Clarify the replacement for MD_ReferenceSystem

[desruisseaux]

ISO 19115-1 figure 12 defines MD_ReferenceSystem with two properties: an Identifier and a MD_ReferenceSystemTypeCode (a code list - see #50). A note said: "Refer to SC_CRS in ISO 19111 when coordinate reference system information is not given through reference system identifier." However there is no details about how to reference ISO 19111. In particular MD_ReferenceSystem has no field for SC_CRS.

In order to allow reference to ISO 19111 and also in order to reduce overlapping between standards, GeoAPI 3.0 uses ISO 19111 RS_ReferenceSystem instead of ISO 19115 MD_ReferenceSystem. It allows us to reference ISO 19111 since SC_CRS is a subtype of RS_ReferenceSystem. If user wants to specify only the identifier, this is possible by using ReferenceSystem.getIdentifiers(). There is currently no way to get the CRS type however, except the way described in #50.

We should clarify the following:

• In Java, should we keep doing that way in GeoAPI 4.0, or should we stick to the ISO 19115 way? (at the cost of adding more types in GeoAPI and extend ISO 19115 e.g. by adding an optional field holding the SC_CRS).
• In Python, the type is currently unspecified. Should we keep it unspecified? ReferenceSystem type appears in:
  • base.py: Metadata.reference_system_info (as a sequence)
  • representation.py: GCPCollection.coordinate_reference_system
  • lineage.py: Source.source_reference_system

[desruisseaux]

Proposal applied on Metadata.reference_system_info:

Description of the spatial and temporal reference systems used in the dataset. The reference system may be:

• An ISO 19111 object such as CoordinateReferenceSystem.
• A ReferenceSystem with the identifier property (from ISO 19111) sets to a list of Identifier values such as ["EPSG::4326"].
• An object with the referenceSystemIdentifier property (from ISO 19115) sets to a single Identifier value such as "EPSG::4326", optionally with a referenceSystemType property sets to a value such as geodeticGeographic2D, compoundProjectedTemporal, etc.

[desruisseaux]

ISO 19115-1:2014/Amd 2:2020 adds the following properties in ReferenceSystem:

• crs of type ISO 19111 CRS.
• coordinateEpoch

If we keep the current GeoAPI approach, which is to use RS_ReferenceSystem as the base class of CRS instead of using MD_ReferenceSystem as a container class for a crs property, then above ISO 19115/Amd 2 does not apply. See #77 for discussion about those amendments.