[OSM-talk] Fixing broken multipolygons

[Walter Nordmann]

Hi sarah,

i'm doing the missing boundaries task trying to fix as many bad boundary 
polygons as possible.

Working with osm2pgsql too i decided some month ago to disallow 
osm2pgsql  to automatically fix these geometries by adding 
--exclude-invalid-polygon. In the first days i got a lot of missings but 
the commiunity helped me to fix all of them :)

go on and disable as much of automatic "fixing" as possible. sooner or 
later these real errors - which are unvisible to end users - will be fixed.

see you in karlsruhe, walter


Am 15.02.2017 um 23:51 schrieb Sarah Hoffmann:
> Hi,
>
> let me add a bit of motivation to this. For osm2pgsql, the software
> that process the OSM data for rendering the map on osm.org, we are
> currently discussing about changing the algorithm that assembles the
> polygons[1]. The new algorithms will be a lot faster but that comes
> at the price that it is less tolerant with invalid geometries. A lot
> of bad geometries that are currently still drawn some way or another
> will be simply dropped. I'm convinced that in the long run this
> stricter handling will be good not only for data consumers but also
> for mappers, who will see immediately when they made a mistake.
> However, in the short run a switch from the old algorithm to the
> new one will leave a few bald patches on the map.
>
> There is a comparison map where you can see the changes:
>
> https://osmium.osm2pgsql.paulnorman.ca
>
> There are some notable holes, for example in the woods of
> Scandinavia. It would be great if they are gone by the time we
> switch the software.
>
> Sarah
>
>
> [1] https://github.com/openstreetmap/osm2pgsql/pull/684
>
> On Wed, Feb 15, 2017 at 04:20:53PM +0100, Jochen Topf wrote:
>> There are a lot of (multi)polygons in OSM that are broken in one way or
>> another. And we have to fix them. While some of the broken ones appear
>> on the map just fine, some don't appear and some mess up the map. And
>> some of those that appear fine on the main OSM map will not show up on
>> other maps where different software is used.
>>
>> A while ago I set up a web page at http://area.jochentopf.com/ and a
>> Github repository at https://github.com/osmlab/fixing-polygons-in-osm
>> devoted to that issue that I never announced properly. Go there and read
>> up in more detail where the problems are and how we are going to fix
>> them.
>>
>> Yesterday I launched several Maproulette challenges that allow you to
>> easily help with the "cleaning up" effort. Read
>> http://area.jochentopf.com/fixing.html for more details on those
>> Maproulette challenges. This is a huge effort that will take a long time
>> and we really need any help we get. The challenges posted today are only
>> the beginning. They only show the about 6,500 ways worldwide tagged as
>> buildings (with less than 100 nodes) where the way intersects itself.
>>
>> I have decided to start with a simple problem like this, to learn how
>> the Maproulette stuff works and how well, you, the community, responds.
>> Especially for beginners fixing those building is much easier than
>> starting with 10,000 node multipolygons with possibly multiple errors in
>> them. (If you want to, you can still do that. All multipolygon errors
>> show up in the OSM Inspector areas view at
>> http://tools.geofabrik.de/osmi/?view=areas )
>>
>> Jochen
>> -- 
>> Jochen Topf  jochen at remote.org  https://www.jochentopf.com/  +49-351-31778688
>>
>> _______________________________________________
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>> talk at openstreetmap.org
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