[OSM-dev] Polygons in OSM don't necessarily comply to simple feature specification

Christopher Schmidt crschmidt at metacarta.com
Wed Apr 16 22:41:40 BST 2008

On Wed, Apr 16, 2008 at 11:28:32PM +0200, Martijn van Exel wrote:
> Hi all,
> The only exceptions apparently  being that self-intersecting polygons
> are allowed in OSM. He found a  couple hundred in the Netherlands' OSM
> data.
> Is this something that is being considered? I guess it would be easy  
> to check for self-intersection upon adding / changing a polygon. 

"Easy"? I don't think that this is true... determining self-intersection
is hard.

Also, note that many many 'GIS' implementations also allow
self-intersecting polygons. Shapefiles, GML, etc. etc. and the tools
around them all 'allow' this insofar as there is typically no
technological limitation preventing it.

It seems likely that the netherlands 'self intersecting polygons' are
simply the result of poorly curated coastline imports or something
similar: in general, self-intersecting polygons aren't useful for
anything I can think of in OSM, but preventing it is hard, and not
really within the spirit of *most* simple geographic data tools. 

Christopher Schmidt

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