[OSM-talk] Turn restrictions ambiguity

[SteveC]

On 23 Apr 2009, at 12:32, Teemu Koskinen wrote:

> On Thu, 23 Apr 2009 22:25:36 +0300, SteveC <steve at asklater.com> wrote:
>
>>
>> On 23 Apr 2009, at 12:17, Teemu Koskinen wrote:
>>
>>> On Thu, 23 Apr 2009 21:34:05 +0300, SteveC <steve at 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?






>
>
> Regards Teemu Koskinen
>

Best

Steve