[OSM-dev] Moving to stricter multipolygon parsing

[Peter Wendorff]

+10
if we could enforce the strict usage in multipolygon relations this
might as well be a step forward to a future area datatype as it would
straighten the definition of how areas are defined currently, and start
by a less ambiguous definition for the subset of areas described by
multipolygon relations.

regards
Peter

Am 13.06.2014 01:25, schrieb Paul Norman:
> 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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