[OSM-dev] Moving to stricter multipolygon parsing

Yves yvecai at gmail.com
Fri Jun 13 07:53:50 UTC 2014


+1 for consistency
MP would be easier to learn from example if a single method 'works'.
Yves

On 13 juin 2014 01:25:42 UTC+02:00, Paul Norman <penorman at mac.com> wrote:
>Osm2pgsql currently tries *very* hard to turn multipolygon relations
>into
>geometries. It currently detects two types of MP relations, new-style
>and
>old-style. A new-style MP has tags on the relation while an old-style
>MP
>only has type=multipolygon on the relation and relies on the ways for 
>the tags.
>
>It then tries to deal with odd tagging in various ways. MP handling is
>one
>of the biggest sources of osm2pgsql bug reports[1] and a big time-sink.
>
>One of the bigger issues is moving tags from ways to MPs that are
>falsely
>detected as old-style. This is an attempt to interpret flawed tagging.
>
>I think we need to move to a more strict parsing of MPs, accepting only
>new-style MPs and old-style MPs where all outers have identical 
>non-deleted[2]
>tags and the relation itself has no non-deleted tags.
>
>Osm2pgsql is not just a consumer of data, it is one of the main
>feedback
>tools, so it is strongly integrated into the feedback cycle, so if 
>osm2pgsql
>doesn't process a multipolygon, a mapper will likely correct the
>tagging. By
>doing this, it will make it easier for those interpreting raw OSM data.
>
>To support this, I looked for some numbers. Using a shortened deleted
>tags
>list, there are 1 million new-style and 261k old-style MPs. Of the 
>old-style,
>256k have a member with role outer. 251k of these have entirely
>consistent
>tags on outers, while 2.3k have two sets of tags among the ways. About
>180
>have three or more.[3] An old-style MP without entirely consistent tags
>on
>outers is ambiguous and in error.
>
>[1]: 
>https://github.com/openstreetmap/osm2pgsql/search?q=multipolygon&type=Issues
>[2]: A deleted tag is one such as source that osm2pgsql is dropping
>[3]: https://gist.github.com/pnorman/ebd41f5a1759916a48b5
>
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