CF Conformance Requirements and Recommendations 1.7 draft

  • The following is a list of requirements and recommendations for a CF conforming netCDF file. They are organized by the section of the CF document that they pertain to.

  • This document is intended to be a concise summary of the CF Conventions document. If there are any discrepencies between the two, the conventions document is the ultimate authority.

  • This document will updated as required to correct mistakes or add new material required for completeness or clarity.

2.1 Filename

Requirements:

  • Filename must have ".nc" suffix.

2.2 Data Types

Requirements:

  • CF attributes that take string values must be 1D character arrays.

2.3 Naming Conventions

Requirements:

  • Variable, dimension and attribute names must begin with a letter and be composed of letters, digits, and underscores.

Recommendations:

  • No two variable names should be identical when case is ignored.

2.4 Dimensions

Requirements:

  • The dimensions of a variable must all have different names.

Recommendations:

  • If any or all of the dimensions of a variable have the interpretations (as given by their units or axis attribute) of time (T), height or depth (Z), latitude (Y), or longitude (X) then those dimensions should appear in the relative order T, then Z, then Y, then X in the CDL definition corresponding to the file.

  • In files that are meant to conform to the COARDS subset of CF, any dimensions of a variable other than space and time dimensions should be added "to the left" of the space and time dimensions as represented in CDL.

2.5.1 Missing data, valid and actual range of data

Requirements:

  • The valid_range attribute must not be present if the valid_min and/or valid_max attributes are present.

  • The _FillValue attribute must be the same type as its associated variable.

  • The missing_value attribute must be the same type as its associated variable.

  • The actual_range attribute must be of the same type as its associated variable unless there is a scale_factor and/or add_offset attribute, in which case it must be of the same type as those attributes.

  • The actual_range attribute must have two elements, of which the first exactly equals the minimum non-missing value occurring in the associated variable after any scale_factor and add_offset are applied, and the second exactly equals the maximum value in the same way.

  • There must not be an actual_range attribute if all the data values of the associated variable equal the missing value.

  • If both the actual_range and valid_range/valid_min/valid_max are specified, the values of the actual_range must be valid values.

Recommendations:

  • The value of the _FillValue attribute should not be within a specified valid range.

  • If both missing_value and _FillValue be used, they should have the same value.

2.6.1 Identification of Conventions

Requirements:

  • Files that conform to the CF version 1.7 conventions must indicate this by setting the global Conventions attribute to the string value "CF-1.7".

  • The Conventions attribute may be a single text string containing a list of convention names separated by blank space or commas, one of which shall be the full CF string as described above.

2.6.2 Description of File Contents

Requirements:

  • The title, history, institution, source, references, and comment attributes are all type string.

Recommendations:

  • The title and history attributes are only defined as global attributes. If they are used as per variable attributes a CF compliant application should treat them exactly as it would treat any other unrecognized attribute.

2.6.3 External variables

Requirements:

  • The external_variables attribute is of string type and contains a blank-separated list of variable names.

  • No variable named by external_variables is allowed in the file.

3 Description of the Data

Recommendations:

  • All variables should use either the long_name or the standard_name attributes to describe their contents. Exceptions are boundary and climatology variables.

3.1 Units

Requirements:

  • The units attribute is required for all variables that represent dimensional quantities (except for boundary variables defined in section 7.1 and climatology variables defined in section 7.4 ).

  • The type of the units attribute is a string that must be recognizable by the udunits package. Exceptions are the units level, layer, and sigma_level.

  • The units of a variable that specifies a standard_name must be physically equivalent to the canonical units given in the standard name table, as modified by the standard_name modifier, if there is one, according to Appendix C, and then modified by all the methods listed in order by the cell_methods attribute, if one is present, according to Appendix E.

Recommendations:

  • The units level, layer, and sigma_level are deprecated.

3.3 Standard Name

Requirements:

  • The standard_name attribute takes a string value comprised of a standard name optionally followed by one or more blanks and a standard name modifier.

  • The legal values for the standard name are contained in the standard name table.

  • The legal values for the standard name modifier are contained in Appendix C, Standard Name Modifiers.

  • If a variable has a standard_name of region or area_type, it must have value(s) from the permitted list.

Recommendataions:

  • Use of the standard_name modifiers status_flag and number_of_observations is deprecated, and the corresponding standard_names are recommended instead.

3.5 Flags

Requirements:

  • The flag_values attribute must have the same type as the variable to which it is attached.

  • If the flag_values attribute is present then the flag_meanings attribute must be specified.

  • The type of the flag_meanings attribute is a string whose value is a blank separated list of words or phrases,  each consisting of characters from the alphanumeric set and the following five: '_', '-', '.', '+', '@'.

  • The number of flag_values attribute values must equal the number of words or phrases appearing in the flag_meanings string.

  • The number of flag_masks attribute values must equal the number of words or phrases appearing in the flag_meanings string.

  • Variables with a flag_masks attribute must have a type that is compatible with bit field expression (char, byte, short and int), not floating-point (float, real, double), and the flag_masks attribute must have the same type.

  • The flag_masks attribute values must be non-zero.

  • The flag_values attribute values must be mutually exclusive among the set of flag_values attribute values defined for that variable.

Recommendations:

  • When flag_masks and flag_values are both defined, the Boolean AND of each entry in flag_values with its corresponding entry in flag_masks should equal the flag_values entry, ie, the mask selects all the bits required to express the value.

4 Coordinate Types

Requirements:

  • The axis attribute may only be attached to a coordinate variable.

  • The only legal values of axis are X, Y, Z, and T (case insensitive).

  • The axis attribute must be consistent with the coordinate type deduced from units and positive.

  • The axis attribute is not allowed for auxiliary coordinate variables.

  • A data variable must not have more than one coordinate variable with a particular value of the axis attribute.

4.3 Vertical (height or depth) Coordinate

Requirements:

  • The only legal values for the positive attribute are up or down (case insensitive).

Recommendations:

  • The positive attribute should be consistent with the sign convention implied by the definition of the standard_name, if both are provided.

4.3.3 Parameterized Vertical Coordinate

Requirements:

  • The formula_terms attribute is only allowed on a coordinate variable which has a standard_name listed in Appendix C.

  • The type of the formula_terms attribute is a string whose value is list of blank separated word pairs in the form term: var. The legal values term are contained in Appendix C for each valid standard_name. The values of var must be variables that exist in the file.

  • Where indicated by the appropriate definition in Appendix D, the standard_name attributes of variables named by the formula_terms attribute must be consistent with the standard_name of the coordinate variable it is attached to, according to the appropriate definition in Appendix D.

  • The computed_standard_name attribute is only allowed on a coordinate variable which has a formula_terms attribute.

  • The computed_standard_name attribute is a string whose value must be consistent with the standard_name of the coordinate variable it is attached to, and in some cases also with the standard_name attributes of variables named by the formula_terms attribute, according to the appropriate definition in Appendix D.

4.4 Time Coordinate

Requirements:

  • The time units of a time coordinate variable must contain a reference time.

  • The reference time of a time coordinate variable must be a legal time in the specified calendar.

Recommendations:

  • The use of a reference time in the year 0 to indicate climatological time is deprecated. This restriction only applies to the real-world calendar as used by the udunits package.

  • Units of year and month and any equivalent units should be used with caution.

4.4.1 Calendar

Requirements:

  • The attributes calendar, month_lengths, leap_year, and leap_month may only be attached to time coordinate variables.

  • The standardized values of the calendar attribute are gregorian, standard, proleptic_gregorian, noleap, 365_day, all_leap, 366_day, 360_day, julian, and none (case insensitive). If the calendar attribute is given a non-standard value, then the attribute month_lengths is required, along with leap_year and leap_month as appropriate.

  • The type of the month_lengths attribute must be an integer array of size 12.

  • The values of the leap_month attribute must be in the range 1-12.

  • The values of the leap_year and leap_month attributes are integer scalars.

Recommendations:

  • The attribute leap_month should not appear unless the attribute leap_year is present.

  • The time coordinate should not cross the date 1582-10-15 when the default mixed Gregorian/Julian calendar is in use.

5 Coordinate Systems

Requirements:

  • All of a variable’s dimensions that are latitude, longitude, vertical, or time dimensions must have corresponding coordinate variables.

  • A coordinate variable must have values that are strictly monotonic (increasing or decreasing).

  • A coordinate variable must not have the _FillValue or missing_value attributes.

  • The type of the coordinates attribute is a string whose value is a blank separated list of variable names. All specified variable names must exist in the file.

  • The dimensions of each auxiliary coordinate must be a subset of the dimensions of the variable they are attached to, with two exceptions. First, a label variable which will have a trailing dimension for the maximum string length. Second a ragged array (Chapter 9, Discrete sampling geometries and Appendix H) uses special, more indirect, methods to connect the data and coordinates.

Recommendations:

  • The name of a multidimensional coordinate variable should not match the name of any of its dimensions.

  • All horizontal coordinate variables (in the Unidata sense) should have an axis attribute.

  • All horizontal coordinate variables (in the unidata sense) should have an axis attribute.

5.6 Grid Mappings and Projections

Requirements:

  • The type of the grid_mapping attribute is a string whose value is of the following form, in which brackets indicate optional text:

    grid_mapping_name[: coord_var [coord_var ...]] [grid_mapping_name: [coord_var ... ]]
  • Note that in its simplest form the attribute comprises just a grid_mapping_name as a single word.

  • Each grid_mapping_name is the name of a variable (known as a grid mapping variable), which must exist in the file.

  • Each coord_var is the name of a coordinate variable or auxiliary coordinate variable, which must exist in the file. If it is an auxiliary coordinate variable, it must be listed in the coordinates attribute.

  • The grid mapping variables must have the grid_mapping_name attribute. The legal values for the grid_mapping_name attribute are contained in Appendix F.

  • The data types of the attributes of the grid mapping variable must be specified in Table 1 of Appendix F.

  • If present, the crs_wkt attribute must be a text string conforming to the CRS WKT specification described in reference [OGC_CTS].

  • reference_ellipsoid_name, prime_meridian_name, horizontal_datum_name and geographic_crs_name must be all defined if any one is defined.

  • If projected_crs_name is defined then geographic_crs_name must be also.

Recommendations:

  • The grid mapping variables should have 0 dimensions.

6.1 Labels

Requirements:

  • A variable of character type that is named by a coordinates attribute is a label variable. This variable must have one or two dimensions. The trailing (CDL order) or sole dimension is for the maximum string length. If there are two dimensions, leading dimension (CDL order) must match one of those of the data variable.

7.1 Cell Boundaries

Requirements:

  • The type of the bounds attribute is a string whose value is a single variable name. The specified variable must exist in the file.

  • A boundary variable must have the same dimensions as its associated variable, plus have a trailing dimension (CDL order) for the maximum number of vertices in a cell.

  • A boundary variable must be a numeric data type.

  • If a boundary variable has units or standard_name attributes, they must agree with those of its associated variable.

Recommendations:

  • The points specified by a coordinate or auxiliary coordinate variable should lie within, or on the boundary, of the cells specified by the associated boundary variable.

  • Boundary variables should not have the _FillValue or missing_value attributes.

7.2 Cell Measures

Requirements:

  • The type of the cell_measures attribute is a string whose value is list of blank separated word pairs in the form measure: var. The valid values for measure are area or volume. The var token specifies a variable that must either exist in the file or be named by the external_variables attribute. The dimensions of the variable specified by var must be the same as, or be a subset of, the dimensions of the variable to which they are related.

  • A measure variable must have units that are consistent with the measure type, i.e., square meters for area measures and cubic meters for volume measures.

7.3 Cell Methods

Requirements:

  • The type of the cell_methods attribute is a string whose value is one or more blank separated word lists, each with the form

    dim1: [dim2: [dim3: ...]] method [where type1 [over type2]] [within|over days|years] [(comment)]

    where brackets indicate optional words. The valid values for dim1 [dim2 [dim3 …​] ] are the names of dimensions of the data variable, names of scalar coordinate variables of the data variable, valid standard names, or the word area. The valid values of method are contained in Appendix E. The valid values for type1 are the name of a string-valued auxiliary or scalar coordinate variable with a standard_name of area_type, or any string value allowed for a variable of standard_name of area_type. If type2 is a string-valued auxiliary coordinate variable, it is not allowed to have a leading dimension (the number of strings) of more than one. When the method refers to a climatological time axis, the suffixes for within and over may be appended.

  • A given dimension name may only occur once in a cell_methods string. An exception is a climatological time dimension.

  • The comment, if present, must take the form ([interval: value unit [interval: …​] comment:] remainder )

    The remainder text is not standardized. If no interval clauses are present, the entire comment is therefore not standardized. There may be zero interval clauses, one interval clause, or exactly as many interval clauses as there are dims to which the method applies. The value must be a valid number and the unit a string that is recognizable by the udunits package.

Recommendations:

  • If a data variable has any dimensions or scalar coordinate variables referring to horizontal, vertical or time dimensions, it should have a cell_methods attribute with an entry for each of these spatiotemporal dimensions or scalar coordinate variables. (The horizontal dimensions may be covered by an area entry.)

  • Except for entries whose cell method is point, all numeric coordinate variables and scalar coordinate variables named by cell_methods should have bounds or climatology attributes.

7.4 Climatological Statistics

Requirements:

  • The climatology attribute may only be attached to a time coordinate variable.

  • The type of the climatology attribute is a string whose value is a single variable name. The specified variable must exist in the file.

  • A climatology variable must have the same dimension as its associated time coordinate variable, and have a trailing dimension (CDL order) of size 2.

  • A climatology variable must be a numeric data type.

  • If a climatology variable has units, standard_name, or calendar attributes, they must agree with those of its associated variable.

  • A climatology variable must not have _FillValue or missing_value attributes.

8.1 Packed Data

Requirements:

  • The scale_factor and add_offset attributes must be the same numeric data type.

  • If scale_factor and add_offset are a different type than the variable, then they must be either type float or type double.

  • If scale_factor and add_offset are a different type than the variable, then the variable must be type byte, short or int.

Recommendations:

  • If scale_factor and add_offset are type float, the variable should not be of type int.

8.2 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).