Changes between Version 16 and Version 17 of Ticket #152


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Timestamp:
11/21/16 11:49:20 (2 years ago)
Author:
martin.juckes
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  • Ticket #152 – Description

    v16 v17  
    33The proposal is to make it clear that use of `where` for non-spatial dimensions is allowed by adding examples in section 7. It is also necessary to provide these examples to clarify the subtle differences implied by different formulations of the `cell_methods` statement.
    44
    5 == Replace text of section 7.3.3 ==
     5== Replace text of section 7.3.3 (first 3 paragraphs) ==
    66
    77By 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). The portion concerned is constant in time in some cases, but it could be time-varying. Grid cell “portions” that can be considered are only those permitted to be associated with the   `standard_name` of `area_type`. There are two options for indicating when a quantity represents a portion of a cell.
    88
    99The first method can be used for the common case that the cell_method applies to 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. As an example, if the method is `area: mean where sea_ice`, then the data would represent a mean over only the sea ice portion of the grid cell. When this first option is adopted, none of the variables in the netCDF file should be given a name identical to the string that names the `area_type`.  This restriction is imposed so that it will be clear that the metadata should not be interpreted following the second option (described in the next paragraph), which takes precedence.
     10
    1011The second method for indicating that a statistic applies to only a portion of a cell is more general because it can reference multiple area-types.  This may be needed when a variable has a dimension that ranges across various area types.  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 Section 6.1, "Labels") 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. This method 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.).
     12
     13== Caption of example 7.6
     14
     15If the method is `mean`, various ways of calculating the mean can be distinguished in the `cell_methods` attribute with a string of the form `mean where type1 [over type2]`. Here, type1 can be any of the possibilities allowed for typevar or type (as specified in the two paragraphs preceding above Example). The same options apply to type2, except it is not allowed to be the name of an auxiliary coordinate variable with a dimension greater than one (ignoring the dimension accommodating the maximum string length).
     16
     17A cell_methods attribute with a string of the form `area: mean where type1 over type2` indicates the mean is calculated by integrating over the type1 portion of the cell and dividing by the area of the type2 portion.  When `over type2` is omitted, it is assumed to be the same as type1.
    1118
    1219== Clarification at start of section 7.3.3 (not needed if above is accepted) ==