diff --git a/ch03.adoc b/ch03.adoc index 9e54fce0..baafdc2e 100644 --- a/ch03.adoc +++ b/ch03.adoc @@ -178,7 +178,7 @@ Knowledge of the XML format is only necessary for application writers who plan t A formatted text version of the table is provided at link:$$https://cfconventions.org/Data/cf-standard-names/current/build/cf-standard-name-table.html$$[https://cfconventions.org/Data/cf-standard-names/current/build/cf-standard-name-table.html], and this table may be consulted in order to find the standard name that should be assigned to a variable. -Some standard names (e.g. **`region`** and **`area_type`**) are used to indicate quantities which are permitted to take only certain standard values. +Some standard names (e.g. **`region`**, <>, and **`area_type`**, <>) are used to indicate quantities which are permitted to take only certain standard values. This is indicated in the definition of the quantity in the standard name table, accompanied by a list or a link to a list of the permitted values. Standard names by themselves are not always sufficient to describe a quantity. diff --git a/ch07.adoc b/ch07.adoc index d71db454..bcc24417 100644 --- a/ch07.adoc +++ b/ch07.adoc @@ -379,11 +379,11 @@ Notice that a parenthesized comment in the `cell_methods` attribute provides the By default, the statistical method indicated by **`cell_methods`** is assumed to have been evaluated over the entire horizontal area of the cell. Sometimes, however, it is useful to limit consideration to only a portion of a cell (e.g. a mean over the sea-ice area). -To indicate this, one of two conventions may be used. +Cell portions are referred to by means of standardised **`area_type`** strings, maintained in the link:$$https://cfconventions.org/Data/area-type-table/current/build/area-type-table.html$$[area-type table], using one of two conventions. The first convention is a method that can be used for the common case of a single area-type. In this case, the **`cell_methods`** attribute may include a string of the form "__name: method__ **`where`** __type__". -Here __name__ could, for example, be **`area`** and __type__ may be any of the strings permitted for a variable with a **`standard_name`** of **`area_type`**. +Here __name__ could, for example, be **`area`** and __type__ may be any of the standardised **`area_type`** strings. As an example, if the method were **`mean`** and the **`area_type`** were **`sea_ice`**, then the data would represent a mean over only the sea ice portion of the grid cell. If the data writer expects __type__ to be interpreted as one of the standard **`area_type`** strings, then none of the variables in the netCDF file should be given a name identical to that of the string (because the second convention, described in the next paragraph, takes precedence). @@ -391,6 +391,7 @@ The second convention is the more general. In this case, the **`cell_methods`** entry is of the form "__name: method__ **`where`** __typevar__". Here __typevar__ is a string-valued auxiliary coordinate variable or string-valued scalar coordinate variable (see <>) with a **`standard_name`** of **`area_type`**. The variable __typevar__ contains the name(s) of the selected portion(s) of the grid cell to which the __method__ is applied. +These name(s) must be a subset of the standardised **`area_type`** strings. This convention can accommodate cases in which a method is applied to more than one area type and the result is stored in a single data variable (with a dimension which ranges across the various area types). It provides a convenient way to store output from land surface models, for example, since they deal with many area types within each surface gridbox (e.g., **`vegetation`**, **`bare_ground`**, **`snow`**, etc.). diff --git a/history.adoc b/history.adoc index bfca341c..65e15016 100644 --- a/history.adoc +++ b/history.adoc @@ -7,6 +7,7 @@ === Working version (most recent first) +* {issues}550[Issue #550]: Include a link to CF area-type table and make explicit the need to use standardized area-type strings in Section 7.3.3. * {issues}367[Issue #367]: Remove the AMIP and GRIB columns from the standard name table format defined by Appendix B. * {issues}403[Issue #403]: Metadata to encode quantization properties * {issues}530[Issue #530]: Define "the most rapidly varying dimension", and use this phrase consistently with the clarification "(the last dimension in CDL order)".