[geos-devel] OverlayNG for Testing

[Paul Ramsey]

> Some things we already know don't work:
> 
> - 3D! GEOS uniquely has supported adding in Z coordinates to introduced coordinates, and that code hasn't been added to JTS (and thus overlayng). How this will affect existing expectations for 3D output is unknown, but it's sure to be broken in some way.
> - Union on linestrings! OverlayNG extracts maximal length linestrings from the unioned graph. Old overlay extracted minimal length linestrings. This change is probably "for the best" but it's a change which might impact existing logic.

In general linestring output. 

select st_astext(st_difference('LINESTRING(-11.1111111 70,70 -11.1111111)'::geometry, 'LINESTRING (20.1 90, 20.1 -90)'::geometry));

                         st_astext                         
-----------------------------------------------------------
 LINESTRING(-11.1111111 70,20.1 38.7888889,70 -11.1111111)

The old overlay would output a MULTILINESTRING. This has broken PostGIS old split hack, which depends on the output of a collection to carry out splitting.