[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.

--
View this message in context: http://gis.19327.n5.nabble.com/New-OGR-driver-to-read-OpenStreetMap-osm-pbf-files-tp5715906p5716533.html
Sent from the Developer Discussion mailing list archive at Nabble.com.



More information about the dev mailing list