How to set geometry of feature as union of multiple geometries

Question

I want to organise my administrative geographies based on the GeoSPARQL standard. This question is moved by the obvious consideration that administrative units are hierarchical and nested. So for example, the geo:feature (see http://www.opengis.net/ont/geosparql#) Italy geo:hasGeometry IT

Then I have other 20 features corresponding to the 20 regions (first-level administrative divisions).

Each feature has a corresponding geometry (based on the ISO_3166-2:IT standard they are named: IT-65, IT-77, IT-78, etc.)

Let's say I have three levels of administrative divisions in Italy, so for ADM0 n=1, ADM1 n=20, ADM2 n=100 and ADM3 n=10000. In total, I should define 10121 geometries but, in practice, I only need 10000, since ADM0 geo:hasGeometry that corresponds to the union of the 20 geometries of the ADM1 features. How to formalise this in an RDF serialisation?

ex:Italy a geo:Feature
  geo:hasGeometry ex:IT 

or instead can I point to the multiple geometries

ex:Italy a geo:Feature;
  geo:hasGeometry ex:IT-65 ;
  ...
  geo:hasGeometry ex:IT-78 .

Would it be implicit in the second declaration that ex:Italy is not defined by any individual geometry but only by the full union of all the geometries?

Answer 1

Actually, the best way to get the union of geometries is the union function proposed by OGC in GeoSPARQL.

Here is the link to the documentation : GeoFunction UNION

Obviously, take care of the CRS as usual.