On 23 Apr 2009, at 22:56, andrzej zaborowski wrote: > 2009/4/23 SteveC <st...@asklater.com>: >> >> On 23 Apr 2009, at 12:32, Teemu Koskinen wrote: >> >>> On Thu, 23 Apr 2009 22:25:36 +0300, SteveC <st...@asklater.com> >>> wrote: >>> >>>> >>>> On 23 Apr 2009, at 12:17, Teemu Koskinen wrote: >>>> >>>>> On Thu, 23 Apr 2009 21:34:05 +0300, SteveC <st...@asklater.com> >>>>> wrote: >>>>> >>>>>> I don't see a clear explanation as to why there is ambiguity if >>>>>> you >>>>>> don't do turn restrictions at the end of ways on the wiki. >>>>>> There is >>>>>> some stuff in the talk page >>>>>> >>>>>> http://wiki.openstreetmap.org/wiki/Talk:Relation:restriction >>>>>> >>>>>> Anyone care to provide an explanation? >>>>>> >>>>>> The reason I ask is that I've come across some roads where there >>>>>> is a >>>>>> restriction every other turn in both directions... and >>>>>> splitting a >>>>>> mile long road in to 30 pieces seems nuts. As a follow up, I can >>>>>> guess, but what will the renderer do in that situation? I'm >>>>>> guessing >>>>>> mapnik will give up trying to put 30 names on a one mile road and >>>>>> won't notice they're the same name? >>>>> >>>>> >>>>> If both from and to ways continue after the via point and neither >>>>> is one-way, there's two possible ways to interpret it: the >>>>> restriction could apply when coming from either of the ends of the >>>>> from-way. This of course doesn't matter if there is similar >>>>> restriction coming from both directions, but that's not nearly >>>>> always the case. And even if there is symmetry in the real life >>>>> restrictions, it's not appropriate in my opinion to map those with >>>>> just one restriction. >>>> >>>> eh? don't you assign direction by saying 'from' and 'to' ? >>>> >>> >>> Yes in the sense of which of the two ways you are coming from, but >>> if the way is not one-way and it doesn't end at the via-node, >>> there's two possible directions from where you can come to the via- >>> node using the way. >> >> Um... no. >> >> The restriction has handedness - left or right... and the way coming >> off it has an angle.. lets try some ascii >> >> B >> | >> | >> |----------C >> | >> | >> | >> A >> >> I am going from A to B. There is no 'right_turn' restriction on the >> corner that stops me turning to C. >> >> That cannot be interpreted as a restriction from B to A as it would >> be >> a left turn, not a right turn. To figure that out you just need to >> compute the angle it makes with your direction of travel to see if >> it's left or right? > > The no_left_turn, no_right_turn is only to indicate the type of > streetsign to show AFAIU. > > Practically, adding angles to the specification will be a hell to > implementers, and there are few use cases that would benefit from > this. Sometimes you will have a way splitting off to C that first > turns slightly left, enters a tunnel or viaduct and then goes on the > other side of AB, something that at low zoom level looks as in your > drawing, and the streetsign might stilll be no_right_turn. > > Or something like this is common: > > B C > \ | > \ | > \| > | > | > A > > where the straight line is considered a turn even though it's > straight, and the turn from A to B is considered straight even though > it's an arc :P >
This is yet another kettle of fish of how do you get the routing engine to tell you when the general flow of traffic is from A - B, even so the road name of A is the same a C, but different to B. I have come across a lot of these on my travels, and still haven't come up with a way to tag it. Shaun _______________________________________________ talk mailing list talk@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk