Terminology#

HEALPix grid#

HEALPix#: Hierarchical Equal Area isoLatitude Pixelization. Divides the sphere (or an ellipsoid) into equal-area cells identified by (refinement level, cell id).
DGGS#: Discrete Global Grid System: a hierarchy of grids covering the whole globe. HEALPix is the DGGS used by GRID4EARTH.
Refinement level#: HEALPix resolution parameter. At level k there are 12 × 4^k cells. Range: 0 (12 cells) to 29.
Cell ID#: Integer identifier of a HEALPix cell. Its meaning depends on the indexing scheme (nested, ring or zuniq).
Nested scheme#: HEALPix indexing where the ids of the children cells are derived from the id of their parent cell. Required for chunking the output along HEALPix cells.
Ring scheme#: HEALPix indexing ordered by iso-latitude rings.
Zuniq scheme#: Unique identifier encoding both the refinement level and the cell id in a single integer.
WGS84#: World Geodetic System 1984: the reference ellipsoid of GPS and of the EPSG:4326 coordinate reference system.

Conversion#

Spatial group#: A group of the input Zarr datasets with spatial data (projected x / y or longitude / latitude coordinates), converted to HEALPix.
Staging cache#: The information parsed from the input datasets and the conversion settings (spatial information, groups to convert, output extent), reused by all the conversion steps. See ConvertStagingCache.
Chunk cell#: A HEALPix cell at a coarser refinement level than the output data, whose children cells form one chunk of the output arrays.
Chunk buffer#: Width added around a chunk cell when selecting the input data points to resample onto the cells of that chunk.
Multiscale group#: A group of the input Zarr dataset(s) holding several children “scale” groups, each converted at its own HEALPix refinement level (see multiscale_settings in Conversion settings).
Pyramid multiscale group#: A multiscale group of the output Zarr dataset holding the levels converted from the input data (native levels) and the coarser levels derived from them (derived levels). Its multiscales metadata describes them all. See generate_pyramids().
Derived level#: A refinement level of a pyramid multiscale group that is aggregated from its native levels, as opposed to being converted from the input data.
Derived level group#: A group holding one derived level, named after its refinement level. It is a child of the pyramid multiscale group, next to the native level groups, or of a group of its own within it (the subgroup_name pyramid setting).
Native level#: A refinement level of a multiscale group that is converted from the input data, as opposed to the coarser levels derived from it.
Parent cell#: The HEALPix cell of a coarser refinement level that contains a given cell (its ancestor in the nested scheme). Each cell of a derived level aggregates all the converted cells it is the ancestor of.
Resampler#: The method that computes the values of the output HEALPix cells from the input data points (nearest, bilinear, k-nearest neighbors, PSF, cell-point), provided by healpix-resample.

Metadata#

EOPF#: ESA’s Earth Observation Processing Framework, whose Zarr products are the main input of healpix-convert.
STAC#: SpatioTemporal Asset Catalog. The stac_discovery attribute of the input and output datasets is a STAC item.
Zarr conventions#: Community conventions for structuring metadata in Zarr datasets (https://github.com/zarr-conventions). healpix-convert writes the DGGS and Multiscales conventions.
CF conventions#: Climate and Forecast metadata conventions. Version 1.13 defines a HEALPix grid mapping.