[OSM-talk] talk Digest, Vol 64, Issue 20
Richard Bullock
rb357 at cantab.net
Fri Dec 4 02:47:43 GMT 2009
> 2) There is a divided road that has been sketched out roughly, simply
> to indicate the division. (Very common, I think) Converting this to a
> simple divided=* tag doesn't lose information, and better indicates
> the actual level of information stored.
> Here's one that looks like 2):
> http://www.openstreetmap.org/edit?lat=-37.77825&lon=144.830416&zoom=19
>
> There are two lanes to indicate the divided highway, but the mapper
> hasn't marked out the cut throughs, so this results in worse mapping
> than if they'd just made a single road. - the router can't tell that
> you can turn left across the median strip.
>
> If you follow that road south a bit, you'll find cases of 3 and 2:
> painted lines, some of which are marked as separate lanes, some
> aren't. IMHO it would be cleaner to represent all of those with a
> divided=* tag, but tthat's just my opinion.
Right; If you find a road which has been sketched out very roughly - and you
have excellent NearMap imagery; then perhaps the best solution is to draw it
in more accurately...
Note that if you follow your 2nd example further south, you'll reach two
junctions with service roads into the Cinema car park.
http://osm.org/go/uGy6p24ha
The northernmost junction only allows traffic heading South on the road to
exit to the left, towards the car-park - and traffic entering the car park
to exit onto the road heading South. Northbound traffic on the road cannot
turn, traffic from the car park cannot exit to the North.
The southernmost junction allows traffic heading in both directions to enter
the car park - and allows traffic from the car park to exit in both
directions.
Using single ways to represent the dual carriageway doesn't distinguish
between these two junctions - but the possible movements are different - the
topology is not the same. Using turn-restrictions is not ideal as
restrictions are really designed for physically possible turnings which are
legally forbidden - so for instance, an emergency vehicle might be able to
make the turning - but an ordinary 'civilian' driver could be fined for
doing so. Mapping as two ways removes any possible doubt here. For the
northernmost junction, the north-bound carriageway of the dual-carriageway
has no shared nodes with the access road to the car park - so there is no
need for any "fudge" of the routing to stop it using those roads.
It's surely a lot simpler to map out separate carriageways as separate
ways - it really doesn't take that long - and I can't believe there are so
many to map that the task is overwhelming to map two ways. There are
probably enough messages and replies on this topic that you could have
mapped several already. If you don't have two carriageways; some roads have
something in between the two carriageways - e.g. part of a footpath, there's
another road (not motorway) between the two carriageways of the UK's M6 for
a short section in Cumbria.
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