[postgis-users] Intersection errors

[Markus Schaber]

Hi, Ethan,

Ethan Alpert wrote:
> 
> While we're on the topic of multipolygons. I've noticed that the
> shapefile specification doesn't have any convention for communicating
> outer and inner rings. That said how do apps like shp2pgsql and ogr2ogr
> determine whether the next polygon in a list is an inner or a new outer?

If you need a really stable algorithm, current JTS releases contain a
polygonize function that converts arbitrary linework into polygons. This
should work without any dependencies on ring order or clockwise
ordinates, but I suspect this to be rather inefficient.

Markus