ISO 19115 defines two ways to define the vertical coordinate reference system in a EX_VerticalExtent:
- verticalCRS which is an association to ISO 19111 SC_VerticalCRS.
- verticalCRSId which is an association to the simpler MD_ReferenceSystem.
GeoAPI intentionally omits the MD_ReferenceSystem type in order to have a single framework for handling coordinate reference systems, namely ISO 19111. Consequently GeoAPI VerticalExtent interface (and Apache SIS DefaultVerticalExtent class) omits the verticalCRSId property. We will not add that property in public API, but we should at least be able to recognize it at unmarshalling with a private JAXB method.