| interpolation | 
                                    gml:SurfaceInterpolationType | 
                                    planar | 
                                     | 
                                    optional | 
                                    
                                        The attribute "interpolation" specifies the interpolation mechanism used for this surface patch. Currently only planar surface
                                          patches are defined in GML 3, the attribute is fixed to "planar", i.e. the interpolation method shall return points on a single
                                          plane. The boundary of the patch shall be contained within that plane.
                                        
                                     |