Mark Burton wrote:
Hi Steve,
Please look at
http://www.openstreetmap.org/?mlat=51.44827&mlon=-3.49581&zoom=16&layers=B000FTF
If you approach the marked junction on the B4270 from the south,
intending to continue northwards on the B4268 then you get told "turn
left" as you approach the junction with 300m or so to go. Trouble is,
there is a minor road junctioning with the B4270 at about the same point
on the road, and the geometry of the combined junction most certainly
does not agree with a "turn left" for the B4268!
The portion of the B4270 that continues to the north-east makes a
junction with the B4268/B4270 as a "give way" on the road. The
transition from the B4270 to the B4268 is seamless on the road. The
minor road going west does of course join the B4270 as a "give way" on
the road. You can see the physical details on Google Earth if you need
more detail.
But to hear "turn left in 300m" is most certainly a surprise! I'd expect
no verbal instruction in this case.
Yes, that's not ideal is it?
:-)
Tricky case this one. The proximity of the minor road on the left to
the bigger junction does confuse the issue. However, [...] the GPS doesn't know
that the road continues in a straight line
and that the turn to the NE is in fact the side road.
Now, if the OSM data could actually express the fact that the straight
on direction was the "main road" (even though the road to the NE has
the same ref as the incoming road), then we could avoid this problem.
Perhaps we should introduce a relation type (prefixed with mkgmap:,
perhaps) that could be used in cases like this to express that two ways
are "the same road" even though their refs are different.
I was going to suggest that myself, so that's two of us at least!
However - I'd not suggest prefixing with "mkgmap:" because probably
every other user of OSM data to create routing instructions would also
be glad of such a mark-up. I guess that the mkgmap community could just
agree amongst ourselves to try out such an idea, and once it proves
itself to be useful, get it ratified by the main OSM people?
Always easier to ratify something that is know to work and be useful
than to try and ratify a wild idea that's not been tried!
You're the guru, Mark, what do you suggest as a relation, and do you
reckon you could implement such a thing "just for a try-out" anytime soon?
I'd think that maybe a relation called "throughway" or "thoroughfare"
would be the thing to use, and get it just to list the ways that make up
the major-route from the P.O.V of a road-user. "Thoroughfare" is a bit
hard to type though, though at least in English it should be fairly
obvious what it is supposed to mean (Anglo Saxon roots there?). I'd also
allow such a relation to plough straight through several junctions if
necessary, indicating the primary way for all of them, to save having to
split ways up and use different relations to disambiguate a multitude of
physically-close junctions.
99% of junctions won't need them, but pathological cases will always
exist - as that one did.
Steve
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