Terminology#

Data formats#

HEALPix#: Hierarchical Equal Area isoLatitude Pixelization. Divides the sphere into equal-area cells identified by (level, cell_id). healpix-plot uses healpix-geo for all HEALPix ↔ lon/lat conversions.
Cell ID#: Integer identifier for a single HEALPix cell. Its meaning depends on the indexing scheme (nested, ring, or zuniq).
Level#: HEALPix resolution parameter. At level k there are 12 × 4^k cells. Range: 0 (12 cells) to 29 (~3 × 10^17 cells).
Nested scheme#: HEALPix indexing where child cells have IDs derived from their parent. Efficient for hierarchical operations.
Ring scheme#: HEALPix indexing ordered by iso-latitude rings. Required by some harmonic-transform libraries (e.g. HEALPy).
Zuniq scheme#: Compact unique identifier encoding both level and cell ID in a single integer.
WGS84#: World Geodetic System 1984 — the standard GPS ellipsoid (EPSG:4326).

Sampling grid#: The regular pixel grid that HEALPix data is resampled onto before rendering. Described by a shape (pixels), a spatial extent (degrees), and optionally an affine transform.
Nearest-neighbour resampling#: Each output pixel takes the value of the closest HEALPix cell centre. Fast, preserves sharp boundaries.
Bilinear resampling#: Each output pixel is a distance-weighted blend of neighbouring cell values. Slower, produces smoother output for continuous fields.
Aggregation#: When multiple cell IDs map to the same cell, their values are combined with a reduce function (mean, max, min, sum, …) before resampling.
Background value#: Fill value used for output pixels that fall outside the spatial coverage of the input cell_ids.

Cartopy CRS#: Coordinate Reference System object from the cartopy library.
Mollweide#: Equal-area pseudocylindrical map projection.
PlateCarree#: Simple equirectangular projection (lon/lat as x/y).