[OSM-talk] Roundabouts - why is a separate segment required?

Mark Wagner mark+osm at carnildo.com
Wed Feb 14 20:44:41 UTC 2018


On Wed, 14 Feb 2018 16:39:29 +0000
Dave F <davefoxfac63 at btinternet.com> wrote:

> I think I have read it correctly.
> 
> https://www.openstreetmap.org/node/5408566797
> 
> It is easy to determine this shared node is part of the roundabout as 
> well as the entrance from Wapping & can exit along Commercial, or if 
> required, continue around the roundabout:
> How is this different from, say, two side roads joining a main road
> at the same node?,

If I'm judging the angles correctly, OsmAnd will not even announce that
intersection: the angle between Wapping and Commercial is shallow
enough that OsmAnd sees it as a single road, while the angle between
Wapping and the roundabout is sharp enough to not require a "keep
left" instruction.

In the general case, a router only needs to consider the ways that a
route actually passes over when creating directions.  By mapping a
roundabout entrance and exit sharing a single node, you've
introduced a special case: the router now needs to check all ways
connected to that node to see if any of them is part of a roundabout.

-- 
Mark



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