On Sat, Aug 17, 2019 at 9:44 AM Andy Townsend <ajt1...@gmail.com> wrote: > It does sound, however, that you don't have a concrete use-case at all - you > have a view of how things "should" be, but this doesn't seem to be driven by > a real-world requirement. That's why I've been asking for specifics > throughout this thread (and Richard has too).
There are some trails for which I wish eventually to produce map books: a series of large-scale maps showing the route of the trail, together with tables of linear distances along the trail (to obtain distance between waypoints), and elevation profiles. Of course, this has the requirement that the "main stem" of the trail (I recognize that there may be links, alternatives, etc.) needs to be _reducible_ to a coherent linear topology. (Note that I say _reducible_: I can tolerate its not being actually _reduced_ in the database.) Answering a question such as, "how far is it along the trail from the Route 28 highway crossing to the campsite at Salmon River?" needs such a topology. My operational definition when I'm mapping: "sorted well enough for JOSM to show route continuity except where discontinuities exist in the field or the mapping is incomplete, and for Waymarked Trails (or similar software) to produce the elevation profile." My operational definition when I'm consuming data: "the route segments have endpoints that are sufficiently close to contiguous that a reasonably simple, greedy algorithm can assemble a linear route" (obviously, if there are route segments with specific roles in the relation, that fact can imply a different topology). I shouldn't have to resort to simulated annealing or machine-learning techniques to assemble a route from a mess. If I get into needing to "approximate longest Hamiltonian path", I'd be spending my time more effectively in cleaning up the underlying data. Ideally, I shouldn't have to resort to very much computation at all to assemble a linear route, which is why OSRM flags all discontinuities. But I don't complain when route segments are scramble. I fire up JOSM, unscramble them, and leave it at that; I recognize that there are editors that are less capable of maintaining a coherent route relation, and that there are data consumers that require a stricter one, so I just go in and repair it. I get rather more cross at way ends that don't quite meet, because closing the tiny gaps requires considerably more computation. I grumble considerably more at closing gaps of a few metres. I appreciate the work that the iD team has done so that I have to repair these things much less often. "Be strict in what you emit and permissive in what you accept" is a reasonably good principle for data interchange in general. -- 73 de ke9tv/2, Kevin _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
