controlSurface must contain a gml:Polygon element
gml:Polygon must have "srsName" attribute set
Polygon exterior must contain a gml:Ring or a gml:LinearRing
At least one segment must be a gml:Bezier, gml:LineStringSegment, gml:GeodesicString.
On other segment allowed besides gml:Bezier, gml:LineStringSegment, gml:GeodesicString.
The interior must contain at least one a gml:Ring or a gml:LinearRing.
Degree of the Bezier must be "3"
Beziers must have exactly two knots
The value of the first knot must be 0.0
The value of the second knot must be 1.0
Multiplicity of each must be degree+1 (=4)
Weight of each knot must be 1.0