From 89fbb67987a78b9b9f5bec8768613a89d1123aa4 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Mon, 2 Jun 2025 19:20:00 +0100
Subject: [PATCH 1/8] dev

---
 Changelog.rst                |   2 +
 cf/__init__.py               |   1 +
 cf/cfimplementation.py       |   3 +
 cf/field.py                  |   3 +-
 cf/fieldancillary.py         |  39 +++++-
 cf/quantization.py           |  59 +++++++++
 cf/test/test_Quantization.py |  42 +++++++
 cf/test/test_quantization.py | 231 +++++++++++++++++++++++++++++++++++
 8 files changed, 377 insertions(+), 3 deletions(-)
 create mode 100644 cf/quantization.py
 create mode 100644 cf/test/test_Quantization.py
 create mode 100644 cf/test/test_quantization.py

diff --git a/Changelog.rst b/Changelog.rst
index c8baf9913b..9f6ab30322 100644
--- a/Changelog.rst
+++ b/Changelog.rst
@@ -3,6 +3,8 @@ Version NEXTVERSION
 
 **2025-??-??**
 
+* Implement lossy compression via quantization
+  (https://github.com/NCAS-CMS/cf-python/issues/870)
 * Read Zarr datasets with `cf.read`
   (https://github.com/NCAS-CMS/cf-python/issues/863)
 * Update CF aggregation keywords
diff --git a/cf/__init__.py b/cf/__init__.py
index 2df23b25e5..4004955cbb 100644
--- a/cf/__init__.py
+++ b/cf/__init__.py
@@ -241,6 +241,7 @@
 from .nodecountproperties import NodeCountProperties
 from .partnodecountproperties import PartNodeCountProperties
 from .interiorring import InteriorRing
+from .quantization import Quantization
 from .tiepointindex import TiePointIndex
 
 from .bounds import Bounds
diff --git a/cf/cfimplementation.py b/cf/cfimplementation.py
index ece90bde2c..543a1fca9b 100644
--- a/cf/cfimplementation.py
+++ b/cf/cfimplementation.py
@@ -23,6 +23,7 @@
     List,
     NodeCountProperties,
     PartNodeCountProperties,
+    Quantization,
     TiePointIndex,
 )
 from .data import Data
@@ -147,6 +148,7 @@ def set_construct(self, parent, construct, axes=None, copy=True, **kwargs):
     H5netcdfArray=H5netcdfArray,
     NetCDF4Array=NetCDF4Array,
     PointTopologyArray=PointTopologyArray,
+    Quantization=Quantization,
     RaggedContiguousArray=RaggedContiguousArray,
     RaggedIndexedArray=RaggedIndexedArray,
     RaggedIndexedContiguousArray=RaggedIndexedContiguousArray,
@@ -203,6 +205,7 @@ def implementation():
      'H5netcdfArray': cf.data.array.h5netcdfarray.H5netcdfArray,
      'NetCDF4Array': cf.data.array.netcdf4array.NetCDF4Array,
      'PointTopologyArray': <class 'cf.data.array.pointtopologyarray.PointTopologyArray'>,
+     'Quantization': cf.quantization.Quantization,
      'RaggedContiguousArray': cf.data.array.raggedcontiguousarray.RaggedContiguousArray,
      'RaggedIndexedArray': cf.data.array.raggedindexedarray.RaggedIndexedArray,
      'RaggedIndexedContiguousArray': cf.data.array.raggedindexedcontiguousarray.RaggedIndexedContiguousArray,
diff --git a/cf/field.py b/cf/field.py
index c9eb0c19bd..c8b2bc9a48 100644
--- a/cf/field.py
+++ b/cf/field.py
@@ -22,6 +22,7 @@
     Flags,
     Index,
     List,
+    Quantization,
     mixin,
 )
 from .constants import masked as cf_masked
@@ -280,7 +281,7 @@ def __new__(cls, *args, **kwargs):
         instance._Domain = Domain
         instance._DomainAncillary = DomainAncillary
         instance._DomainAxis = DomainAxis
-        #        instance._Data = Data
+        instance._Quantization = Quantization
         instance._RaggedContiguousArray = RaggedContiguousArray
         instance._RaggedIndexedArray = RaggedIndexedArray
         instance._RaggedIndexedContiguousArray = RaggedIndexedContiguousArray
diff --git a/cf/fieldancillary.py b/cf/fieldancillary.py
index ca32eeda3c..90a1643535 100644
--- a/cf/fieldancillary.py
+++ b/cf/fieldancillary.py
@@ -1,7 +1,42 @@
 import cfdm
 
-from . import mixin
+from . import Quantization, mixin
 
 
 class FieldAncillary(mixin.PropertiesData, cfdm.FieldAncillary):
-    pass
+    """A field ancillary construct of the CF data model.
+
+    The field ancillary construct provides metadata which are
+    distributed over the same sampling domain as the field itself. For
+    example, if a data variable holds a variable retrieved from a
+    satellite instrument, a related ancillary data variable might
+    provide the uncertainty estimates for those retrievals (varying
+    over the same spatiotemporal domain).
+
+    The field ancillary construct consists of an array of the
+    ancillary data, which is zero-dimensional or which depends on one
+    or more of the domain axes, and properties to describe the
+    data. It is assumed that the data do not depend on axes of the
+    domain which are not spanned by the array, along which the values
+    are implicitly propagated. CF-netCDF ancillary data variables
+    correspond to field ancillary constructs. Note that a field
+    ancillary construct is constrained by the domain definition of the
+    parent field construct but does not contribute to the domain's
+    definition, unlike, for instance, an auxiliary coordinate
+    construct or domain ancillary construct.
+
+    **NetCDF interface**
+
+    {{netCDF variable}}
+
+    {{netCDF dataset chunks}}
+
+    .. versionadded:: 3.0.0
+
+    """
+
+    def __new__(cls, *args, **kwargs):
+        """Store component classes."""
+        instance = super().__new__(cls)
+        instance._Quantization = Quantization
+        return instance
diff --git a/cf/quantization.py b/cf/quantization.py
new file mode 100644
index 0000000000..4c36183f85
--- /dev/null
+++ b/cf/quantization.py
@@ -0,0 +1,59 @@
+import cfdm
+
+
+class Quantization(cfdm.Quantization):
+    """A quantization variable.
+
+    A quantization variable describes a quantization algorithm via a
+    collection of parameters.
+
+    The ``algorithm`` parameter names a specific quantization
+    algorithm via one of the keys in the `algorithm_parameters`
+    dictionary.
+
+    The ``implementation`` parameter contains unstandardised text that
+    concisely conveys the algorithm provenance including the name of
+    the library or client that performed the quantization, the
+    software version, and any other information required to
+    disambiguate the source of the algorithm employed. The text must
+    take the form ``software-name version version-string
+    [(optional-information)]``.
+
+    The retained precision of the algorthm is defined with either the
+    ``quantization_nsb`` or ``quantization_nsd`` parameter.
+
+    For instance, the following parameters describe quantization via
+    the BitRound algorithm, retaining 6 significant bits, and
+    implemented by libnetcdf::
+
+       >>> q = {{package}}.{{class}}(
+       ...         parameters={'algorithm': 'bitround',
+       ...                     'quantization_nsb': 6,
+       ...                     'implementation': 'libnetcdf version 4.9.4'}
+       ... )
+       >>> q.parameters()
+       {'algorithm': 'bitround',
+        'quantization_nsb': 6,
+        'implementation': 'libnetcdf version 4.9.4'}
+
+    See CF section 8.4. "Lossy Compression via Quantization".
+
+    **NetCDF interface**
+
+    {{netCDF variable}}
+
+    {{netCDF group attributes}}
+
+    .. versionadded:: NEXTVERSION
+
+    """
+
+    def __repr__(self):
+        """Called by the `repr` built-in function.
+
+        x.__repr__() <==> repr(x)
+
+        .. versionadded:: NEXTVERSION
+
+        """
+        return super().__repr__().replace("<", "<CF ", 1)
diff --git a/cf/test/test_Quantization.py b/cf/test/test_Quantization.py
new file mode 100644
index 0000000000..18b32484c8
--- /dev/null
+++ b/cf/test/test_Quantization.py
@@ -0,0 +1,42 @@
+import datetime
+import unittest
+
+import cf
+
+
+class QuantizationTest(unittest.TestCase):
+    """Unit test for the Quantization class."""
+
+    q = cf.Quantization({"quantization_nsd": 4, "algorithm": "bitgroom"})
+
+    def test_Quantization_algorithm_parameters(self):
+        """Test Quantization.algorithm_parameters."""
+        self.assertEqual(
+            cf.Quantization().algorithm_parameters(),
+            {
+                "bitgroom": "quantization_nsd",
+                "bitround": "quantization_nsb",
+                "digitround": "quantization_nsd",
+                "granular_bitround": "quantization_nsd",
+            },
+        )
+
+    def test_Quantization__str__(self):
+        """Test Quantization.__str__."""
+        self.assertEqual(str(self.q), "algorithm=bitgroom, quantization_nsd=4")
+
+    def test_Quantization_dump(self):
+        """Test Quantization.dump."""
+        self.assertEqual(
+            self.q.dump(display=False),
+            "Quantization: \n"
+            "    algorithm = 'bitgroom'\n"
+            "    quantization_nsd = 4",
+        )
+
+
+if __name__ == "__main__":
+    print("Run date:", datetime.datetime.now())
+    cf.environment()
+    print("")
+    unittest.main(verbosity=2)
diff --git a/cf/test/test_quantization.py b/cf/test/test_quantization.py
new file mode 100644
index 0000000000..03960da987
--- /dev/null
+++ b/cf/test/test_quantization.py
@@ -0,0 +1,231 @@
+import atexit
+import datetime
+import faulthandler
+import os
+import tempfile
+import unittest
+
+import netCDF4
+import numpy as np
+
+faulthandler.enable()  # to debug seg faults and timeouts
+
+import cf
+
+# Set up temporary files
+n_tmpfiles = 2
+tmpfiles = [
+    tempfile.mkstemp("_test_quantization.nc", dir=os.getcwd())[1]
+    for i in range(n_tmpfiles)
+]
+[tmpfile1, tmpfile2] = tmpfiles
+
+
+def _remove_tmpfiles():
+    """Remove temporary files created during tests."""
+    for f in tmpfiles:
+        try:
+            os.remove(f)
+        except OSError:
+            pass
+
+
+atexit.register(_remove_tmpfiles)
+
+
+class quantizationTest(unittest.TestCase):
+    """Test the reading and writing with quantization."""
+
+    f1 = cf.example_field(1)
+
+    netcdf3_fmts = (
+        "NETCDF3_CLASSIC",
+        "NETCDF3_64BIT",
+        "NETCDF3_64BIT_OFFSET",
+        "NETCDF3_64BIT_DATA",
+    )
+
+    def test_quantization_read_write(self):
+        """Test reading, writing, and storing quantization."""
+        f = self.f1.copy()
+        # Add some precision to the data
+        f.data[...] = f.array + np.pi
+
+        # Set a quantisation instruction
+        nsd = 2
+        q0 = cf.Quantization(
+            {
+                "algorithm": "granular_bitround",
+                "implementation": "foobar",
+                "quantization_nsd": nsd,
+            }
+        )
+        f.set_quantize_on_write(q0)
+
+        # Write the field and read it back in
+        tmpfile1 = "delme1.nc"
+        cf.write(f, tmpfile1)
+        g = cf.read(tmpfile1)[0]
+
+        # Check that f and g have different data (i.e. that
+        # quantization occured on disk, and not in f in memory)
+        self.assertFalse(np.allclose(f.data, g.data))
+
+        # Check that g has the correct Quantisation component
+        q = g.get_quantization()
+        self.assertIsInstance(q, cf.Quantization)
+        self.assertEqual(
+            q.parameters(),
+            {
+                "_QuantizeGranularBitRoundNumberOfSignificantDigits": nsd,
+                "algorithm": "granular_bitround",
+                "implementation": (
+                    f"libnetcdf version {netCDF4.__netcdf4libversion__}"
+                ),
+                "quantization_nsd": nsd,
+            },
+        )
+
+        # Write the quantized field and read it back in
+        cf.write(g, tmpfile2)
+        h = cf.read(tmpfile2)[0]
+        self.assertIsInstance(h.get_quantization(), cf.Quantization)
+
+        # Check that h and g are equal
+        self.assertTrue(h.equals(g))
+
+        # Check that h and g are not equal when they only differ by
+        # their quantization components
+        h._set_quantization(q0)
+        self.assertFalse(h.equals(g))
+
+    def test_quantize_on_write(self):
+        """Test del/get/set quantize-on-write methods."""
+        f = self.f1.copy()
+        q0 = cf.Quantization(
+            {
+                "algorithm": "digitround",
+                "quantization_nsd": 9,
+                "implementation": "foobar",
+            }
+        )
+
+        self.assertIsNone(f.set_quantize_on_write(q0))
+
+        q1 = q0.copy()
+        q1.del_parameter("implementation")
+
+        self.assertTrue(f.get_quantize_on_write().equals(q1))
+        self.assertTrue(f.del_quantize_on_write().equals(q1))
+        self.assertIsNone(f.get_quantize_on_write(None))
+        self.assertIsNone(f.del_quantize_on_write(None))
+
+        # Check consistency of arguments
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write()
+
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(quantization_nsd=2)
+
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(quantization_nsd=2, quantization_nsb=2)
+
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(algorithm="digitround", quantization_nsb=2)
+
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(
+                algorithm="digitround", quantization_nsd=2, quantization_nsb=2
+            )
+
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(algorithm="bitround", quantization_nsd=2)
+
+        # Can't set quantization metadata when there is a
+        # quantize-on-write instruction
+        f._set_quantization(q0)
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(q0)
+
+    def test_quantization(self):
+        """Test _del/get/_set quantization methods."""
+        f = self.f1.copy()
+        q0 = cf.Quantization(
+            {
+                "algorithm": "digitround",
+                "quantization_nsd": 9,
+                "implementation": "foobar",
+            }
+        )
+
+        self.assertIsNone(f._set_quantization(q0))
+
+        with self.assertRaises(ValueError):
+            f.set_quantize_on_write(q0)
+
+        self.assertTrue(f.get_quantization().equals(q0))
+        self.assertTrue(f._del_quantization().equals(q0))
+        self.assertIsNone(f.get_quantization(None))
+        self.assertIsNone(f._del_quantization(None))
+
+    def test_quantization_write_exceptions(self):
+        """Test writing quantization exceptions."""
+        f = self.f1.copy()
+
+        # digit_round
+        f.set_quantize_on_write(algorithm="digitround", quantization_nsd=2)
+        with self.assertRaises(ValueError):
+            cf.write(f, tmpfile1)
+
+        # NetCDF3 formats
+        for fmt in self.netcdf3_fmts:
+            with self.assertRaises(ValueError):
+                cf.write(f, tmpfile1, fmt=fmt)
+
+        # Integer data type
+        f.data.dtype = int
+        with self.assertRaises(ValueError):
+            cf.write(f, tmpfile1)
+
+        # Out-of-range quantization_nsd
+        f.data.dtype = "float32"
+        f.set_quantize_on_write(algorithm="bitgroom", quantization_nsd=8)
+        with self.assertRaises(ValueError):
+            cf.write(f, tmpfile1)
+
+        f.data.dtype = "float64"
+        f.set_quantize_on_write(algorithm="bitgroom", quantization_nsd=16)
+        with self.assertRaises(ValueError):
+            cf.write(f, tmpfile1)
+
+        # Out-of-range quantization_nsb
+        f.data.dtype = "float32"
+        f.set_quantize_on_write(algorithm="bitround", quantization_nsb=24)
+        with self.assertRaises(ValueError):
+            cf.write(f, tmpfile1)
+
+        f.data.dtype = "float64"
+        f.set_quantize_on_write(algorithm="bitround", quantization_nsb=53)
+        with self.assertRaises(ValueError):
+            cf.write(f, tmpfile1)
+
+    def test_quantization_copy(self):
+        """Test that quantization information gets copied."""
+        f = self.f1.copy()
+
+        f.set_quantize_on_write(algorithm="bitround", quantization_nsb=2)
+        q = f.get_quantize_on_write()
+        g = f.copy()
+        self.assertTrue(g.get_quantize_on_write().equals(q))
+
+        f.del_quantize_on_write(q)
+        f._set_quantization(q)
+        g = f.copy()
+        self.assertTrue(g.get_quantization().equals(q))
+
+
+if __name__ == "__main__":
+    print("Run date:", datetime.datetime.now())
+    cf.environment()
+    print("")
+    unittest.main(verbosity=2)

From a49e8c6fc4fb004c3a9136ecffb2a51171b9bd95 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Tue, 3 Jun 2025 12:09:19 +0100
Subject: [PATCH 2/8] dev

---
 docs/source/class.rst                 |   1 +
 docs/source/class/cf.Quantization.rst | 120 ++++++++++++++++
 docs/source/tutorial.rst              | 189 +++++++++++++++++++++++++-
 3 files changed, 306 insertions(+), 4 deletions(-)
 create mode 100644 docs/source/class/cf.Quantization.rst

diff --git a/docs/source/class.rst b/docs/source/class.rst
index d7941ac067..6031d91c94 100644
--- a/docs/source/class.rst
+++ b/docs/source/class.rst
@@ -102,6 +102,7 @@ Classes that support the creation and storage of compressed arrays.
    cf.RaggedIndexedArray
    cf.RaggedIndexedContiguousArray
    cf.SubsampledArray
+   cf.Quantization
 
 Data UGRID classes
 ------------------
diff --git a/docs/source/class/cf.Quantization.rst b/docs/source/class/cf.Quantization.rst
new file mode 100644
index 0000000000..50709e93a4
--- /dev/null
+++ b/docs/source/class/cf.Quantization.rst
@@ -0,0 +1,120 @@
+.. currentmodule:: cf
+.. default-role:: obj
+
+cf.Quantization
+===============
+
+----
+
+.. autoclass:: cf.Quantization
+   :no-members:
+   :no-inherited-members:
+
+Inspection
+----------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization.dump
+
+Parameter terms
+---------------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization.del_parameter
+   ~cf.Quantization.get_parameter
+   ~cf.Quantization.has_parameter
+   ~cf.Quantization.set_parameter
+   ~cf.Quantization.parameters
+   ~cf.Quantization.set_parameters
+   ~cf.Quantization.clear_parameters
+
+Miscellaneous
+-------------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization.algorithm_parameters
+   ~cf.Quantization.copy
+   ~cf.Quantization.equals
+   ~cf.Quantization.get_original_filenames
+   ~cf.Quantization.creation_commands
+
+NetCDF
+------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization.nc_del_variable
+   ~cf.Quantization.nc_get_variable
+   ~cf.Quantization.nc_has_variable
+   ~cf.Quantization.nc_set_variable
+   ~cf.Quantization.nc_group_attributes
+   ~cf.Quantization.nc_clear_group_attributes
+   ~cf.Quantization.nc_set_group_attribute
+   ~cf.Quantization.nc_set_group_attributes
+
+Groups
+^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization.nc_variable_groups
+   ~cf.Quantization.nc_set_variable_groups
+   ~cf.Quantization.nc_clear_variable_groups
+
+Special
+-------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization.__bool__
+   ~cf.Quantization.__deepcopy__
+   ~cf.Quantization.__repr__
+   ~cf.Quantization.__str__
+
+Docstring substitutions
+-----------------------
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Quantization._docstring_special_substitutions
+   ~cf.Quantization._docstring_substitutions
+   ~cf.Quantization._docstring_package_depth
+   ~cf.Quantization._docstring_method_exclusions
diff --git a/docs/source/tutorial.rst b/docs/source/tutorial.rst
index c741aed4e1..21894bac96 100644
--- a/docs/source/tutorial.rst
+++ b/docs/source/tutorial.rst
@@ -6692,16 +6692,197 @@ The content of the new file is:
      4, 0, 5 ;
    }
 
-----
-   
 .. _Coordinate-subampling:
 
 Coordinate subsampling
 ^^^^^^^^^^^^^^^^^^^^^^
 
 `Lossy compression by coordinate subsampling`_ was introduced into the
-CF conventions at CF-1.9, but is not yet available in cfdm. It will be
-ready in a future 3.x.0 release.
+CF conventions at CF-1.10 for applications for which the coordinates
+can require considerably more storage than the data itself. Space may
+be saved in the netCDF file by storing a subsample of the coordinates
+that describe the data, and the uncompressed coordinate and auxiliary
+coordinate variables are reconstituted by interpolation, from the
+subsampled coordinate values to the domain of the data
+
+This is illustrated with the file ``subsampled.nc`` (found in the
+:ref:`sample datasets <Sample-datasets>`):
+
+
+.. code-block:: console
+   :caption: *Inspect the compressed dataset with the ncdump command
+             line tool.*
+      
+   $ ncdump -h subsampled.nc 
+   netcdf subsampled {
+   dimensions:
+   	time = 2 ;
+   	lat = 18 ;
+   	lon = 12 ;
+   	tp_lat = 4 ;
+   	tp_lon = 5 ;
+   variables:
+   	float time(time) ;
+   		time:standard_name = "time" ;
+   		time:units = "days since 2000-01-01" ;
+   	float lat(tp_lat, tp_lon) ;
+   		lat:standard_name = "latitude" ;
+   		lat:units = "degrees_north" ;
+   		lat:bounds_tie_points = "lat_bounds" ;
+   	float lon(tp_lat, tp_lon) ;
+   		lon:standard_name = "longitude" ;
+   		lon:units = "degrees_east" ;
+   		lon:bounds_tie_points = "lon_bounds" ;
+   	float lat_bounds(tp_lat, tp_lon) ;
+   	float lon_bounds(tp_lat, tp_lon) ;
+   	int lat_indices(tp_lat) ;
+   		lat_indices:long_name = "Tie point indices for latitude dimension" ;
+   	int lon_indices(tp_lon) ;
+   		lon_indices:long_name = "Tie point indices for longitude dimension" ;
+   	int bilinear ;
+   		bilinear:interpolation_name = "bi_linear" ;
+   		bilinear:computational_precision = "64" ;
+   		bilinear:tie_point_mapping =
+		    "lat: lat_indices tp_lat lon: lon_indices tp_lon" ;
+   	float q(time, lat, lon) ;
+   		q:standard_name = "specific_humidity" ;
+   		q:units = "1" ;
+   		q:coordinate_interpolation = "lat: lon: bilinear" ;
+   
+   // global attributes:
+   		:Conventions = "CF-1.11" ;
+   }
+
+
+Reading and inspecting this file shows the latitude and longitude
+coordinates in uncompressed form, whilst their underlying arrays are
+still in subsampled representation described in the file:
+   
+.. code-block:: python
+   :caption: *Read a field construct from a dataset that has been
+             compressed by corodinate subsampling, and inspect
+             coordinates.*
+
+   >>> f = cf.read('subsampled.nc')[0]
+   >>> print(f)
+   Field: specific_humidity (ncvar%q)
+   ----------------------------------
+   Data            : specific_humidity(time(2), ncdim%lat(18), ncdim%lon(12)) 1
+   Dimension coords: time(2) = [2000-01-01 00:00:00, 2000-02-01 00:00:00]
+   Auxiliary coords: latitude(ncdim%lat(18), ncdim%lon(12)) = [[-85.0, ..., 85.0]] degrees_north
+                : longitude(ncdim%lat(18), ncdim%lon(12)) = [[15.0, ..., 345.0]] degrees_east
+   >>> lon = f.construct('longitude')
+   >>> lon
+   <AuxiliaryCoordinate: longitude(18, 12) degrees_east>
+   >>> lon.data.source()
+   <SubsampledArray(18, 12): >
+   >>> print(lon.array)
+   [[15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]
+    [15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]]
+   >>> lon.data.source().source()
+   <Data(4, 5): [[15.0, ..., 345.0]]>
+   >>> print(lon.data.source().source().array)
+   [[ 15. 135. 225. 255. 345.]
+    [ 15. 135. 225. 255. 345.]
+    [ 15. 135. 225. 255. 345.]
+    [ 15. 135. 225. 255. 345.]]
+
+As with all other forms of compression, the field may be treated as if
+were not compressed:
+
+.. code-block:: python
+   :caption: *Get subspaces based on indices of the uncompressed
+             data.*
+
+   >>> g = f[0, 6, :]
+   >>> print(g)
+   Field: specific_humidity (ncvar%q)
+   ----------------------------------
+   Data            : specific_humidity(time(1), ncdim%lat(1), ncdim%lon(12)) 1
+   Dimension coords: time(1) = [2000-01-01 00:00:00]
+   Auxiliary coords: latitude(ncdim%lat(1), ncdim%lon(12)) = [[-25.0, ..., -25.0]] degrees_north
+                   : longitude(ncdim%lat(1), ncdim%lon(12)) = [[15.0, ..., 345.0]] degrees_east
+   >>> print(g.construct('longitude').array)
+   [[15.0 45.0 75.0 105.0 135.0 165.0 195.0 225.0 255.0 285.0 315.0 345.0]]
+
+
+The metadata that define the subsampling are contained within the
+coordinate's `Data` object:
+
+.. code-block:: python
+   :caption: *Get subspaces based on indices of the uncompressed
+             data.*
+
+   >>> lon = f.construct('longitude')
+   >>> d = lon.data.source()
+   >>> d.get_tie_point_indices()
+   {0: <TiePointIndex: long_name=Tie point indices for latitude dimension(4) >,
+    1: <TiePointIndex: long_name=Tie point indices for longitude dimension(5) >}
+   >>> d.get_computational_precision()
+   '64'
+
+It is not yet, as of version 1.10.0.0, possible to write to disk a
+field construct with compression by coordinate subsampling.
+
+.. _Lossy-compression-via-quantization:
+
+Lossy compression via quantization
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+`Lossy compression via quantization`_ eliminates false precision,
+usually by rounding the least significant bits of floating-point
+mantissas to zeros, so that a subsequent compression on disk is more
+efficient. Quantization is described by the following parameters:
+
+* The ``algorithm`` parameter names a specific quantization algorithm.
+
+* The ``implementation`` parameter contains unstandardised text that
+  concisely conveys the algorithm provenance including the name of the
+  library or client that performed the quantization, the software
+  version, and any other information required to disambiguate the
+  source of the algorithm employed. The text must take the form
+  ``software-name version version-string [(optional-information)]``.
+
+* The retained precision of the algortqhm is defined with either the
+  ``quantization_nsb`` or ``quantization_nsd`` parameter.
+
+If quantization has been applied to the data, then it may be described
+with in a `Quantization` object, accessed via the construct's
+`!get_quantization` method. To apply quantization at the time of
+writing the data to disk, use the construct's `!set_quantize_on_write`
+method:
+
+.. code-block:: python
+   :caption: *Lossy compression via quantization.*
+
+   >>> q, t = cf.read('file.nc')
+   >>> t.set_quantize_on_write(algorithm='bitgroom', quantization_nsd=6)
+   >>> cf.write(t, 'quantized.nc')
+   >>> quantized = cf.read('quantized.nc')[0]
+   >>> c = quantized.get_quantization()
+   >>> c
+   <CF Quantization: _QuantizeBitGroomNumberOfSignificantDigits=6, algorithm=bitgroom, implementation=libnetcdf version 4.9.4-development, quantization_nsd=6>
+   >>> c.parameters()
+   {'algorithm': 'bitgroom',
+    'implementation': 'libnetcdf version 4.9.4-development',
+    '_QuantizeBitGroomNumberOfSignificantDigits': np.int32(6),
+    'quantization_nsd': np.int64(6)}
 
 ----
 

From db2f87346e21a7cdb7ca39927ce7bbe37f74260d Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Wed, 4 Jun 2025 14:10:10 +0100
Subject: [PATCH 3/8] dev

---
 Changelog.rst        | 6 ++++++
 README.md            | 2 ++
 cf/fieldancillary.py | 2 +-
 setup.py             | 5 ++++-
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Changelog.rst b/Changelog.rst
index 9f6ab30322..1463a9c4e5 100644
--- a/Changelog.rst
+++ b/Changelog.rst
@@ -5,6 +5,12 @@ Version NEXTVERSION
 
 * Implement lossy compression via quantization
   (https://github.com/NCAS-CMS/cf-python/issues/870)
+* New quantization class: `cf.Quantization`
+  (https://github.com/NCAS-CMS/cf-python/issues/870)
+* New quantization methods: `cf.Field.get_quantization`,
+  `cf.Field.get_quantize_on_write`, `cf.Field.set_quantize_on_write`,
+  `cf.Field.del_quantize_on_write`
+  (https://github.com/NCAS-CMS/cf-python/issues/870)
 * Read Zarr datasets with `cf.read`
   (https://github.com/NCAS-CMS/cf-python/issues/863)
 * Update CF aggregation keywords
diff --git a/README.md b/README.md
index 9f5c75a8b6..ce752fd0af 100644
--- a/README.md
+++ b/README.md
@@ -116,6 +116,8 @@ of its array manipulation and can:
 * create running means from field constructs,
 * apply differential operators to field constructs,
 * create derived quantities (such as relative vorticity).
+* read and write that data that are quantized to eliminate false
+  precision.
 
 Visualization
 =============
diff --git a/cf/fieldancillary.py b/cf/fieldancillary.py
index 90a1643535..6df6dcaa7a 100644
--- a/cf/fieldancillary.py
+++ b/cf/fieldancillary.py
@@ -31,7 +31,7 @@ class FieldAncillary(mixin.PropertiesData, cfdm.FieldAncillary):
 
     {{netCDF dataset chunks}}
 
-    .. versionadded:: 3.0.0
+    .. versionadded:: 2.0
 
     """
 
diff --git a/setup.py b/setup.py
index 706d89af63..495dfa08a8 100755
--- a/setup.py
+++ b/setup.py
@@ -227,7 +227,10 @@ def compile():
 
 * apply differential operators to field constructs,
 
-* create derived quantities (such as relative vorticity).
+* create derived quantities (such as relative vorticity),
+
+* read and write that data that are quantized to eliminate false
+  precision.
 
 """
 

From e7b0421af3cffa2e9be8225f632387e664ff833e Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Thu, 5 Jun 2025 09:25:35 +0100
Subject: [PATCH 4/8] Zarr and quantization updates

---
 docs/source/spelling_false_positives.txt | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/docs/source/spelling_false_positives.txt b/docs/source/spelling_false_positives.txt
index 6d324f615e..6f7ff66d65 100644
--- a/docs/source/spelling_false_positives.txt
+++ b/docs/source/spelling_false_positives.txt
@@ -1,6 +1,7 @@
 absoluted
 absolutised
 abspath
+actify
 addattr
 aggregatable
 al
@@ -293,6 +294,7 @@ lbproc
 lbtim
 le
 len
+libnetcdf
 linebreaks
 Lineplot
 ln
@@ -379,6 +381,8 @@ programmatically
 pseudolevels
 py
 pyplot
+Quantization
+quantization
 radd
 RaggedContiguousArray
 RaggedIndexedArray
@@ -521,6 +525,7 @@ unmapped
 unselected
 Unselected
 unsqueeze
+unstandardised
 url
 uri
 var
@@ -534,8 +539,11 @@ Voronoi
 vorticity	
 wi
 wo
+xarray
 xor
 xy
+Zarr
+ZarrArray
 Zimmermann
 δu
 δv

From 68ac81c828305725706ee5bb353da0dcc73687bd Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Thu, 5 Jun 2025 10:09:23 +0100
Subject: [PATCH 5/8] quantisation

---
 cf/quantization.py                            |  2 +-
 docs/source/class/cf.AuxiliaryCoordinate.rst  | 13 +++++++++++++
 docs/source/class/cf.Bounds.rst               | 13 +++++++++++++
 docs/source/class/cf.CellConnectivity.rst     | 13 +++++++++++++
 docs/source/class/cf.CellMeasure.rst          | 13 +++++++++++++
 docs/source/class/cf.CoordinateConversion.rst | 13 +++++++++++++
 docs/source/class/cf.Count.rst                | 13 +++++++++++++
 docs/source/class/cf.Datum.rst                | 13 +++++++++++++
 docs/source/class/cf.DimensionCoordinate.rst  | 13 +++++++++++++
 docs/source/class/cf.DomainAncillary.rst      | 13 +++++++++++++
 docs/source/class/cf.DomainTopology.rst       | 13 +++++++++++++
 docs/source/class/cf.Field.rst                | 17 +++++++++++++++++
 docs/source/class/cf.FieldAncillary.rst       | 17 +++++++++++++++++
 docs/source/class/cf.Index.rst                | 13 +++++++++++++
 docs/source/class/cf.List.rst                 | 13 +++++++++++++
 docs/source/tutorial.rst                      | 14 +++++++++-----
 16 files changed, 200 insertions(+), 6 deletions(-)

diff --git a/cf/quantization.py b/cf/quantization.py
index 4c36183f85..26e2861ac6 100644
--- a/cf/quantization.py
+++ b/cf/quantization.py
@@ -19,7 +19,7 @@ class Quantization(cfdm.Quantization):
     take the form ``software-name version version-string
     [(optional-information)]``.
 
-    The retained precision of the algorthm is defined with either the
+    The retained precision of the algorithm is defined with either the
     ``quantization_nsb`` or ``quantization_nsd`` parameter.
 
     For instance, the following parameters describe quantization via
diff --git a/docs/source/class/cf.AuxiliaryCoordinate.rst b/docs/source/class/cf.AuxiliaryCoordinate.rst
index e5af934219..6216de643f 100644
--- a/docs/source/class/cf.AuxiliaryCoordinate.rst
+++ b/docs/source/class/cf.AuxiliaryCoordinate.rst
@@ -342,6 +342,19 @@ Cyclicity
    ~cf.AuxiliaryCoordinate.get_original_filenames
    ~cf.AuxiliaryCoordinate.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.AuxiliaryCoordinate.get_quantization
+   ~cf.AuxiliaryCoordinate.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.Bounds.rst b/docs/source/class/cf.Bounds.rst
index 98d8caec77..27a29b1010 100644
--- a/docs/source/class/cf.Bounds.rst
+++ b/docs/source/class/cf.Bounds.rst
@@ -239,6 +239,19 @@ Data
    ~cf.Bounds.has_bounds
    ~cf.Bounds.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Bounds.get_quantization
+   ~cf.Bounds.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.CellConnectivity.rst b/docs/source/class/cf.CellConnectivity.rst
index 0723d8f9af..791479de51 100644
--- a/docs/source/class/cf.CellConnectivity.rst
+++ b/docs/source/class/cf.CellConnectivity.rst
@@ -261,6 +261,19 @@ Data
    ~cf.CellConnectivity.has_bounds
    ~cf.CellConnectivity.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.CellConnectivity.get_quantization
+   ~cf.CellConnectivity.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.CellMeasure.rst b/docs/source/class/cf.CellMeasure.rst
index bdb3ef502e..8e61434557 100644
--- a/docs/source/class/cf.CellMeasure.rst
+++ b/docs/source/class/cf.CellMeasure.rst
@@ -262,6 +262,19 @@ Data
    ~cf.CellMeasure.has_bounds
    ~cf.CellMeasure.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.CellMeasure.get_quantization
+   ~cf.CellMeasure.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.CoordinateConversion.rst b/docs/source/class/cf.CoordinateConversion.rst
index cdc9f293ad..82c178e9e7 100644
--- a/docs/source/class/cf.CoordinateConversion.rst
+++ b/docs/source/class/cf.CoordinateConversion.rst
@@ -10,6 +10,18 @@ cf.CoordinateConversion
    :no-members:
    :no-inherited-members:
    
+Inspection
+----------
+
+.. rubric:: Methods
+	    
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.CoordinateConversion.dump
+
 Parameter terms
 ---------------
 
@@ -58,6 +70,7 @@ Miscellaneous
 
    ~cf.CoordinateConversion.copy
    ~cf.CoordinateConversion.equals
+   ~cf.CoordinateConversion.creation_commands
 
 Special
 -------
diff --git a/docs/source/class/cf.Count.rst b/docs/source/class/cf.Count.rst
index 011938c558..27d04c6cf0 100644
--- a/docs/source/class/cf.Count.rst
+++ b/docs/source/class/cf.Count.rst
@@ -233,6 +233,19 @@ Data
    ~cf.Count.has_bounds
    ~cf.Count.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Count.get_quantization
+   ~cf.Count.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.Datum.rst b/docs/source/class/cf.Datum.rst
index 5dff1d8dfe..ca81f442fd 100644
--- a/docs/source/class/cf.Datum.rst
+++ b/docs/source/class/cf.Datum.rst
@@ -10,6 +10,18 @@ cf.Datum
    :no-members:
    :no-inherited-members:
 
+Inspection
+----------
+
+.. rubric:: Methods
+	    
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Datum.dump
+
 Parameters
 ----------
 
@@ -57,6 +69,7 @@ Miscellaneous
    ~cf.Datum.copy
    ~cf.Datum.get_original_filenames
    ~cf.Datum.equals
+   ~cf.Datum.creation_commands
 
 Groups
 ^^^^^^
diff --git a/docs/source/class/cf.DimensionCoordinate.rst b/docs/source/class/cf.DimensionCoordinate.rst
index d3e4944747..7defc250e6 100644
--- a/docs/source/class/cf.DimensionCoordinate.rst
+++ b/docs/source/class/cf.DimensionCoordinate.rst
@@ -349,6 +349,19 @@ Cyclicity
    ~cf.DimensionCoordinate.anchor
    ~cf.DimensionCoordinate.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.DimensionCoordinate.get_quantization
+   ~cf.DimensionCoordinate.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.DomainAncillary.rst b/docs/source/class/cf.DomainAncillary.rst
index 13f4f78f6f..61d8f795e6 100644
--- a/docs/source/class/cf.DomainAncillary.rst
+++ b/docs/source/class/cf.DomainAncillary.rst
@@ -306,6 +306,19 @@ Data
    ~cf.DomainAncillary.get_original_filenames
    ~cf.DomainAncillary.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.DomainAncillary.get_quantization
+   ~cf.DomainAncillary.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.DomainTopology.rst b/docs/source/class/cf.DomainTopology.rst
index 9e7e84e22b..a651bd72ad 100644
--- a/docs/source/class/cf.DomainTopology.rst
+++ b/docs/source/class/cf.DomainTopology.rst
@@ -262,6 +262,19 @@ Data
    ~cf.DomainTopology.has_bounds
    ~cf.DomainTopology.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.DomainTopology.get_quantization
+   ~cf.DomainTopology.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.Field.rst b/docs/source/class/cf.Field.rst
index 72f9780687..8183b61434 100644
--- a/docs/source/class/cf.Field.rst
+++ b/docs/source/class/cf.Field.rst
@@ -265,6 +265,23 @@ Miscellaneous data operations
    ~cf.Field.persist
    ~cf.Field.persist_metadata
  
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Field.get_quantization
+   ~cf.Field._set_quantization
+   ~cf.Field._del_quantization
+   ~cf.Field.get_quantize_on_write
+   ~cf.Field.set_quantize_on_write
+   ~cf.Field.del_quantize_on_write
+
 Metadata constructs
 -------------------
 
diff --git a/docs/source/class/cf.FieldAncillary.rst b/docs/source/class/cf.FieldAncillary.rst
index 679574045a..62bdbb2381 100644
--- a/docs/source/class/cf.FieldAncillary.rst
+++ b/docs/source/class/cf.FieldAncillary.rst
@@ -237,6 +237,23 @@ Data
    ~cf.FieldAncillary.has_bounds
    ~cf.FieldAncillary.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.FieldAncillary.get_quantization
+   ~cf.FieldAncillary._set_quantization
+   ~cf.FieldAncillary._del_quantization
+   ~cf.FieldAncillary.get_quantize_on_write
+   ~cf.FieldAncillary.set_quantize_on_write
+   ~cf.FieldAncillary.del_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.Index.rst b/docs/source/class/cf.Index.rst
index 4a9a431127..2717ebba13 100644
--- a/docs/source/class/cf.Index.rst
+++ b/docs/source/class/cf.Index.rst
@@ -234,6 +234,19 @@ Data
    ~cf.Index.has_bounds
    ~cf.Index.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.Index.get_quantization
+   ~cf.Index.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/class/cf.List.rst b/docs/source/class/cf.List.rst
index 4f343ecc4c..d96f2f48a5 100644
--- a/docs/source/class/cf.List.rst
+++ b/docs/source/class/cf.List.rst
@@ -234,6 +234,19 @@ Data
    ~cf.List.has_bounds
    ~cf.List.persist
 
+Quantization
+^^^^^^^^^^^^
+
+.. rubric:: Methods
+
+.. autosummary::
+   :nosignatures:
+   :toctree: ../method/
+   :template: method.rst
+
+   ~cf.List.get_quantization
+   ~cf.List.get_quantize_on_write
+
 Miscellaneous
 -------------
 
diff --git a/docs/source/tutorial.rst b/docs/source/tutorial.rst
index 21894bac96..4b6806b619 100644
--- a/docs/source/tutorial.rst
+++ b/docs/source/tutorial.rst
@@ -6859,7 +6859,7 @@ efficient. Quantization is described by the following parameters:
   source of the algorithm employed. The text must take the form
   ``software-name version version-string [(optional-information)]``.
 
-* The retained precision of the algortqhm is defined with either the
+* The retained precision of the algorithm is defined with either the
   ``quantization_nsb`` or ``quantization_nsd`` parameter.
 
 If quantization has been applied to the data, then it may be described
@@ -6872,17 +6872,21 @@ method:
    :caption: *Lossy compression via quantization.*
 
    >>> q, t = cf.read('file.nc')
-   >>> t.set_quantize_on_write(algorithm='bitgroom', quantization_nsd=6)
+   >>> t.set_quantize_on_write(algorithm='bitgroom', quantization_nsd=1)
    >>> cf.write(t, 'quantized.nc')
    >>> quantized = cf.read('quantized.nc')[0]
    >>> c = quantized.get_quantization()
    >>> c
-   <CF Quantization: _QuantizeBitGroomNumberOfSignificantDigits=6, algorithm=bitgroom, implementation=libnetcdf version 4.9.4-development, quantization_nsd=6>
+   <CF Quantization: _QuantizeBitGroomNumberOfSignificantDigits=1, algorithm=bitgroom, implementation=libnetcdf version 4.9.4-development, quantization_nsd=1>
    >>> c.parameters()
    {'algorithm': 'bitgroom',
     'implementation': 'libnetcdf version 4.9.4-development',
-    '_QuantizeBitGroomNumberOfSignificantDigits': np.int32(6),
-    'quantization_nsd': np.int64(6)}
+    '_QuantizeBitGroomNumberOfSignificantDigits': np.int32(1),
+    'quantization_nsd': np.int64(1)}
+   >>> t[0, 0, 0].array
+   array([[[262.8]]])
+   >>> quantized[0, 0, 0].array
+   array([[[256.]]])
 
 ----
 

From 11f3f3c7bb81760679200e947a3e086bc9e6fd9c Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Thu, 5 Jun 2025 10:30:05 +0100
Subject: [PATCH 6/8] cfdm versions

---
 RELEASE.md | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/RELEASE.md b/RELEASE.md
index 070b545b3d..b3cf931b4a 100644
--- a/RELEASE.md
+++ b/RELEASE.md
@@ -17,10 +17,11 @@
 - [ ] Change the version and date in `cf/__init__.py` (`__version__` and
   `__date__` variables)
 
-- [ ] Ensure that the requirements on dependencies & their versions are
-  up-to-date and consistent in both the `requirements.txt` and in
-  `docs/source/installation.rst`; and in the `_requires` list and
-  `Version` checks in `cf/__init__.py`.
+- [ ] Ensure that the requirements on dependencies & their versions
+  are up-to-date and consistent in both the `requirements.txt` and in
+  `docs/source/installation.rst` (paying particular attention to
+  `cfdm`); and in the `_requires` list and `Version` checks in
+  `cf/__init__.py`.
 
 - [ ] Make sure that `README.md` is up to date.
 

From e382d4b4178f39c8dbb67d58076c701dfbbd3453 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Thu, 5 Jun 2025 10:37:24 +0100
Subject: [PATCH 7/8] cfdm versions

---
 requirements.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/requirements.txt b/requirements.txt
index 9d875bb541..1bfcaec175 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 netCDF4>=1.7.2
 cftime>=1.6.4
 numpy>=2.0.0
-cfdm>=1.12.1.0, <1.12.2.0
+cfdm>=1.12.2.0, <1.12.3.0
 psutil>=0.6.0
 cfunits>=3.3.7
 dask>=2025.5.1

From d892bd23d2a6e2bb38981d5a6e315aba480c8f09 Mon Sep 17 00:00:00 2001
From: David Hassell <david.hassell@ncas.ac.uk>
Date: Thu, 5 Jun 2025 10:37:43 +0100
Subject: [PATCH 8/8] quantisation

---
 docs/source/introduction.rst | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/docs/source/introduction.rst b/docs/source/introduction.rst
index 1e9c66ed0d..635aa81157 100644
--- a/docs/source/introduction.rst
+++ b/docs/source/introduction.rst
@@ -61,8 +61,7 @@ may nonetheless be modified in memory.
                    : longitude(128) = [0.0, ..., 357.1875] degrees_east
                    : height(1) = [2.0] m
 
-The `cf` package uses :ref:`Dask <Performance>` for all of its array
-manipulation and can:
+The `cf` package can:
 
 * read :term:`field constructs <field construct>` and :term:`domain
   constructs <domain construct>` from netCDF, CDL, PP and UM datasets
@@ -120,7 +119,10 @@ manipulation and can:
 
 * apply differential operators to field constructs,
 
-* create derived quantities (such as relative vorticity).
+* create derived quantities (such as relative vorticity),
+
+* read and write that data that are quantized to eliminate false
+  precision.
 
 ----
 
@@ -146,9 +148,9 @@ of plotting possibilities with example code.
 **Performance**
 ---------------
 
-As of version 3.14.0 (released 2023-01-31), cf uses :ref:`Dask
-<Performance>` for all of its data manipulations, which provides lazy,
-parallelised, and out-of-core computations of array operations.
+The `cf` package uses :ref:`Dask <Performance>` for all of its data
+manipulations, which provides lazy, parallelised, and out-of-core
+computations of array operations.
 
 ----