[Tagging] Another multipolygon question

Dave Swarthout daveswarthout at gmail.com
Mon Oct 29 07:48:04 UTC 2018


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

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

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.



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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