[OSM-talk] cadastre, ways, rendering
frederik at remote.org
Tue Jan 23 10:40:47 GMT 2007
> I understand the rendering issue, but why is routing a problem?
> Unless I
> missed something routing should only look at the segments?
> I currently put the oneway tag on the segments and not the ways. Do
> I need to
> change that? (by splitting the way and then readding the tags).
As you can see from Mike's response, there is not really consensus on
this. Personally, I think that attributes on segments are evil. I
treat ways and nodes as the "real" elements making up the data, and
segments are (for me) just the low-level modelling of polylines. If
it were possible to have a way with bends in it, I would not need any
I have this opinion because I fear that otherwise all sorts of
ambiguities could arise, especially if tags on segments and tags on
ways using the segment contradict each other. I think of the segment
as a quite low-level thing, basically something that may have been
drawn from a satellite picture, without any semantics; and I view the
creation of a way as the precondition for applying any information to
it. In my eyes, a segment could probably even be used by two ways at
the same time (e.g. a tram line and a road sharing the same asphalted
area), although I've never dared to tag something like that yet ;-)
So I would never tag a segment; least of all with something that
makes only sense in the context of a way being put on it. If I have a
road that is partly one-way, partly not, I always create two ways,
one with oneway=true, one without.
I don't intend to forestall a proper discussion on this (I guess it
must a subject popping up every now and then), I'm just writing down
what I think and how I tag.
(And no, I believe routing should look at ways, not at segments. A
segment doesn't even have a name, so what should the routing
algorithm tell the driver?)
> Ok. So if there's a round-about you advise to split the road?
I do, except of course it is a "mini roundabout" which is only one node.
> somehow related question: I marked the segments of roundabouts as
> being "junction:roundabout". Should this be moved to the "way"-
> level too? (IE
> make the roundabout a way, and put this label in the way level).
That's how I do it. I think it makes no sense otherwise. Imagine you
have 10 segments making up a roundabout. The fact that it is a
roundabout is not instilled in any one of the segments. A segment
alone can never be a roundabout. Only the way, as a combination of
these segments, can be a roundabout. (And it saves storage space on
the database too; you only store "junction=roundabout" once, instead
of 10 times!)
> unrelated question: sometimes I selected the whole roundabout to add
> the "junction:roundabout" tag. Later I discovered, that this
> included the
> nodes too. Does this make any troubles? Do I need to remove this
> tag from the
> nodes and only leave it at the segment/way level?
It doesn't hurt but it is a waste of space.
Frederik Ramm ## eMail frederik at remote.org ## N49°00.09' E008°23.33'
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