Am 14.04.2014 08:28, schrieb Peter Wendorff: > Hi, > > Am 13.04.2014 21:35, schrieb Steve Doerr: >> I'm surprised that so many people are jumping to this conclusion. Let's >> remember that a way is just a series of nodes in a particular order. So >> a node is not necessarily an isolated object. > Agree >> In many cases, it exists solely as part of a way. Thus the concept of >> direction is not >> meaningless for a node which is part of a way. > Agree partly. It's not meaningless, but it get's ambiguous very often.
Exactly, it is not meaningless but ambiguous and can easily lead to wrong results. > Take traffic signals as one example where the direction might be used: > Besides an intersection someone could add the traffic lights on the four > individual ways (instead on the intersecting node itself). > This matches the installation of the individual lights and the stop > positions, but it produces wrong results without a direction tag. > The drawback of that is, that someone crossing the intersection straight > meets two traffic lights, which is wrong of course. The mapper therefore > might decide to add direction-tags to them, as each traffic light node > is relevant and applied only for one of the two directions. > > Looks perfect now - all four traffic lights are mapped separately where > they are, routing for cars works great (provided that the direction tag > is known and supported by routers). > > Enter of the next mapper: He want's to add the footways and cycleways > that cross the streets using the pedestrian traffic lights integrated in > the traffic lights mentioned above. > As a result the nodes previously mapped with a direction are shared by > two ways, and it's hard to determine what the direction tag refers to, > as of course crossing for pedestrians is possible and meaningful for > both directions. Thanks for another example where cardinal coordinates work but forward/backward fails. >> I haven't examined any >> uses of the tag on a node, but I can imagine, for instance, that a node >> in a way with a direction attribute might be used to represent a >> road-sign that applies only to traffic on the way passing that node in a >> particular direction. > For other traffic signs it's the same, and that's why we usually map the > road signs meaning on the road that is affected by it. (The sign itself > may be mapped as such, as an obstacle and a physical object next to the > street), while maximum speed, maximum dimensions (width, height, > weight), oneway access, access restrictions and so on are mapped on the > where they hold. > > Here the direction is useful (look at the oneway=yes tag), meaningful > and not ambiguous; on nodes it is or get's very lightly without tagging > mistakes. Ok, we can take a split between unconnected nodes on the left-/right-hand-side of the road and nodes being part of a way. The first are less ambigious but you still need to know the driving directions where as the latter ones just won't work properly with forward/backward. To make it less ambigious and easier I would deprecate forward/backward completely for nodes and advice to use cardinal coordinates for all nodes. fly _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
