Thanks, Dave, for asking these questions. I am still struggling with the practical consequences of the concept, but it really helped.
I have a question of my own: is it possible to define operations on multipolygons ? E.g. can you join two multipolygons, add a multipolygon to another, split a multipolygon by adding one or two cutting ways? I know you can do it by hand, but can the operations be defined as formulas (with hard checkable preconditions if needed)? Eg. a join of two adjacent multipolygons. It looks to me that the precondition would be that landuse/landcover are the same and there is only one way (or combination) that they share. The operation would then be to move the complete set of ways -including roles- from one multipolygon to the other, removing double ways, save, then delete the empty one. A tool could do this. Right? Probably already exists, then. (Every time I think of something I find that someone else has done the same long ago...the wheel, for instance...) Op ma 29 okt. 2018 om 08:52 schreef Dave Swarthout <daveswarth...@gmail.com >: > Thanks. > > After I posted my question I thought about it again. When I asked the > question I was thinking in terms of a method of loading the entire external > relation into the existing large one with one or two clicks. Then later, as > I was watching Boston win the World Series, I realized that adding all the > ways that make up an inner multipolygon to the large one making sure to set > their roles as "inner" amounts to the same thing. The ways comprising the > inner multipolygon remain "connected" spatially inside the big > multipolygon. > > The Wilcox Lake Wild Forest islet is another good illustrative example > since it simultaneously serves as an inner way and an outer way. That > answers what was going to be my next question, i.e., how would you add a > wetland inner to a multipolygon that is already an inner of an enclosing > multipolygon? > > But I'm still a bit confused about way:427547729. It's tagged as an outer > in the Wilcox WF multipolygon but it's located inside of an enclosing way > that's also an inner to the same relation. Does that mean the inner/outer > roles alternate as you add more and more "nested" objects to the large > multipolygon? For example,iIf there was a block of private property inside > way:427547729 would that be tagged as inner? > > Just to touch on another topic because Kevin mentioned it. Sometimes it's > fairly obvious that certain boundaries were meant to follow a riverbank or > a coastline but at the present time don't. My first impulse is to delete > segments of the original boundary and replace them with the more recent > riverbank or coastline. That would probably be considered wrong by some but > seeing as we do not and can not guarantee perfect accuracy with the > placement of any boundary I don't see it as an absolute no-no. Plus, many > of these boundaries use thousands of nodes that follow every little zig-zag > to achieve legal accuracy. IMO, OSM doesn't need that level of detail. > > Opinions? > > Dave > > PS: This has been a most beneficial conversation. I feel enlightened. > > > On Mon, Oct 29, 2018 at 9:33 AM Kevin Kenny <kevin.b.ke...@gmail.com> > wrote: > >> On Sun, Oct 28, 2018 at 8:12 PM Dave Swarthout <daveswarth...@gmail.com> >> wrote: >> >>> Okay, next question. >>> >>> I added the Laguna Atascosa National Wildlife Refuge to OSM yesterday . >>> (I don't do much mapping in Texas but that place is special because I once >>> did a water quality assessment there as a volunteer.) It's a fairly large >>> multipolygon and the main relation holds the bulk of the refuge territory. >>> However, there are scattered about several other areas, some of which are >>> also multipolygons, that are part of the refuge. >>> >>> Simple areas can be easily included as "outers" in the main relation >>> (Rel ID:885828). But what about other pieces that are multipolygons? I >>> could simply add them as separate relations with identical tags but >>> handling such areas that are connected administratively but not physically >>> would seem to be one reason multipolygons were invented. But I'm thinking >>> there must be a more elegant method. And what about inner areas that are >>> also multipolygons? This case cannot be handled by my simplistic approach. >>> >> >> There's nothing wrong with having more than one segmented outer ring. >> >> Have a look at relation 6362971 (use File->Download Object in JOSM) in >> the relation editor, and you'll see just such an area, with muiltiple >> segmented outer rings, and some of the segmentation is there to have shared >> ways. If you also download 6370357, you'll see how the two relations share >> some, but not all, of the ways. Relation 8428216 might also interest you. >> It's a case where the same protected area shares multiple, noncontiguous >> segments with a lake shore, and multiple, also noncontiguous, segments with >> an adjacent protected area. >> >> Way 427547737 is also interesting. It's tagged place=islet (because it >> is). It's an inner way of Lens Lake, and an outer way of Wilcox Lake Wild >> Forest. Since the lake is not part of the Wild Forest, but is part of a >> private inholding that is completely surrounded by the Wild Forest, its >> west shore is an outer way of the lake and an inner way of the Wild >> Forest. And the inner ring to which that way belongs completely surrounds >> the islet. >> >> (The shoreline looks wrong in places, but I'm not going to fix it, >> because it's way too hard to tell land from water in orthos of beaver swamp. >> >> Because research is needed to find out whether, for instance, a nature >> reserve boundary that appears to run along a shoreline actually follows the >> shoreline or rather follows some survey line that was the shoreline in >> times past, I generally do this sort of conflation only when resolving >> conflicts or reimporting a particular boundary, so you'll see a lot of >> imported borders up in the Adirondacks that don't use shared ways yet. You >> can still use them as examples of how arbitrarily complex the topology can >> get. That Wilcox Lake Wild Forest relation (6360587) to which that islet >> belongs is pretty crazy, because it's a ton of small parcels. >> > > > -- > Dave Swarthout > Homer, Alaska > Chiang Mai, Thailand > Travel Blog at http://dswarthout.blogspot.com > _______________________________________________ > Tagging mailing list > Tagging@openstreetmap.org > https://lists.openstreetmap.org/listinfo/tagging > -- Vr gr Peter Elderson
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