On Thu, Mar 14, 2019 at 10:39 AM Kevin Kenny <kevin.b.ke...@gmail.com> wrote: > The order of ways in the relation definitely determines the direction > to which oneway=* refers. It oneway=yes or oneway=signed (or whatever > we settle on) is present, the ways are traversed from the first > relation member to last - irrespective of the direction of the way.
And I just realized where this rule needs *something* to resolve a possible ambiguity: We already have concluded that the direction a route traverses a way needs to be independent of the direction of the way. (I have examples where two routes share one way in the same direction, and elsewhere share a different way in opposite directions, and that's not achievable otherwise.) If a route is circular, and the relation has fewer than three ways, there's then an ambiguity. For a single way in the relation, the direction of travel could be either with or against the way. For a relation with two ways, the direction of travel on the first way could be either with or against the way, and in either case, the other way will have an endpoint that joins and the route will close. The first case is just a closed way, and the route provides no indication of the direction of travel. The second case is that there are ways x and y joining nodes A and B. The direction of the ways in a route relation, we've already established, needs to be ignored. So the actual route will depend on the choice of which end of 'x' is chosen as the start. It could be B -> x-in-reverse -> A -> y -> B, or A -> x -> B -> y-in-reverse -> A. Both routings visit way x before way y, so the order of the ways in the relation doesn't determine the route. For three or more ways, there's no ambiguity, since only one endpoint of the first way will join to the second, determining the direction of travel on both. The simplest solution is to require that the ways be split if necessary to avoid ambiguity - forbid one-way routes of fewer than three members. An alternative is what I mentioned previously, disambiguate with 'forward' or 'backward' roles on the ways. I can live with either. _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
