[OSM-talk] osm2pgsql multipolygon parsing

[Peter Wendorff]

Am 23.09.2013 11:59, schrieb Paul Norman:
>> From: Martin Koppenhoefer [mailto:dieterdreist at gmail.com]
>> Subject: Re: [OSM-talk] osm2pgsql multipolygon parsing
>>
>> it has a different meaning. tags on a closed way are for the whole area
>> inside the way, tags on a mp relation are for the area of the outer
>> minus the inner ways.
> 
> Unless the closed way is a member of a multipolygon relation with no other
> tags on the relation - then you'll have a resulting area with a hole. This
> is a very well established means of tagging areas with holes (~22% of
> type=multipolygon relations have no other tags).
It's well established, but it's less correct from a theoretical point of
view, and establishment is produced here by software that supports wrong
tagging - in the past.
You propose to leave it as it is, which means software should estimate
the meaning because often there are errors in the tagging, while I
proposed (and as far as I understand at least Martin supports that) not
to ignore these errors with the goal to teach mappers about what's
correct instead of teaching them what's correct enough for application X
(be it the default mapnik style and -pipeline).

> The issues raised originally in the ticket are best explored through
> examples
> 
> The first case is a way with natural=water and a MP relation with no other
> tags. Both old osm2pgsql (0.82) and latest master version from git create an
> area with a hole with natural=water on the area. There are no suggestions of
> changing this.
Problem:
Let's say the multipolygon relation was tagged before as depth=shallow
(and someone wanted to mark parts of the water as not deep - e.g. not
deep enough for boats). Let's say another mapper came along and found
this strange tags and - removing them - accidently left a multipolygon
without tags. This changes the meaning of stuff never touched again, as
you would now count the natural=water only for the area covered by the
multipolygon, excluding the hole.

> The second is a way with natural=water and a MP relation with name=foo (with
> the way as a member). Old osm2pgsql created an area with a hole with
> natural=water, name=foo, latest master does too.
What if the inner part does NOT have that name, but only the outer ring?
What if that is what somebody wanted to say?

regards
Peter