[Tagging] Another multipolygon question

Peter Elderson pelderson at gmail.com
Mon Oct 29 09:51:44 UTC 2018

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 <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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Vr gr Peter Elderson
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