Implement HEALPix grids

Changelog.rst

@@ -1,10 +1,26 @@
 Version NEXTVERSION
--------------------
-
-**2025-12-??**
+--------------
 
+**2025-??-??**
+
+* Support for HEALPix grids
+  (https://github.com/NCAS-CMS/cf-python/issues/909)
+* New HEALPix methods: `cf.Field.healpix_info`,
+  `cf.Field.healpix_decrease_refinement_level`,
+  `cf.Field.healpix_increase_refinement_level`,
+  `cf.Field.healpix_indexing_scheme`, `cf.Field.healpix_to_ugrid`,
+  `cf.Domain.create_healpix`
+  (https://github.com/NCAS-CMS/cf-python/issues/909)
+* New method: `cf.Field.create_latlon_coordinates`
+  (https://github.com/NCAS-CMS/cf-python/issues/909)
+* New method: `cf.Data.coarsen`
+  (https://github.com/NCAS-CMS/cf-python/issues/909)
+* New function: `cf.locate`
+  (https://github.com/NCAS-CMS/cf-python/issues/909)
 * Reduce the time taken to import `cf`
   (https://github.com/NCAS-CMS/cf-python/issues/902)
+* New optional dependency: ``healpix>=2025.1``
+* Changed dependency: ``cfdm>=1.13.0.0, <1.13.1.0``
 
 ----
 
@@ -34,8 +50,8 @@ Version 3.18.1
 * Allow multiple conditions for the same axis in `cf.Field.subspace`
   and `cf.Field.indices`
   (https://github.com/NCAS-CMS/cf-python/issues/881)
-* Fix bug in `cf.Field.collapse` that causes a ``ValueError`` to be raised
-  for missing external cell measures data
+* Fix bug in `cf.Field.collapse` that causes a ``ValueError`` to be
+  raised for missing external cell measures data
   (https://github.com/NCAS-CMS/cf-python/issues/885)
 * New dependency: ``distributed>=2025.5.1``
 * Changed dependency: ``cfdm>=1.12.3.0, <1.12.4.0``

README.md

@@ -89,6 +89,7 @@ of its array manipulation and can:
 * create new field constructs in memory,
 * write and append field and domain constructs to netCDF datasets on disk,
 * read, create, and manipulate UGRID mesh topologies,
+* read, write, and manipulate HEALPix grids,
 * read, write, and create coordinates defined by geometry cells,
 * read netCDF and CDL datasets containing hierarchical groups,
 * inspect field constructs,
@@ -105,11 +106,12 @@ of its array manipulation and can:
 * manipulate field construct data by arithmetical and trigonometrical
   operations,
 * perform weighted statistical collapses on field constructs,
-  including those with geometry cells and UGRID mesh topologies,
+  including those with geometry cells, UGRID mesh topologies, and
+  HEALPix grids,
 * perform histogram, percentile and binning operations on field
   constructs,
-* regrid structured grid, mesh and DSG field constructs with
-  (multi-)linear, nearest neighbour, first- and second-order
+* regrid structured grid, UGRID, HEALPix, and DSG field constructs
+  with (multi-)linear, nearest neighbour, first- and second-order
   conservative and higher order patch recovery methods, including 3-d
   regridding, and large-grid support,
 * apply convolution filters to field constructs,

cf/__init__.py

@@ -119,7 +119,6 @@
 from .tiepointindex import TiePointIndex
 
 from .bounds import Bounds
-from .domain import Domain
 from .datum import Datum
 from .coordinateconversion import CoordinateConversion
 
@@ -138,6 +137,7 @@
 from .cellconnectivity import CellConnectivity
 from .cellmethod import CellMethod
 from .cellmeasure import CellMeasure
+from .domain import Domain
 from .domainancillary import DomainAncillary
 from .domainaxis import DomainAxis
 from .domaintopology import DomainTopology

cf/constants.py

@@ -493,6 +493,37 @@
     },
 }
 
+# --------------------------------------------------------------------
+# CF cell methods
+# --------------------------------------------------------------------
+cell_methods = set(
+    (
+        "point",
+        "sum",
+        "maximum",
+        "maximum_absolute_value",
+        "median",
+        "mid_range",
+        "minimum",
+        "minimum_absolute_value",
+        "mean",
+        "mean_absolute_value",
+        "mean_of_upper_decile",
+        "mode",
+        "range",
+        "root_mean_square",
+        "standard_deviation",
+        "sum_of_squares",
+        "variance",
+    )
+)
+
+
+# --------------------------------------------------------------------
+# CF HEALPix indexing schemes supported by cf
+# --------------------------------------------------------------------
+healpix_indexing_schemes = ("nested", "ring", "nuniq")
+
 
 # --------------------------------------------------------------------
 # Logging level setup