[Tagging] Another multipolygon question

Dave Swarthout daveswarthout at gmail.com
Wed Oct 24 13:33:12 UTC 2018

They say a picture is worth a thousand words and IMO that video says
bushels about mapping relations. I was always a bit scared when fooling
with them because as it turns out, their reputation is worse than the
reality. The Reltoolbox is similar to the normal relation editor as Kevin
points out but for me it makes the task ever so much faster. You can move
along picking up pieces of whatever multipolygon you're constructing and it
just magically adds them. I have one place where the same way is used for a
place=island with its name, a NWR boundary, a wood multipolygon and a sand
multipolygon. Freakin' awesome! I've learned more in the past day while
mapping islands in the Kodiak Archipelago than in the past 5 years of
working with multipolygons.

Now all we need is a video tutorial showing how to analyze one during a
debugging episode. Talk abut sorting, perhaps, an a walk through of a
session where there is a "gap" in some relation that you cannot locate.
What techniques and/or tools would one use in that case? And what about
those little squiggles that appear at the end of each member's line in the
relation editor. I know they indicate connectivity (a closed loop?) but
what do you do if there is a problem, a "gap."??

Anyway, thanks again to Adam. You've advanced my understanding immensely.


On Tue, Oct 23, 2018 at 10:39 PM Kevin Kenny <kevin.b.kenny at gmail.com>

> On Mon, Oct 22, 2018 at 12:36 PM Adam Franco <adamfranco at gmail.com> wrote:
>> Hi Dave, all,
>> Based on this discussion I just recorded this short tutorial
>> <https://youtu.be/x7SPb0JtheA> of how I use JOSM and its Relation
>> Toolbox plugin to to add adjoining land-cover areas as multipolygons with
>> shared boundary ways to reduce duplication and overlapping ways.
> Thanks for recording that! Now I don't have to. :)
> Your workflow is essentially the same as mine, except that I use the
> regular old relation editor to add and delete ways. Works well enough for
> me, and I think it's only one or two clicks more than what you're doing. I
> also make a lot of use of 'replace geometry' from utlilsplugin2, since a
> lot of what I'm editing was born as imports and is being replaced with
> updated data from the same sources. Yes, I'm very careful not to step on
> the work of local mappers when I do it.
> Depending on what's going on in the field, I might have called that
> hedgerow a tree_row or a hedge and used a linear feature to map it.
> Similarly, at breaks in tree cover for things like power lines and
> pipelines, I might use man_made=cutline. Speeds up the process a little bit
> more. For what it's worth, I tend to restrict the 'cutline' tag to a
> standard (in NY) four-rod right-of-way; if the cutting is larger than that,
> it gets a polygon.
> Hopefully this will begin to show that for complex landcover, or similarly
> complex admin boundaries, that multipolygons with shared ways are actually
> quicker and easier to maintain than simple areas.  I know that they're
> still controversial, even among experienced mappers, but for something
> complicated like West Point https://www.openstreetmap.org/relation/175474,
> with a whole bunch of shared borders, rights-of-way cut out of it and what
> not, I'd be really handicapped without shared ways.  I didn't get very far
> on the landcover because I seldom map landcover other than in my own
> neighbourhood or when fixing other people's mistakes.
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Dave Swarthout
Homer, Alaska
Chiang Mai, Thailand
Travel Blog at http://dswarthout.blogspot.com
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