[OSM-dev] multipolygon relation and non-closed ways

[Frederik Ramm]

Hi,

On 13.10.2012 21:35, Pedro Larroy wrote:
> Thanks for the link. I have updated the wiki with requirements for
> valid multipolygon relations:
>
> http://wiki.openstreetmap.org/wiki/Relation:multipolygon#Valid_Multipolygon_conditions

I made a few changes to your list in the Wiki. (Frankly I am surprised 
that you thought the list was necessary, as I believed everything to be 
explained properly already.)

First of all, the "role" is not actually required to determine validity, 
and I have removed your references of "... belonging to the same role". 
The algorithm laid out on the multipolygon page takes care of any valid 
multipolygon whatever the role; and algorithms relying on the role are 
broken. The use of the inner/outer roles is recommended because it adds 
redundancy and will often make it clearer what the mapper intended to 
do, but these roles are not a condition for validity.

Also, the ways in a mutlipolygon relation do not have to form "a closed 
chain" (your words) but "one or more closed chains".

You wrote that inner polygons "should" not overlap with outers; I 
changed that to "must" because the polygon is invalid otherwise.

Also, your rule that no more than two unclosed ways may meet in a point 
is not correct in the case of touching inner rings, where any even 
number of unclosed ways might touch in one point.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"