[Talk-ca] Multipolygon problems

[Jochen Topf]

Hi!

In the last days the OpenStreetMap Carto Style 4.0 is being deployed on
the OSMF tile servers. This new version of the style doesn't take
old-style multipolygons (where the tags are on the outer ways instead of
on the relation) into account any more. In a huge effort in the last
months we have converted all old-style multipolygons to the modern
tagging, so this is a good step!

Unfortunately, as a side-effect of this change, many multipolygon
relations now appear wrong on the map. This is the case for multipolygon
relations that have the same tags on the relation as well as on (some of
the) outer or inner ways. This is *wrong* tagging, and needs to be
fixed. (Note that this always was wrong tagging, even before we
deprecated old-style multipolygons, but the way the software worked with
old-style multipolygons, this problem was not visible on the map. But
now it is.)

Here is an example: http://www.openstreetmap.org/relation/1330741 . As
you can see (unless somebody fixes this :-) the clearing in the forest
that should just have grass, also has tree symbols on it. In many other
cases it is not this obvious, there are just islands in a river missing
or so.

There are about 50,000 cases like this worldwide, forests, waterways,
all sorts of areas. But the worst problem is in Canada. There are about
15,000 affected relations, most from the CanVec imports.

First, we have to make sure that there are no further imports of broken
data. I hope the people who have done those imports (and might still
continue) are here on this mailing list. If not please make them aware
of this issue and/or put me in touch with them. Second, somebody needs
to clean up the broken data, either automatically or manually. 99% of
the data has not been changed since the import, so it might be feasible
to do an automatic cleanup, but somebody has to do this. Otherwise we'll
have to do a manual cleanup, through tools such as Maproulette and the
OSM Inspector. I am currently in the process of creating Maproulette
challenges for other areas of the planet, but will not do this for
Canada at this time. Lets discuss this here first.

I can provide OSM data extracts, statistics, etc. if somebody wants to
look at the data.

All of this is part of a larger effort to fix areas in OSM. See
http://area.jochentopf.com/ for more information. There is also a thread
on the talk mailinglist at
https://lists.openstreetmap.org/pipermail/talk/2017-June/078203.html 
and this issue
https://github.com/osmlab/fixing-polygons-in-osm/issues/36 .
News of the effort are posted regularly to
https://github.com/osmlab/fixing-polygons-in-osm/issues/15 .

Jochen
-- 
Jochen Topf  jochen at remote.org  https://www.jochentopf.com/  +49-351-31778688