[Tagging] [OSM-talk] Roundabouts and routing

David Earl david at frankieandshadow.com
Fri Sep 9 21:00:47 BST 2011

On Friday, 9 September 2011, Bryce Nesbitt <bryce2 at obviously.com> wrote:
> On 09/09/2011 04:14 AM, Martin Koppenhoefer wrote:
>> 2011/9/9 Graham Stewart (GrahamS)<graham at dalmuti.net>:
>>> David Earl wrote:
>>>> The reason I needed such a tag was to avoid one way arrows cluttering
>>>> the map on those little Y-shaped approaches to roundabouts
> That's an issue you can take up with the rendering teams.  Based on the
length of the ways and the connections, they should be able to safely
suppress the undesired rendering.
> The routers need the right information more than the rendering packages.

you seem to be conflating two things. Explicitly identifying the approaches
to a junction is completely harmless, and potentially helpful to routers as
well as renderers as well as marking something distinctive as what it is
rather than relying on its shape which only humans are good at seeing.

what you and others dont like is this idiom where people put a single
one-way with a sharp bend to represent this. I dont like it much either
(though no router should take the continuity of a way or otherwise as an
indication of anything at all because of the way ways are broken up in our
model). Nevertheless, it seems to be a rather common idiom - and I stress
again, I didn't introduce this. Even without this though, you still have two
one-ways (or you simply omit a large ground feature), and I still want to
identify such as junction approaches.

Incidentally, I did try some rendering heuristics to decide when this is a
feature of this kind, not a 'real' one way street, and it is fiendishly
difficult. Make it too long, and you miss out real one ways, make it too
short and you get some approaches included and not others. It is so, so much
easier just ro call this spafe a spade than try to deduce it.

Something similar applies on dual carriageways. People dont need the
reference or road name appearing on both carriageways, or one way arrows. it
is visual clutter on what is already often a busy graphic, especially on
paper where you need to exaggerate road widthsvrather more than on screen.
But, except for motorways, it is exceptionally hard to know that this is the
case in a renderer which doesnt have the advantage of the human visual
system, so to do is properly, you need to know that two ways are related as
a pair of cariageways. There are of course roads which run parallel but
which arent pairs of carriageways (actually by cpincidentce there is an
example of exactly that just north of e roundabout example I originaly
quoted) so it is well nigh impossible to do this heuristically. In Mapnik
and the like, the answer is simply to draw all the arrows and repeat the
names/refs, but itnis very unsatisfactory for the idiosyncrasies of the
model we happen to use to force a particular style of graphic, especially if
you want something beautiful not just functional.

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