[Tagging] Another multipolygon question

Kevin Kenny kevin.b.kenny at gmail.com
Mon Oct 29 02:33:34 UTC 2018


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.
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