Defining a domain for a cell_method
|Reported by:||markh||Owned by:||cf-conventions@…|
Description (last modified by cofino)
Defining the domain of operation as part of a cell_method definition.
The information contained within a data variable's coordinate may have value in defining a cell_method for a result data variable calculated from the initial data variable.
4. Initial Statement of Technical Proposal
7.3.2. Recording the spacing of the original data and other information
To indicate more precisely how the cell method was applied, extra information may be included in parentheses () after the identification of the method. This information includes standardized and non-standardized parts.
Currently the only standardized information isThe interval spacing keyword is used to provide the typical interval between the original data values to which the method was applied, in the situation where the present data values are statistically representative of original data values which had a finer spacing. The syntax is (interval: value unit), where value is a numerical value and unit is a string that can be recognized by UNIDATA's Udunits package [UDUNITS]. The unit will usually be dimensionally equivalent to the unit of the corresponding dimension, but this is not required (which allows, for example, the interval for a standard deviation calculated from points evenly spaced in distance along a parallel to be reported in units of length even if the zonal coordinate of the cells is given in degrees). Recording the original interval is particularly important for standard deviations. For example, the standard deviation of daily values could be indicated by cell_methods="time: standard_deviation (interval: 1 day)" and of annual values by cell_methods="time: standard_deviation (interval: 1 year)".
If the cell method applies to a combination of axes, they may have a common original interval e.g. cell_methods="lat: lon: standard_deviation (interval: 10 km)". Alternatively, they may have separate intervals, which are matched to the names of axes by position e.g. cell_methods="lat: lon: standard_deviation (interval: 0.1 degree_N interval: 0.2 degree_E)", in which 0.1 degree applies to latitude and 0.2 degree to longitude.
To explicitly define the domain over which the statistic was calculated, the syntax (domain: varname [varname]) may be used. In this case, each 'varname' listed is the name of an ancillary variable, explicitly referenced by the data variable. Each of the ancillary variables referenced contributes to the definition of the domain over which the cell_method operation was conducted.
The community will benefit by having enhanced capabilities for defining cell_methods for data variables calculated across complex domains.
aggregateExampleMH presents a use case for this capability.
6. Status Quo
The capability to define complex domains for cell_methods will not be standardised.