Description: The Reference Framework is not a thematic sphere. It holds what other data is located by. There are two kinds of reference:
Coordinate frames: coordinate reference systems and the geodetic points that realise them. Data with its own coordinates is positioned through these.
Reference units: stable identities that other data refers to by ID, such as municipalities, NUTS regions, statistical units and addresses. Non-spatial data, for example a table of population per municipality, carries only the ID and borrows the geometry from here.
One identity, many geometries. A reference unit keeps its ID while its geometry exists in several versions for different scales. The Danish municipality 0101 has a full-resolution DAGI boundary and a generalised Eurostat LAU boundary at 1:1M. The NUTS region DK01 exists at 1:1M, 1:3M, 1:10M, 1:20M and 1:60M. Reference realisations record their ID scheme and their geometries per scale and version (reference_units). Data that refers to them records which unit and ID scheme it uses (anchored_by).
Subclasses (Complexes):
reference_crs: Coordinate reference systems and vertical datums (e.g., EPSG:25832, DVR90, ETRS89-LAEA).
reference_geodetic_control: Physical points that realise the reference systems (e.g., geodetic control points, benchmarks, GNSS reference stations).
reference_administrative_units: Units of governance with a legal ID (e.g., Danish regions, municipalities and parishes from DAGI).
reference_statistical_units: Units defined for reporting statistics (e.g., NUTS, LAU, census tracts, statistical grids).
reference_addresses: Official addresses and access points, the finest unit most registers refer to.
reference_grids_map_sheets: Systematic partitions used to index and tile space (e.g., map sheet divisions, tiling schemes, DGGS indices).
Common Overlaps & Distinctions:
Frame vs Theme: A leaf belongs here when its purpose is to locate or frame other data, not to describe a phenomenon. A DTM describes the land surface, so it belongs to geosphere_landform_topography.
Not imagery: Orthophotos and satellite imagery are a collection method (passive remote sensing), not a reference. See Passive Remote Sensing and the Orthoimagery leaf.
Authority: Reference units are mandated by authorities (SDFI, Danmarks Statistik, Eurostat). Leaves here often also belong to anthroposphere_governance.
Statistical vs indexing grids: A grid used to report statistics (e.g., population per km² cell) is a statistical unit. A grid used to index or tile space belongs to Grids and Map Sheets.