"OGC EPSG Coordinate Reference Systems Register" too narrow?

[heidivanparys]

The specification mentions the "OGC EPSG Coordinate Reference Systems Register" in several places, such as the descriptions of the geodcatap:referenceSystem property.

Entity	Property	Range	Property label	Property definition	Property usage note
dcat:Catalog	geodcatap:referenceSystem	dct:Standard	reference system	The reference system used in accordance with OGC EPSG Coordinate Reference Systems Register [[OGC-EPSG]].
dcat:Distribution	geodcatap:referenceSystem	dct:Standard	reference system	The reference system used in the Distribution.	The reference system used in accordance with OGC EPSG Coordinate Reference Systems Register [[OGC-EPSG]].
dcat:DataService	geodcatap:referenceSystem	skos:Concept	reference system	The reference system used in the Data Service.	Spatial reference systems SHOULD be specified by using the corresponding URIs from the "EPSG coordinate reference systems" register operated by OGC [[OGC-EPSG]].
dcat:Dataset	geodcatap:referenceSystem	skos:Concept	reference system	The reference system used in accordance with OGC EPSG Coordinate Reference Systems Register [[OGC-EPSG]].
dcat:DatasetSeries	geodcatap:referenceSystem	skos:Concept	reference system	The reference system used in accordance with OGC EPSG Coordinate Reference Systems Register [[OGC-EPSG]].	Spatial reference systems SHOULD be specified by using the corresponding URIs from the "EPSG coordinate reference systems" register operated by OGC [[OGC-EPSG]].

The OGC CRS register contains more CRS descriptions than the ones defined in IOGP’s EPSG Geodetic Parameter Dataset:

The OGC CRS register concentrates on providing a machine readable interface. For the entries defined in the EPSG register, it serves as a URI resolver to those entries in their IOGP source registry. OGC also includes CRSs defined in its own Standards and CRSs that OGC Member organizations request OGC to manage.

(Source: Guide to Coordinate Reference System (CRS) Resources)

Several of the examples in the GeoDCAT-AP specification refer to CRS http://www.opengis.net/def/crs/OGC/1.3/CRS84, so I suppose the intention is not to restrict the CRSs to only the ones defined by IOGP?

If the use of URIs defined in the OGC CRS register is only a recommendation, it should be in the usage note for every description of geodcatap:referenceSystem. Currently, the definitions of geodcatap:referenceSystem in dcat:Catalog, dcat:Dataset and dcat:DatasetSeries seem to require that those URIs are used.

I would also suggest to add a link to the Guide to Coordinate Reference System (CRS) Resources somewhere in the specification (this guide is referred to from the front page of the ISO Geodetic Registry).

[jakubklimek]

Currently, the OGC EPSG Coordinate Reference Systems Register [OGC-EPSG] codelist is what MUST be used with geodcatap:referenceSystem, as per https://semiceu.github.io/GeoDCAT-AP/drafts/3.1.0/#properties-with-controlled-vocabularies-that-must-be-used-for-the-listed-properties.

It is true that http://www.opengis.net/def/crs/OGC/1.3/CRS84 is not listed in there, so this is an inconsistency.

It is, however, a question to the community, whether the examples should be changed to the ones from the registry, or whether this requirement should be loosened and the codelist should be only recommended.

[heidivanparys]

Currently, the OGC EPSG Coordinate Reference Systems Register [OGC-EPSG] codelist is what MUST be used with geodcatap:referenceSystem, as per https://semiceu.github.io/GeoDCAT-AP/drafts/3.1.0/#properties-with-controlled-vocabularies-that-must-be-used-for-the-listed-properties.

The specification is inconsistent on this point:

• https://semiceu.github.io/GeoDCAT-AP/drafts/3.1.0/#properties-with-controlled-vocabularies-that-must-be-used-for-the-listed-properties states indeed that CRSs from http://www.opengis.net/def/crs/EPSG/0/ must be used.
• According to A.7.23 Coordinate reference systems and Temporal reference systems – *Reference System:
  • the use of the OGC EPSG Coordinate Reference Systems Register codelist is recommended, not required;
  • specifying reference systems in other ways than via HTTP URIs is allowed;
  • geodcatap:referenceSystem can be used to specify temporal reference systems too (not present in the OGC EPSG Coordinate Reference Systems Register codelist).

  In GeoDCAT-AP, these elements are mapped to property geodcatap:referenceSystem. Moreover, in order to indicate which type of a reference system the object of geodcatap:referenceSystem denotes, an additional statement with predicate dct:type is added, with a code list value defining the notion of (spatial / temporal) reference system, taken from the glossary operated by the INSPIRE Registry
  [...]
  The reference system identifier SHOULD be preferably represented with an HTTP URI. In particular, spatial reference systems should be specified by using the corresponding URIs from the “EPSG coordinate reference systems” register operated by the Open Geospatial Consortium
  [...]
  In case the reference system is not specified as an HTTP URI, the mapping covers [...] If not represented with an HTTP URI, the reference system identifier MUST be mapped to dct:identifier [...]

• The descriptions of the geodcatap:referenceSystem properties, including whether it is a requirement or recommendation to use the OGC EPSG Coordinate Reference Systems Register codelist, are inconsistent:
  

[heidivanparys]

It is, however, a question to the community, whether the examples should be changed to the ones from the registry, or whether this requirement should be loosened and the codelist should be only recommended.

There are a lot of good arguments to require the use of other CRSs than http://www.opengis.net/def/crs/OGC/1.3/CRS84 in quite some use cases. However, in my view, a requirement like that does not belong in a standard for describing spatial datasets, but belongs e.g. in a data product specification. Excluding http://www.opengis.net/def/crs/OGC/1.3/CRS84 as a valid value would mean that datasets that are actually stored in http://www.opengis.net/def/crs/OGC/1.3/CRS84 cannot be described in a conformant way with GeoDCAT-AP.

Note that, according to https://www.w3.org/TR/sdw-bp/#bp-crs:

[alitka]

Perhaps to be changed to: If the resource to be described has an absolute spatial reference, then the specification of the reference system as HTTP URI identifier is mandatory, provided that the identifier to be specified is listed at http://www.opengis.net/def/crs/OGC/1.3/ or http://www.opengis.net/def/crs/EPSG/0/.

The wording is similar to the INSPIRE TG MD, section 3.2.1.1. Coordinates reference systems

TG Requirement 2.2: metadata/2.0/req/isdss/crs-id
If the coordinate reference system is listed in the table Default Coordinate Reference System Identifiers in Annex D.4, the value of the HTTP URI Identifier column shall be used as the value of gmd:referenceSystemInfo/gmd:MD_ReferenceSystem/gmd:referenceSystemIdentifier/gmd:RS_Identifier/gmd:code element.