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From "Martin Desruisseaux (JIRA)" <j...@apache.org>
Subject [jira] [Created] (SIS-448) Extension to CF-conventions for localization grid smaller than data in netCDF
Date Sun, 17 Mar 2019 14:35:00 GMT
Martin Desruisseaux created SIS-448:
---------------------------------------

             Summary: Extension to CF-conventions for localization grid smaller than data
in netCDF
                 Key: SIS-448
                 URL: https://issues.apache.org/jira/browse/SIS-448
             Project: Spatial Information Systems
          Issue Type: Improvement
          Components: Storage
            Reporter: Martin Desruisseaux
            Assignee: Martin Desruisseaux
             Fix For: 1.0


The _Global Change Observation Mission - Climate (GCOM-C)_ data have content like below (simplified):

{noformat}
group: Geometry_data {
    variables:
        float Latitude(161, 126)
        string Unit = "degree"
        string Dim0 = "Line grids"
        string Dim1 = "Pixel grids"
        int Resampling_interval = 10
    float Longitude(161, 126)
        string Unit = "degree"
        string Dim0 = "Line grids"
        string Dim1 = "Pixel grids"
        int Resampling_interval = 10
}
group: Image_data {
    variables:
        ushort SST(1599, 1250)        // Note: different size than (latitude, longitude) variables.
        string dim0 = "Line grids"
        string dim1 = "Pixel grids"
        string Unit = "degree"
}
{noformat}

The size of latitude and longitude variables is not the same than the size of image data.
In this case, even if reader correctly identifies {{Latitude}} and {{Longitude}} as the variables
to use for building a localization grid, we are still unable to associate the {{SST}} variable
to those axes because they have no dimension in common. However if we interpret {{dim0}} and
{{dim1}} attributes as _"Name of dimension 0"_ and _"Name of dimension 1"_ respectively, then
we can associate the same dimension *names* to all those variables: namely {{"Line grids"}}
and {"Pixel grids"}} in above example. Using those names, we deduce that the ({{data_y}},
{{data_x}}) dimensions in the {{SST}} variable are mapped to the ({{grid_y}}, {{grid_x}})
dimensions in the localization grid.

The feature is an extension to CF-conventions, as I'm not aware of equivalent mechanism in
CF at this time. The {{coordinates}} attributes is not exactly the same since it tells us
which variable to use, but not which dimensions (an ambiguity still exists if the variable
has 2 or more dimensions).




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