Lossy compression through coordinate sampling

appa.adoc

@@ -115,6 +115,12 @@ formula in the definition.
 | link:$$http://www.unidata.ucar.edu/software/netcdf/docs/attribute_conventions.html$$[NUG Appendix A, "Attribute Conventions"]
 | Name of the conventions followed by the dataset.
 
+| **`coordinate_interpolation`**
+| S
+| D, Do
+| <<compression-by-coordinate-subsampling>>
+| Indicates that coordinates have been compressed by sampling and identifies the tie point coordinate variables and their associated interpolation variables.
+
 | **`coordinates`**
 | S
 | D, M, Do

bibliography.adoc

@@ -14,6 +14,8 @@
 - [[[FGDC]]]  link:$$http://www.fgdc.gov/standards/projects/FGDC-standards-projects/metadata/base-metadata/v2_0698.pdf$$[Content Standard for Digital Geospatial Metadata] .
 				Federal Geographic Data Committee,  FGDC-STD-001-1998
 			. 
+
+- [[[IEEE_754]]]  link:$$https://ieeexplore.ieee.org/servlet/opac?punumber=8766227$$[IEEE Standard for Floating-Point Arithmetic], in __IEEE Std 754-2019 (Revision of IEEE 754-2008)__, vol., no., pp.1-84, 22 July 2019, doi: 10.1109/IEEESTD.2019.8766229.
 - [[[NetCDF]]]  link:$$http://www.unidata.ucar.edu/netcdf/index.html$$[ NetCDF Software Package] .
 				UNIDATA Program Center of the University Corporation for Atmospheric Research
 			. 

cf-conventions.adoc

@@ -40,6 +40,10 @@ include::toc-extra.adoc[]
 * Alan D. Snow, Corteva Agriscience
 * Tobias Kölling, MPIM
 * Dave Allured, NOAA
+* Aleksandar Jelenak, HDF Group
+* Anders Meier Soerensen, EUMETSAT
+* Lucile Gaultier, OceanDataLab
+* Sylvain Herlédan, OceanDataLab
 
 Many others have contributed to the development of CF through their participation in discussions about proposed changes.
 
@@ -107,6 +111,9 @@ include::apph.adoc[]
 :numbered!:
 include::appi.adoc[]
 
+:numbered!:
+include::appj.adoc[]
+
 :numbered!:
 include::history.adoc[]
 

ch01.adoc

@@ -76,6 +76,8 @@ It is a one-dimensional variable with the same name as its dimension [e.g., **`t
 
 grid mapping variable:: A variable used as a container for attributes that define a specific grid mapping. The type of the variable is arbitrary since it contains no data.
 
+interpolation variable:: A variable used as a container for attributes that define a specific interpolation method for uncompressing tie point variables. The type of the variable is arbitrary since it contains no data.
+
 latitude dimension:: A dimension of a netCDF variable that has an associated latitude coordinate variable.
 
 local apex group:: The nearest (to a referring group) ancestor group in which a dimension of an out-of-group coordinate is defined. The word "apex" refers to position of this group at the vertex of the tree of groups formed by it, the referring group, and the group where a coordinate is located. 
@@ -103,6 +105,8 @@ sibling group:: Any group with the same parent group as the referring group
 
 spatiotemporal dimension:: A dimension of a netCDF variable that is used to identify a location in time and/or space.
 
+tie point variable:: A netCDF variable that contains coordinates that have been compressed by sampling. There is no relationship between the name of a tie point variable and the name(s) of its dimension(s).
+
 time dimension:: A dimension of a netCDF variable that has an associated time coordinate variable.
 
 vertical dimension:: A dimension of a netCDF variable that has an associated vertical coordinate variable.

conformance.adoc

@@ -678,6 +678,134 @@ in the file.
 starting with 0 and going up to the product of the compressed dimension
 sizes minus 1 (CDL index conventions).
 
+[[compression-by-gathering]]
+=== 8.2 Lossless Compression by Gathering
+
+*Requirements:*
+
+* The **`compress`** attribute may only be attached to a coordinate variable
+with an integer data type.
+* The type of the **`compress`** attribute is a string whose value is a blank
+separated list of dimension names. The specified dimensions must exist
+in the file.
+* The values of the associated coordinate variable must be in the range
+starting with 0 and going up to the product of the compressed dimension
+sizes minus 1 (CDL index conventions).
+
+[[compression-by-coordinate-subsampling]]
+=== 8.3 Lossy Compression by Coordinate Subsampling
+
+*Requirements:*
+
+* When attached to a data variable, the type of the **`tie_points`**
+  attribute is a string whose value is a list of blank separated word
+  groups of the following form, in which brackets indicate optional
+  text: **`tie_point_variable: [tie_point_variable: ...]
+  interpolation_variable`**. Each **`tie_point_variable`** token
+  specifies a tie point variable that must exist in the file, and each
+  **`interpolation_variable`** token specifies a variable that must
+  exist in the file.
+
+* An interpolation variable must have one of the string-valued
+  attributes **`interpolation_name`** or
+  **`interpolation_description`**, but not both. The legal values for
+  the **`interpolation_name`** attribute are contained in the
+  Interpolation Methods section of http://cfconventions.org/cf-conventions/cf-conventions.html#appendix-coordinate-subsampling[Appendix J].
+
+* An interpolation variable must have the attribute **`computational_precision`**.
+  The legal values for the **`computational_precision`** attribute are contained in the Interpolation Method Implementation subsection of the Lossy Compression by Coordinate Subsampling section of chapter 8.
+
+* An interpolation variable must have a **`tie_point_dimensions`**
+  attribute that is a string whose value is a list of blank separated
+  word groups of the following form, in which brackets indicate
+  optional text: **`interpolation_dimension:
+  tie_point_interpolation_dimension [interpolation_zone_dimension]`**.
+  Each **`interpolation_dimension`** token specifies a unique
+  interpolation dimension of the parent data variable, each
+  **`tie_point_interpolation_dimension`** token specifies the tie
+  point interpolation dimension of a unique tie point index variable,
+  and each **`interpolation_zone_dimension`** token specifies a unique
+  interpolation zone dimension. The tie point interpolation dimensions
+  and interpolation zone dimensions must not be dimensions of the
+  parent data variable.
+
+* The tie point variables associated with each
+  **`interpolation_variable`** token must all span the same dimensions,
+  which comprise a subset of zero or more dimensions of the parent
+  data variable with the addition of all of the tie point
+  interpolation dimensions identified by the **`tie_point_dimensions`**
+  attribute of the interpolation variable. A tie point variable must
+  not span both a tie point interpolation dimension and its
+  corresponding interpolation dimension, as defined by the
+  **`tie_point_dimensions`** mapping.
+
+* An interpolation variable must have a **`tie_point_indices`**
+  attribute that is a string whose value is a list of blank separated
+  word pairs of the following form: **`interpolation_dimension:
+  tie_point_index_variable`**. The **`interpolation_dimension`**
+  tokens specify the same interpolation dimensions as the
+  **`tie_point_dimensions`** attribute, and each
+  **`tie_point_index_variable`** token specifies a tie point index
+  variable that must exist in the file.
+
+* A tie point index variable must be a one-dimensional variable with
+  an integer data type.
+
+* The dimension of a tie point index variable must be a tie point
+  interpolation dimension identified by the **`tie_point_dimensions`**
+  attribute.
+
+* The values of a tie point index variable must be non-negative
+  integers. The first value must be zero, and each subsequent value
+  must be greater than or equal to the previous value. If a value
+  differs by zero or one from its previous value, then it must differ
+  by two or more from its subsequent value.
+
+* The size of an interpolation zone dimension must be equal to the
+  size of the corresponding tie point interpolation dimension minus
+  the number of interpolation areas for that tie point interpolation
+  dimension. The number of interpolation areas is equal one plus the
+  number of occurences of adjacent values differing by zero or one in
+  the corresponding tie point index variable.
+
+* When attached to an interpolation variable, the type of the
+  **`interpolation_parameters`** attribute is a string whose value is
+  list of blank separated word pairs in the form **`term: var`**. For
+  each valid **`interpolation_name`**, the legal values for **`term`**
+  are described by the "Interpolation Parameter terms" table entry in
+  the Interpolation Methods section of http://cfconventions.org/cf-conventions/cf-conventions.html#appendix-coordinate-subsampling[Appendix J].
+  The values of
+  **`var`** must be interpolation parameter variables that exist in
+  the file.
+
+* The dimensions of an interpolation parameter variable must be a
+  subset of zero or more of the dimensions of the corresponding tie
+  point variables, with the exception that a tie point interpolation
+  dimension may be replaced with its corresponding interpolation zone
+  dimension, as defined by the **`tie_point_dimensions`** mapping.
+
+* If a tie point variable has **`bounds_tie_points`** attribute then
+  it must be a string whose value is a single variable name. The
+  specified variable must exist in the file.
+
+* A bounds tie point variable must have the same dimensions as its
+  associated tie points coordinate variable.
+
+* A bounds tie point variable must be a numeric data type.
+
+* A bounds tie point variable must not have the **`_FillValue`** or
+  **`missing_value`** attributes. The requirements on all other bounds
+  tie point variable attributes are the same as for bounds variables
+  described in <<cell-boundaries>>.
+
+*Recommendations:*
+
+* An interpolation variable should have 0 dimensions.
+
+* The recommendations on bounds tie point variable attributes are the
+  same as for bounds variables described in <<cell-boundaries>>.
+
+
 === Appendix D Parametric Vertical Coordinates
 
 *Requirements:*

history.adoc

@@ -285,3 +285,9 @@ node coordinate variables to be one of the dimensions of the data variable.
 .2 July 2021
 . link:$$https://github.com/cf-convention/cf-conventions/issues/298$$[Issue #298]: Interpretation of negative years in the units attribute
 . link:$$https://github.com/cf-convention/cf-conventions/issues/319$$[Issue #319]: Restrict "gregorian" label to only dates in the Gregorian calendar
+
+.24 August 2021
+. Added <<compression-by-coordinate-subsampling, Section 8.3, "Lossy Compression by Coordinate Subsampling">>.
+. Updated definitions in chapter 1 and Appendix A
+. Added <<appendix-coordinate-subsampling, Appendix J, "Coordinate Subsampling Methods">>.
+