[OSM-talk-be] Large multipolygons for Forêt d'Anlier

[Julien Minet]

Hi Marc (and other interested),

When trying to add a multipolygon relation into another multipolygon (MP)
relation, JOSM gives the following warning: Non-Way in multipolygon. So to
me it is clear: we should only put polygon inside MP relation, and not
other MP relation.

The Forêt d'Anlier is 7000 ha wide and it cannot be map as a single
polygon: as you said, there are meadows, scrubs, dozens of ponds inside it.
At the moment, it is mapped as several multipolygons that are separated by
highways. So I planned to do what I make for the Forêt de Rulles
<http://www.openstreetmap.org/relation/1653686#map=13/49.7498/5.5791>: put
together these polygons in a MP relation, with several outers (not only one
outer).

But I was wondering if it is really a good idea because of the large
numbers of items to put into one MP relation. And because I have heard that
huge MP makes some issues for the use of OSM data downstream in some
applications. I also had a a look at  big forests in Germany, and I did not
find huge MP encompassing the same forests: for instance, the 10000's ha
Pfälzerwald <http://overpass-turbo.eu/s/oWs> is mapped with several
distinct MP (so similarly to Forêt d'Anlier right now).

Thanks to your email, I decided to draw a new single polygon, and use the
tag place=forest, name=Forêt d'Anlier
<http://www.openstreetmap.org/way/492279158>. At least now the name "Forêt
d'Anlier" is known in OSM. But I looked at taginfo for place=forest and
there is no such value worldwide! I only found 21 place=woodland items, all
located in Norway! So in conclusion now I'm thinking about doing something
more similar to the Pfälzerwald example above: I mean putting the name tag
to each MP and single polygons belonging to Forêt d'Anlier.

As further zooming into the OSM project, further questions are coming!

Julien

On Fri, May 5, 2017 at 10:28 PM, Marc Gemis <marc.gemis at gmail.com> wrote:

> Julien,
>
> As for the warnings, can you let us know the exact warning ?
> Most likely it has something to do with an impossible combination of 2
> multi polygons
> e.g. nested outer polygons or touching inner/outer polygons.
>
>
As for the mapping, it depends on whether Forêt d'Anlier can be be mapped as
>
> -1 polygon. This would mean that the area is without holes or gaps.
> - otherwise you have to use a multi polygon, but then it depends on
> whether all the different parts are indeed landuse=forest or whether
> to want to include grass fields scrub etc to the area. In the former
> case you can just put landuse=forest on the relation. In that latter
> case you might have to tag the relation as type=multipolygon;
> place=locality or perhaps as some protected area.
> - If it is a closed area, you could also use a multi polygon relation
> of many ways, just as we do for administrative boundaries. The rest of
> the tagging still depends on whether you have to combine multiple
> landuses or not.
>
>



> The latter case is what the Dutch community tried to solve with the
> toponym-relation. However, this is not documented and not understood
> by data consumers. I had a private conversation about this with
> dieterdreist that I will send you in a private mail.
>
> So there are still many choices, but I hope this information helps you
> a bit to pick a solution that suits you and the situation best.
>
> have a nice weekend
>
> m
>
>
> On Fri, May 5, 2017 at 12:00 PM, Julien Minet <juminet at gmail.com> wrote:
> > Hi list,
> >
> > Recently, I merge several polygons together to create a multipolygon
> "Forêt
> > de Rulles", which has now a osm id for itself. I'd like now to merge
> several
> > polygons together to form the Forêt d'Anlier, 7000 ha, the biggest mostly
> > deciduous forest of Belgium. My goal is to create a unique object named
> > "Forêt d'Anlier", and eventually link this object with its wikipedia
> page.
> >
> > But to do this, I identified 7 multi-polygons and 3 simple polygons to
> > merge, accounting for a total of 49 closed ways! I plan to do it by
> taking
> > the multipolygon with the highest number of polygon and add the other
> ways
> > of the others MP, but...
> >
> > it takes time to do so, can I simply add a multipolygon to another
> > multipolygon? it seems possible in JOSM (I tried) but it gives a warning.
> > is it a good idea to create such a big MP?  If not, what can be the
> > alternative to give the name "Forêt d'Anlier" and other tags to this
> large
> > forest?
> >
> >
> > Julien
> >
> > _______________________________________________
> > Talk-be mailing list
> > Talk-be at openstreetmap.org
> > https://lists.openstreetmap.org/listinfo/talk-be
> >
>
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