[Tagging] Another multipolygon question

Joseph Eisenberg joseph.eisenberg at gmail.com
Mon Oct 29 08:57:58 UTC 2018

Re: “Does that mean the inner/outer roles alternate as you add more and
more "nested" objects to the large multipolygon?“

Yes, you’ve got it. This situation is rare but it does happen.
On Mon, Oct 29, 2018 at 4:52 PM Dave Swarthout <daveswarthout at 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.kenny at gmail.com>
> wrote:
>> On Sun, Oct 28, 2018 at 8:12 PM Dave Swarthout <daveswarthout at 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
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