g_centroid()
returns a vector of point X, point Y.
Details
The GDAL documentation states "This method relates to the SFCOM
ISurface::get_Centroid()
method however the current implementation based
on GEOS can operate on other geometry types such as multipoint, linestring,
geometrycollection such as multipolygons. OGC SF SQL 1.1 defines the
operation for surfaces (polygons). SQL/MM-Part 3 defines the operation for
surfaces and multisurfaces (multipolygons)."