[Tagging] Another multipolygon question

Dave Swarthout daveswarthout at gmail.com
Sun Oct 21 01:09:37 UTC 2018

Wow, the West Point and the Hudson Highlands State Park multipolygons are
impressive and yes, I see how using multipolygons has made it simpler.
Except if, as Mark points out, one of the boundaries changes and then it's
going to be an awful mess to fix. In my particular use case, it's highly
unlikely that developers will move anything seeing as most of my Alaska
work is in country so wild I'll never see any of it in my lifetime. But I
can certainly appreciate the difficulty in changing a beast like that.

On some of the larger islands where there are areas without trees I'll use
the coastline way as a border by clicking along it and using (F)ollow in
JOSM thus using those nodes for two purposes. When a clearing or beach
occurs I simply move off the coastline, trace around the clear area, and
then rejoin the coastline way. Then later, if adjustments need to be made I
can simply unglue the wood area nodes where I need to.

As for Warin's comment, I'm not sure it makes any difference as to which of
natural=wood or natural=coastline gets tagged at top level. Unless, of
course, the coastline is already a multipolygon.


On Sun, Oct 21, 2018 at 2:12 AM Mark Wagner <mark+osm at carnildo.com> wrote:

> On Sat, 20 Oct 2018 09:49:57 -0500
> Kevin Kenny <kevin.b.kenny at gmail.com> wrote:
> > Not only legitimate,  but recommended!
> >
> > If you haven't stumbled on it yet, another useful procedure is to map
> > areas of landuse use or landcover by drawing each border only once,
> > and having each area be a multipolygon with the shared border way as
> > a member. With that approach there's no need to retrace an irregular
> > boundary. You just add it to the multipolygon on either side.
> Works great, right up until you need to maintain it.  So, you've got
> your "natural=wood" multipolygon sharing a way with an adjoining
> "natural=scrub".  And then, some inconsiderate developer bulldozes his
> way across the boundary and puts up a housing development.  Now what do
> you do?  You can't unglue the boundary and shrink the two affected
> areas to make room for the "landuse=residential" because there's only
> one way.
> The only option I've found is to remove the affected section of
> boundary from one of the multipolygons, move it to the new location,
> create a new boundary way for the other multipolygon in the proper
> place and add it, create a new multipolygon for the development and add
> the relevant boundary ways to it, and then confirm that you haven't
> broken any of the multipolygons involved.  It's painful enough that
> it's usually faster and easier just to delete everything and re-create
> them from scratch as ordinary closed ways.
> --
> Mark
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Dave Swarthout
Homer, Alaska
Chiang Mai, Thailand
Travel Blog at http://dswarthout.blogspot.com
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