While creating GML files and download services for a statistical grid in the Central Statistical Office of Poland we have encountered the following problem:

In the Statistical Units data model the geometry attribute of StatisticalGridCell is defined as GM_Polygon. See: http://inspire.ec.europa.eu/data-model/approved/r4618-ir/html/index.htm?goto=2:3:19:2:8823

However according to ISO 19107 GM_Polygon is a specialization of the GM_SurfacePatch abstract type. ISO definitions state the following:

"A GM_Polygon is a surface patch that is defined by a set of boundary curves and an underlying
surface to which these curves adhere."

"GM_SurfacePatch defines a homogeneous portion of a GM_Surface"

See: http://inspire-twg.jrc.ec.europa.eu/data-model/draft/r2563/index.html?goto=1:3:6:1:1:899

Since GM_Polygon is not an independent type it cannot be used as value type for geometry of StatisticalGridCell. Therefore WFS using the current SU schema does not generate geometry for features.

To enable GML file creation and WFS publishing we modified the schema changing the Geometry attribute's value type to GM_Surface. I think this also should be modified in the Technical Guidelines for SU.