[OSM-talk] Multipolygon fixing effort done

Jochen Topf jochen at remote.org
Tue Aug 29 13:35:46 UTC 2017

On Tue, Aug 29, 2017 at 12:10:47PM +0200, Christoph Hormann wrote:
> Date: Tue, 29 Aug 2017 12:10:47 +0200
> From: Christoph Hormann <osm at imagico.de>
> To: talk at openstreetmap.org
> Subject: Re: [OSM-talk] Multipolygon fixing effort done
> On Tuesday 29 August 2017, Jochen Topf wrote:
> > We have completed the 7-months effort to switch away from old-style
> > multipolygons and fix a lot of broken (multi)polygons. More about
> > this on my blog:
> >
> > https://blog.jochentopf.com/2017-08-28-polygon-fixing-effort-conclude
> >d.html
> First of all congratulations to what was achieved, i.e. a fairly massive 
> reduction in the number of errors.
> The sad news however seems to be that given the current circumstances 
> the number of errors will likely be back to near the pre-cleanup levels 
> in 2-3 years for many types of errors.
> From my point of view this is because in contrast to the old style 
> multipolygons where
> * the problem was fully eliminated in the data
> * the most important data user (the standard map) was changed to not 
> interpret old style MPs any more after that
> * the major editors had already ceased to generate old style MPs long 
> ago
> the circumstances that lead to the large number of broken multipolygons 
> are essentially unchanged.  We certainly got rid of a number of errors 
> from bad imports and can hope that future imports will be better in 
> that regard but the problem that mappers introduce this kind of error 
> in manual mapping and don't realize they are making an error is 
> unchanged.

What has changed is that now without the complication of the old-style 
MPs it is easier to check for correctness of the data in editors and
other tools. I hope that especially editor developers are looking at
this problem more closely to identify common problems that can be fixed
by better UI or better checking on their code.

> Of the points above both completely eliminating MP geometry errors and 
> changing the editors not to upload broken geometries are things that 
> are very hard to accomplish.  This leaves the third point and therefore 
> my question:
> Would the number of visible problems in the map due to dropping broken 
> geometries now, after the fixing effort, be low enough so this change 
> could be made to the main map to give mappers a better feedback about 
> broken geometries?

Osm2pgsql is switching to the libosmium MP code which is more strict
than the older code before. As far as I know the code is finished and
should be in the next osm2pgsql version.

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

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