[OSM-dev] Coastlines and water - contact and technical

[Ben Supnik]

Hi Jon,

Jon Burgess wrote:

>> ... if a user incorrectly tries to make a lake by creating a 
>> multipolygon and putting the water-related attribute (e.g.
>> landuse=water 
>> or natural=lake) on the relation and __not__ at least one outer
>> polygon
>>
>> ... then pgsql will create a polygon without those water tags.

> No. If the relation is tagged then the tags on the outer ways are
> ignored. If the user puts all the relevant tags on the relation instead
> of the outer ways then osm2pgsql is perfectly happy with that and should
> do the right thing.

Sorry to ask the same thing about 6 ways...I think my brain be crossed. 
  Is this a situation where:

(a) the "spec" allows the tagging of area attributes on the 
multi-polygon relation and pgsql understand this or...

(b) the "spec" disallows the tagging of area attributes on multi-polygon 
relations, but people do it anyway, so pgsql is lenient and grabs the 
data anyway to try not to lose the features?

I realize it is a slightly moot question...since my goal is to reproduce 
what I see on the map, if pgsql produces sane polygons from meta data on 
the relation, then that's what users see when they browse the map.  But 
in the long term I could provide some validation to my user base to 
limit how much the practice continues.

> If you have specific relation or way IDs then I can lookup how they have
> been handled.

I do not yet...I found one multipolygon water body in an area I am 
familiar with for the purpose of debugging...but I am processing the 
entire planet file so...I could pretty easily produce a list of 
multipolygons where the relation passed the attribute filter that would 
have been used on the outer ways, but the ways did not.

For that matter, if a multipolygon relation has multiple outer ways (for 
exapmle, the outer boundary has for some reason been split) and there 
are conflicting outer boundary tags, how is that case handle?  (E.g. one 
is landuse=water and one is landuse=forest.)

cheers
Ben


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