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conformance-1.9.html
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<title>CF Conformance Requirements and Recommendations 1.9</title>
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<div id="header">
</div>
<div id="content">
<div class="sect1">
<h2 id="cf-conformance-requirements-and-recommendations-1.9">CF Conformance Requirements and Recommendations 1.9</h2>
<div class="sectionbody">
<div class="ulist">
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>This document is intended to be a concise summary of the
<a href="http://cfconventions.org/cf-conventions/cf-conventions.html">CF Conventions document</a>.
If there are any discrepencies between the two, the
conventions document is the ultimate authority.</p>
</li>
<li>
<p>This document will be updated as required to correct mistakes or add new
material required for completeness or clarity.</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="filename">2.1 Filename</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>Filename must have ".nc" suffix.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="data-types">2.2 Data Types</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>CF attributes that take string values must be 1D character arrays or single
atomic strings.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="naming-conventions">2.3 Naming Conventions</h3>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>Variable, dimension and attribute names should begin with a letter and
be composed of letters, digits, and underscores.</p>
</li>
<li>
<p>No two variable names should be identical when case is ignored.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="dimensions">2.4 Dimensions</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The dimensions of a variable must all have different names.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>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.</p>
</li>
<li>
<p>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.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="missing-data-valid-and-actual-range-of-data">2.5.1 Missing data, valid and actual range of data</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>valid_range</code></strong> attribute must not be present if the <strong><code>valid_min</code></strong> and/or
<strong><code>valid_max</code></strong> attributes are present.</p>
</li>
<li>
<p>The <strong><code>_FillValue</code></strong> attribute must be the same type as its associated
variable.</p>
</li>
<li>
<p>The <strong><code>missing_value</code></strong> attribute must be the same type as its associated
variable.</p>
</li>
<li>
<p>The <strong><code>actual_range</code></strong> attribute must be of the same type as its associated
variable unless there is a <strong><code>scale_factor</code></strong> and/or <strong><code>add_offset</code></strong> attribute, in
which case it must be of the same type as those attributes.</p>
</li>
<li>
<p>The <strong><code>actual_range</code></strong> attribute must have two elements, of which the first
exactly equals the minimum non-missing value occurring in the associated
variable after any <strong><code>scale_factor</code></strong> and <strong><code>add_offset</code></strong> are applied, and the
second exactly equals the maximum value in the same way.</p>
</li>
<li>
<p>There must not be an <strong><code>actual_range</code></strong> attribute if all the data values of
the associated variable equal the missing value.</p>
</li>
<li>
<p>If both the <strong><code>actual_range</code></strong> and <strong><code>valid_range/valid_min/valid_max</code></strong> are
specified, the values of the <strong><code>actual_range</code></strong> must be valid values.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The value of the <strong><code>_FillValue</code></strong> attribute should not be within a specified
valid range.</p>
</li>
<li>
<p>If both <strong><code>missing_value</code></strong> and <strong><code>_FillValue</code></strong> be used, they should have the
same value.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="identification-of-conventions">2.6.1 Identification of Conventions</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>Conventions</code></strong> attribute must 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 below.</p>
</li>
<li>
<p>Files that conform to the CF version 1.8 conventions must indicate this by setting the global <strong><code>Conventions</code></strong> attribute to contain the CF string value "CF-1.8".</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="description-of-file-contents">2.6.2 Description of File Contents</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>title</code></strong>, <strong><code>history</code></strong>, <strong><code>institution</code></strong>, <strong><code>source</code></strong>, <strong><code>references</code></strong>, and <strong><code>comment</code></strong>
attributes are all type string.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>title</code></strong> and <strong><code>history</code></strong> attributes are only defined as global or groups 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.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_2_6_3_external_variables">2.6.3 External variables</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>external_variables</code></strong> attribute is of string type and contains a blank-separated
list of variable names.</p>
</li>
<li>
<p>No variable named by <strong><code>external_variables</code></strong> is allowed in the file.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="groups">2.7 Groups</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>Conventions</code></strong> and <strong><code>external_variables</code></strong> attributes must not be used in non-root groups.</p>
</li>
<li>
<p>If any dimension of an out-of-group variable has the same name as a dimension of the referring variable, the two must be the same dimension (i.e. they must have the same netCDF dimension ID).</p>
</li>
<li>
<p>Variable or dimension paths must follow a UNIX style file convention. They must be formed of words (composed of letters, digits and underscores) and be separated by the slash character ('/'). Paths may begin with either '/', '…​' or a word.</p>
</li>
<li>
<p>The variable or dimension referenced must exist in the file unless it is an external variable. References can be absolute, relative or with no path, in which case, the variable or dimension must be found in one of the following (in order of precedence):</p>
<div class="ulist">
<ul>
<li>
<p>In the referring group</p>
</li>
<li>
<p>In the ancestor group (starting from the direct ancestor and proceeding toward the root group, until it is found)</p>
</li>
<li>
<p>By the lateral search algorithm for coordinate variables only.</p>
</li>
</ul>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>NUG-coordinate variables that are not in the referring group or one of its direct ancestors should be referenced by absolute or relative paths rather than relying on the lateral search algorithm.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="description-of-the-data">3 Description of the Data</h3>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>All variables should use either the <strong><code>long_name</code></strong> or the <strong><code>standard_name</code></strong>
attributes to describe their contents. Exceptions are boundary and
climatology variables.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="units">3.1 Units</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>units</code></strong> attribute is required for all variables that represent
dimensional quantities (except for boundary variables defined in
<a href="http://cfconventions.org/cf-conventions/cf-conventions.html#cell-boundaries">section 7.1</a>
and climatology variables defined in
<a href="http://cfconventions.org/cf-conventions/cf-conventions.html#climatological-statistics">section 7.4</a>
).</p>
</li>
<li>
<p>The type of the <strong><code>units</code></strong> attribute is a string that must be recognizable
by the udunits package. Exceptions are the units <strong><code>level, layer, and
sigma_level</code></strong>.</p>
</li>
<li>
<p>The <strong><code>units</code></strong> of a variable that specifies a <strong><code>standard_name</code></strong> must be
physically equivalent to the canonical units given in the standard name
table, as modified by the <strong><code>standard_name</code></strong> modifier, if there is one,
according to Appendix C, and then modified by all the methods listed in
order by the <strong><code>cell_methods</code></strong> attribute, if one is present, according to
Appendix E.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The units <strong><code>level</code></strong>, <strong><code>layer</code></strong>, and <strong><code>sigma_level</code></strong> are deprecated.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="standard-name">3.3 Standard Name</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>standard_name</code></strong> attribute takes a string value comprised of a
standard name optionally followed by one or more blanks and a standard
name modifier.</p>
</li>
<li>
<p>The legal values for the standard name are contained in the standard
name table.</p>
</li>
<li>
<p>The legal values for the standard name modifier are contained in
Appendix C, Standard Name Modifiers.</p>
</li>
<li>
<p>If a variable has a <strong><code>standard_name</code></strong> of <strong><code>region</code></strong> or <strong><code>area_type</code></strong>, it must have value(s)
from the permitted list.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendataions:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>Use of the <strong><code>standard_name</code></strong> modifiers <strong><code>status_flag</code></strong> and <strong><code>number_of_observations</code></strong>
is deprecated, and the corresponding <strong><code>standard_names</code></strong> are recommended instead.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="flags">3.5 Flags</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>flag_values</code></strong> attribute must have the same type as the variable to
which it is attached.</p>
</li>
<li>
<p>If the <strong><code>flag_values</code></strong> attribute is present then the <strong><code>flag_meanings</code></strong>
attribute must be specified.</p>
</li>
<li>
<p>The type of the <strong><code>flag_meanings</code></strong> 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: '_', '-', '.', '+',
'@'.</p>
</li>
<li>
<p>The number of <strong><code>flag_values</code></strong> attribute values must equal the number of
words or phrases appearing in the <strong><code>flag_meanings</code></strong> string.</p>
</li>
<li>
<p>The number of <strong><code>flag_masks</code></strong> attribute values must equal the number of
words or phrases appearing in the <strong><code>flag_meanings</code></strong> string.</p>
</li>
<li>
<p>Variables with a <strong><code>flag_masks</code></strong> 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 <strong><code>flag_masks</code></strong> attribute must
have the same type.</p>
</li>
<li>
<p>The <strong><code>flag_masks</code></strong> attribute values must be non-zero.</p>
</li>
<li>
<p>The <strong><code>flag_values</code></strong> attribute values must be mutually exclusive among the
set of <strong><code>flag_values</code></strong> attribute values defined for that variable.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>When <strong><code>flag_masks</code></strong> and <strong><code>flag_values</code></strong> are both defined, the Boolean AND of
each entry in <strong><code>flag_values</code></strong> with its corresponding entry in <strong><code>flag_masks</code></strong>
should equal the <strong><code>flag_values</code></strong> entry, ie, the mask selects all the bits
required to express the value.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="coordinate-types">4 Coordinate Types</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>axis</code></strong> attribute may only be attached to coordinate variables and geometry node coordinate variables (Chapter 7).</p>
</li>
<li>
<p>The only legal values of axis are <strong><code>X</code></strong>, <strong><code>Y</code></strong>, <strong><code>Z</code></strong>, and <strong><code>T</code></strong> (case insensitive).</p>
</li>
<li>
<p>The <strong><code>axis</code></strong> attribute must be consistent with the coordinate type deduced
from <strong><code>units</code></strong> and <strong><code>positive</code></strong>.</p>
</li>
<li>
<p>The <strong><code>axis</code></strong> attribute is not allowed for auxiliary coordinate variables.</p>
</li>
<li>
<p>A data variable must not have more than one coordinate variable with a
particular value of the <strong><code>axis</code></strong> attribute.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="vertical-height-or-depth-coordinate">4.3 Vertical (height or depth) Coordinate</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The only legal values for the <strong><code>positive</code></strong> attribute are <strong><code>up</code></strong> or <strong><code>down</code></strong> (case
insensitive).</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>positive</code></strong> attribute should be consistent with the sign convention implied by the
definition of the <strong><code>standard_name</code></strong>, if both are provided.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="dimensionless-vertical-coordinates">4.3.3 Parameterized Vertical Coordinate</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <strong><code>formula_terms</code></strong> attribute is only allowed on a coordinate variable
which has a <strong><code>standard_name</code></strong> listed in Appendix C.</p>
</li>
<li>
<p>The type of the <strong><code>formula_terms</code></strong> attribute is a string whose value is
list of blank separated word pairs in the form <strong><code>term: var</code></strong>. The legal
values <strong><code>term</code></strong> are contained in Appendix C for each valid <strong><code>standard_name</code></strong>.
The values of <strong><code>var</code></strong> must be variables that exist in the file.</p>
</li>
<li>
<p>Where indicated by the appropriate definition in Appendix D, the <strong><code>standard_name</code></strong>
attributes of variables named by the <strong><code>formula_terms</code></strong> attribute must be consistent
with the <strong><code>standard_name</code></strong> of the coordinate variable it is attached to, according to
the appropriate definition in Appendix D.</p>
</li>
<li>
<p>The <strong><code>computed_standard_name</code></strong> attribute is only allowed on a coordinate variable
which has a <strong><code>formula_terms</code></strong> attribute.</p>
</li>
<li>
<p>The <strong><code>computed_standard_name</code></strong> attribute is a string whose value must be consistent
with the <strong><code>standard_name</code></strong> of the coordinate variable it is attached to, and in some cases
also with the <strong><code>standard_name</code></strong> attributes of variables named by the <strong><code>formula_terms</code></strong> attribute,
according to the appropriate definition in Appendix D.</p>
</li>
<li>
<p>The units of a variable named by the <strong><code>formula_terms</code></strong> attribute
must be consistent with the units defined in Appendix D.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="time-coordinate">4.4 Time Coordinate</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The time <strong><code>units</code></strong> of a time coordinate variable must contain a reference
date/time.</p>
</li>
<li>
<p>The reference date/time of a time coordinate variable must be a legal date/time
in the specified calendar.</p>
</li>
<li>
<p>The reference date/time in time <strong><code>units</code></strong> is not allowed to contain seconds
equal to or greater than 60.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>Recommendations:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The use of time coordinates in year 0 and reference date/times in year 0 to indicate climatological time is deprecated.</p>
</li>
<li>
<p>Units of <strong><code>year</code></strong> and <strong><code>month</code></strong> and any equivalent units should be used with caution.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="calendar">4.4.1 Calendar</h3>
<div class="paragraph">
<p><strong>Requirements:</strong></p>
</div>
<div class="ulist">
<ul>
<li>
<p>The attributes <strong><code>calendar</code></strong>, <strong><code>month_lengths</code></strong>, <strong><code>leap_year</code></strong>, and <strong><code>leap_month</code></strong> may only be attached to time coordinate variables.</p>
</li>
<li>
<p>The standardized values (case insensitive) of the <strong><code>calendar</code></strong> attribute are <strong><code>standard</code></strong>, <strong><code>gregorian</code></strong> (deprecated), <strong><code>proleptic_gregorian</code></strong>, <strong><code>noleap</code></strong>, <strong><code>365_day</code></strong>, <strong><code>all_leap</code></strong>, <strong><code>366_day</code></strong>,
<strong><code>360_day</code></strong>, <strong><code>julian</code></strong>, and <strong><code>none</code></strong>. If the <strong><code>calendar</code></strong> attribute
is given a non-standard value, then the attribute <strong><code>month_lengths</code></strong> is
required, along with <strong><code>leap_year</code></strong> and <strong><code>leap_month</code></strong> as appropriate.</p>
</li>
<li>
<p>The type of the <strong><code>month_lengths</code></strong> attribute must be an integer array of