[talk-au] Roundabouts and routing

[Sam Couter]

morb.gis at beagle.com.au <morb.gis at beagle.com.au> wrote:
> "The first exit (west on Ginninderra Drive) isn't being counted as an exit
> because that exit point is also my entry point."
> 
> Well, I would rather represent geometrical reality than play tricks for routers.
> 
> Take a comparative example:
> 
> http://www.nearmap.com/?ll=-28.0917,153.3945&z=19&t=k
> http://www.openstreetmap.org/?lat=-28.0917&lon=153.3945&zoom=18&layers=B000FTT
> 
> You can see that to get from S to W, W to N and N to E it's a graceful sweep
> left, more or less tangential to the roundabout.  Whereas from W to S, there's
> the left bend into the roundabout, a bit of right bend in the roundabout,
> before turning left again out of the roundabout.
> 
> In the first case I have edited the entry, exit and roundabout as meeting at
> exactly one node.  IMHO this represents reality and if the router can't handle
> it then the router should be upgraded to suit (or its OSM-to-router-format
> script suitably upgraded).
> 
> We shouldn't be "mapping for the router" to the extent it disagrees with
> reality, should we?

You can't represent a "graceful sweep" with two straight lines and one
angle.

It seems clear to me that even if you don't turn the steering wheel due
to the width of the road, the entry and exit points on that roundabout
are separate. If you were driving a hypothetical and impossible point
vehicle on a hypothetical impossible zero-width line road, you'd be
turning the wheel to go around the roundabout. And it just so happens
that what we are mapping is impossible zero-width line roads.

Also, if you're driving tangetially to the roundabout and the circle of
the roundabout has been simplified to straight lines, it's not hard to
arrange one of the straight lines from the circle to coincide with your
tangent.
-- 
Sam Couter         |  mailto:sam at couter.id.au
OpenPGP fingerprint:  A46B 9BB5 3148 7BEA 1F05  5BD5 8530 03AE DE89 C75C
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