[OSM-dev] (Multi)Polygon handling

Frederik Ramm frederik at remote.org
Fri Jul 13 16:06:17 BST 2012


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.

I don't know if that is an ESRI curiosity or following the standard, and 
if so, what standard. If it should turn out to be a convention for shape 
files but not for Simple Features generally, then any program that 
creates shape files should fix the winding order.


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

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