges or tunnels).
I do not really have faith that mappers will reliably add correct
layers... The relation at least makes the relationship explicit and
works just the same way e.g. for multi-level bridges.
But then the relation is also a lot more flexible. As you already hinted
at, it makes knowing the outline optional, which is useful for tunnels
where you cannot easily see it from aerial imagery. It also lets you map
the edges instead of or in addition to an outline. This makes it
potentially a lot easier to achieve the desired rendering.
The flexibility would further extend to possible future additions. For
example individually mapped bridge piers, as have been proposed in the
"bridge types" proposal, could be easily associated with the bridge by
the relation. Association by layer wouldn't really work as these are
_under_ the bridge.
So I think that this is a case where a relation is actually a good
representation. With a decent preset instead of our (unfortunately)
massively-overcomplex relation editors, editing this could be pretty
intuitive even for beginners. In my opinion, the fact that a bridge is a
physical entity actually makes understanding the relation easier.
Tobias
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