On Wed, 25 Nov 2020 at 08:45, Ture Pålsson via Tagging < [email protected]> wrote:
> > (And I agree with Kevin about reconstructing an area from a point + > surrounding coastline. I'd like to see at least an outline of an > algorithm for that! Having said that, I also recognise that > gazillion-point polygons to outline Skagerrak, Kattegatt, the North Sea > and what-have-you may not be the prettiest state of things either...) > I'm not convinced that point + coastline will give a reasonable result enough of the time. But I could be wrong about that. Polygons that are contiguous with the coastline are a pain to add, even with generalized coastline (and even worse if Slartibartfast has added crinkly bits to the coastline). It's a lot of work. If the polygon is crude and not contiguous with the coastline that can give bogus results when trying to determine if a given co-ordinate is in a named bay or not. However, it is often the case that the ends of bays are known (local knowledge that village X is in Y bay) or are obvious from inspection. Since at least one person is confident that a single point is enough to create a workable algorithm, two points should be twice as good! Yeah, I was joking, but a lot less code and a lot less algorithmic guesswork would be involved in marking two points on a coastline that define the extent of the bay. An algorithm can generate a bounding polygon from those two points, the coastline between them, and a straight line connecting them. The hardest part would be ensuring that the algorithm takes the shortest segment of coastline between the two points and not the longest segment. Better than two points would be a way joining those two points. In the absence of further knowledge, map a simple straight line. A straight line is an approximation because currents and water depths might mean hydrographers and/or mariners regard the seaward extent of the bay to be wibbly-wobbly (one of the examples posted on the list showed a convex seaward extent of a bay). So use a way rather than two points to allow for curvy seaward extents, where known. Using a way rather than a polygon avoids the problems of nested bays. There are many small bays within Cardigan Bay (mapped by somebody as a polygon): https://www.openstreetmap.org/way/651881240 It would also deal with the potential problem of overlapping bays (imperfect nesting), should that ever be necessary, without mappers having to jump through hoops constructed of multipolygons. As far as I can see we can use a point, placed by visual inspection, and add a tag for importance which determines (in cartos that make use of it) the size of the label and at what zooms the label appears. Or we use a way to determine closure of the bay and let an algorithm handle placement and importance of the label. An algorithm would give greater consistency than mappers using their best guess at how important the label should be. Yes, the way could be abused by people wanting to control placement of the label. As could the point. As could any other way of mapping bays that we come up with. I don't think we should reject solutions because somebody could abuse them, otherwise we wouldn't have any tags at all. -- Paul
_______________________________________________ Tagging mailing list [email protected] https://lists.openstreetmap.org/listinfo/tagging
