Explain use of DGGS to represent extent of a resource in metadata

[PeterParslow]

Some times a good way to express the extent of a resource could be by reference to an area defined by a DGGS grid element(s). Where this is a geographic extent, would it be a polygon or identifier i.e. EX_BoundingPolygon or EX_GeographicDescription, or something else, to be defined during revision?

@rob-metalinkage or @geofizzydrink may be good people to contribute; Rob suggested it during ISO 19115-4 project meeting

[geofizzydrink]

Referencing by DGGS index references can be expressed in a number of different ways.

As per the core data model defined in ISO 19170-1, each DGGS zone represents a specific spatio-temporal region of space time. This ranges from 1D, 2D and 3D Pure spatial regions to 1D + time, 2D + time and 3D + time spatio-temporal regions.

So the expression of data via DGGS referencing in a metadata record is not as simple as just defining a simple geometry object, because that will be dependant on a number of factors related to the structure of the DGGS in question.

As a general statement, data that has been indexed to a DGGS can be expressed as:

1. A direct position point location centred on the centroid of the DGGS zone;
2. An unbounded area representation. In other words, an area statement that is not constrained by a specific geometry. E.g. extent = 45 m^2 for a 2D DGGS; extent = 23.5 m^3 for a 3D DGGS; or, extent = 123.23 m^2s for a 2D + time DGGS.
3. A bounding geometry that defines the shape of the zone. E.g. a polygon object, a prism or polyhedron object, or a tesseract that describes the spatial geometry plus a 4-space expression of a time interval.

Granted for most implementations of DGGSs we are talking in either 2D or 3D spatial contexts with the temporal dimension addressed as a data/feature parameter. So most of the time we will be taking about expressions of points, polygons or prisms/polyhedra.

So if someone wanted to minimise the complexity of a metadata record expressing DGGS referencing, I would suggest to purely reference by the zone identifier and a link/reference (possibly even expressed as a uri or curie path as part of the identifier name parameter) to the DGGS resource (e.g. via the OGC API DGGS) that contains the full structural definition of that DGGS zone.