On Thu, 23 Apr 2009 21:56:09 +0200, andrzej zaborowski <balr...@gmail.com> 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 > > Cheers > So how do you mean to tag a no_left_turn, where it is marked with a fully drawn yellow line in the center of a road, but no sign? The restriction to tag must correspodent with the actual restriction, so that a routing engine will route you correctly even if there are no visible signs. Sometimes restrictions can be painted in the lanes (one lane with arrow to the right, and one straight ahead, but no lane with arrow to the left). The choise of lane will than correspodent with where you are going, I guess that type of routing might come in another relation.
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