The HEALPix grid

healpix-plot uses healpix-geo internally for all HEALPix coordinate conversions. The healpix-geo documentation covers everything you need to know about the HEALPix grid: resolution levels, indexing schemes (nested, ring, zuniq), ellipsoids, and coordinate conversions.

Parameter	Type	Description
level	int	HEALPix depth. Must be in [0, 29].
indexing_scheme	str	Cell numbering scheme: "nested", "ring", or "zuniq".
ellipsoid	str or dict	Reference ellipsoid: "sphere" (default), "WGS84", or a custom dict with radius (sphere) or semimajor_axis + inverse_flattening (ellipsoid).

The HealpixGrid object

In healpix-plot, you describe your data with a HealpixGrid object that wraps the healpix-geo parameters:

import healpix_plot

grid = healpix_plot.HealpixGrid(
    level=4,  # resolution level, 0-29
    indexing_scheme="nested",  # "nested", "ring", or "zuniq"
    ellipsoid="sphere",  # default; "WGS84" for geodesic accuracy
)

It exposes a .operations property that delegates directly to healpix-geo:

import numpy as np

cell_ids = np.arange(12 * 4**grid.level, dtype="uint64")
lon, lat = grid.operations.healpix_to_lonlat(cell_ids, **grid.as_keyword_params())

For everything else — what levels mean, how indexing schemes differ, ellipsoid support — refer to the healpix-geo documentation.