[OSM-talk] Advanced relationships

[Robert (Jamie) Munro]

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Nic Roets wrote:
> On 6/13/07, Abigail Brady <morwen at evilmagic.org> wrote:
>> Not really true.  How can you model a simple no-right-turn (or
>> no-left-turn), which has just been posted by a sign?  How do you tell
> 
> Well, if you break the road down into two oneway ways in the area
> around the junction / intersection and then have a short oneway way
> that connects one of them with the perpendicular street. Then it may
> become necessary to break the perpendicular street also into two
> oneways. Just because it crosses the street does not mean there needs
> to be an intersecting node.
> 
> The journey planner may then frequently recommend a U-turn at the node
> where you recombine the two oneways back into one bidirectional way.
> To prevent that you'll need a lot of oneways... Then you need an
> editor that makes it all managable at lowzoom...

Or you need relationships.

Oh, and when you have relationships you also get the 3 other benefits I
already listed and hundreds of others that we haven't thought of yet.

Another idea I have had is you could put textual instructions for
complicated junctions where a normal sat-nav would not be helpful.

e.g. at the "hamburger" roundabout here:
http://www.openstreetmap.org/?lat=51.76278619346854&lon=-1.1964009771259083&zoom=17

If you are approaching on the dual carriageway from the east, and you
want to head towards the unclassified road that heads north east, you
need to be in the correct lane, take the middle prong of the 3-way fork,
and take the forth exit of the roundabout. If the instructions just said
"Take the forth exit of the roundabout" which they probably would
without a relationship telling them not to, you would probably get into
the wrong lane and end up heading north west, continuing on the dual
carriageway. We could define a relationship k=special_instruction on two
segments that tells sat-navs what to say when you are on the "from"
segment (in this case the last segment of dual carriageway before the
fork) and your journey involves the "to" segment (in this case the first
segment of the unclassified road).

Robert (Jamie) Munro

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