[talk-au] Roundabouts and routing

[Craig Feuerherdt]

I have been watching the discussion with interest. My perspective on mapping
is getting the topology absolutely correct. All routing is driven by the
topology of the underlying data (and the routing engine of the specific
manufacturer). What I say below is said without criticism of the original
mapper.

The intersection shown in the links below is an interesting case. As it is
currently mapped in OSM, the mapper is showing the turning posibilities at
the intersection (and to a lesser extent that the turning lanes have their
own separate traffic light sequences). To my mind this is over mapping.
While the intersection is displayed neatly at smaller scales, at the largest
scale it is not pleasant (not saying it is wrong!). I am wondering if anyone
has routed through this intersection with a garmin - what happens? Is it
correct? I suspect in the scenario where you were travelling south and
wished to turn right (west) into Christine Avenue you would be told to "turn
right into 'to Christine Avenue', continue along Christine Avenue" which
isn't correct. The intent of the mapper (as far as I can ascertain) was to
indicate "move into right lane, turn right into Christine Avenue".

I have also been questioning what to do in a few circumstances, not unlike
the above. I have taken the simple approach and introduced relations for any
turn restrictions that may exist and just leaving the intersection as two
separated ways joining to separated ways (ie no explicit turning lanes)
which, I believe, is the currently stated way of doing it on the wiki -
happy to be corrected otherwise.

While OSM is essentially a wiki and people can contribute as they see fit,
we have to be consistent.

Craig

> 2009/12/18 Ross Scanlon <info at 4x4falcon.com>:
> > Wrong, on everyone of these you have to enter the roundabout and then
take the next exit, it is not a graceful sweep.
>
> We really shouldn't have to map, if they are an exit they're an exit!
> It shouldn't really matter if they are attached to an adjoining entry
> or not. Until this thread came up I wasn't aware this was even an
> issue and have tagged roundabouts this way myself.
>
> We might be able to do something if it is the pre-processor that is to
> blame for that bad routing information, although if it's a problem
> with garmin software we can't do anything about this other than to
> inform people about the issue and hope they will be understanding that
> even though we shouldn't have to go out of our way to tag a roundabout
> in a special kind of way we do for those that have garmin gear.

Were not tagging it in a special kind of way it's as per the wiki and as per
how it is on the ground.

It's also aparent in intersections like this:

http://www.nearmap.com/?ll=-28.085727,153.425859&z=19&t=k
http://www.openstreetmap.org/?lat=-28.085727&lon=153.425859&zoom=18&layers=B000FTF

What's currently entered into osm is not whats on the ground. There are not
separate ways going through the center of the intersection but someone has
entered them there for the turning lanes.

Josm's validator and turnright throw so many warnings for this intersection
it's not funny.
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