[OSM-dev] (Multi)Polygon handling

nimix melchiormoos at gmail.com
Fri Jul 13 17:31:17 BST 2012

Frederik Ramm wrote
> On 07/13/2012 04:59 PM, nimix wrote:
>> If I remember it right mapnik requires normalized geometries, so outer in
>> clockwise and inners in counterclockwise order. I'm not sure if that is
>> still the case, but osm2pgsql still does the normalization. That's why I
>> defined the result in clockwise orientation...
> Mapnik doesn't require that as far as I know (have to check though). 
> However, if you e.g. create a shape file and open that in ArcGIS and 
> compute the polygon area, it will come up with negative areas if your 
> polygon goes the wrong way round.

Hi, I just has a look at this... Mapnik indeed has no such requirement. But
it requires inner rings of multipolygons do be in a different orientation
than the outer ring. Hence it is reasonable to ensure that by just making
all outers cw and inners ccw.

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