[Tagging] Route members: ordered or not

Colin Smale colin.smale at xs4all.nl
Thu May 3 16:49:30 UTC 2018


Why does a loop make it impossible to sort the ways? It implies that a
section of the route is present twice in the relation, but there is
surely no distinction between the first traversal of a way and the
second traversal?

On 2018-05-03 18:42, Volker Schmidt wrote:

> I will try to explain this in a more systematic way:
> 
> Routes belong to either of two categories: (A) Those whose members can be sorted into a single ordered sequence (B) Those that cannot be sorted into a single ordered sequence of members Sorting makes only sense for category (A) Routes of type (B) can be subdivided into routes of type (A), each of which can be sorted, but the overall route can not be sorted.
> 
> Routes are of type (A) if 
> (1) the path from begin to end is identical to the reverse path with  all members traversed in the reverse order and in the opposite direction 
> or 
> (2) all members have the role forward 
> or 
> (3) all members have the role backward
> 
> Any route 
> (1) that has more than two ends 
> or 
> (2) that contains any loop (except the case that the entire route is a single loop) 
> or 
> (3) that contains any element with role forword or role backward (except the cases of all-forward or all-backward) 
> or
> (4) that contains node or area elements 
> is of type B
> 
> I am not sure if I have taken care of all cases - please complete as necessary 
> 
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