[OSM-fi] Fixing broken and old-style multipolygons

Sami Haapaniemi sami.haapaniemi at gmail.com
Mon Apr 3 15:56:41 UTC 2017


I'm not sure but I think we have the whole country laser scanned and the
huge data available from MML. Maybe someone skilled enough could write a
forest detector and import the forest data from there? Just my 2 cents.

3.4.2017 3.21 ip. "Jochen Topf" <jochen at remote.org> kirjoitti:

> On Mon, Apr 03, 2017 at 02:13:23PM +0300, Jukka Rahkonen wrote:
> > Right, those seem to come from the notorious Corine Landcover import that
> > was never completed nor reversed even it does have loads of topological
> > errors. It does bring some colour to the map in about 25% of the Finnish
> > territory, though. You may know that we do not have very good alternative
> > sources for land cover imports because 71,6% of the area of Finland is
> > covered by forests. Therefore our topographic map database does not even
> > have a class for forests - they are background, not interesting at all.
> >
> > I had a look at some polygons and I do not know exactly how they should
> be
> > corrected. For example relation
> > https://www.openstreetmap.org/relation/1481481#map=8/62.740/25.565
> >
> > It has an inner member https://www.openstreetmap.org/way/104644001 which
> > somehow looks the same as
> > https://www.openstreetmap.org/way/104646035
> >
> > The inner polygon, that is coniferous wood, is a simple polygon with only
> > outer ring. So as a stand-alone it could carry its own tags. But because
> the
> > ring is re-used as a hole of the bigger mixed wood polygon it should not
> > have tags. Instead the single ring coniferous area should be changed
> into a
> > multipolygon relation that has only one member, and that new relation
> would
> > get the coniferous tag. Is this correct?
>
> No, simple closed ways as polygons are okay. The problem in this case is
> that the relation has one outer ring and three inner rings, all of which
> have the same tags (natural=wood, leaf_type=mixed). This is a not
> uncommon sub-problem of old-style tagging and is not allowed any more by
> new-style tagging. We would interpret this now as additional polygons
> filling
> the holes with exactly the same type of landcover the outer polygon has,
> which, of course, doesn't make much sense. To fix this the tags from the
> inner and outer ways have to be removed and put on the relation. There
> might or might not be a different closed way or multipolygon relation
> covering the whole to get some different type of landcover. If there is
> a different way that covers the inner, it could be removed and the inner
> tags can be put on the way that is the inner ring.
>
> > I really much doubt that anybody is interested in correcting the Corine
> > polygons but people has not been too eager to revert the import  either.
> > Because of the colour, you know. But let's see how other members of the
> > Finnish community react. I fear also that OSM will never get rid of the
> > hand-written multipolygon relation system and because the reason for the
> > trouble will not be fixed you have been forced to start  your huge effort
> > with fixing the broken data
> > https://wiki.openstreetmap.org/wiki/Area/The_Future_of_Areas. Good luck
> for
> > you.
>
> We are currently not looking at changing anything in the basic OSM data
> model, but osm2pgsql software *will* change in the not too far future and
> this *will* have a visible impact on the map in Finland if nothing is
> done. You can see what these changes will bring by looking at this map:
>
> http://area.jochentopf.com/map/index.html#8/61.967/27.554
>
> Switch off the overlay and move the slider around and you'll see that a
> lot of the green will be gone. There will also be areas that don't show
> up as forest now, but will in the future. So if nobody cares about this
> issue, the map will look even more wrong in the future than it is now.
>
> I hope the Finish community will decide on some plan for action here. I
> am sure the global OSM community will help to the best of their ability,
> but doubt many people want to spend their time fixing up imported data
> that isn't that high quality anyway.
>
> Jochen
> --
> Jochen Topf  jochen at remote.org  https://www.jochentopf.com/
> +49-351-31778688
>
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