[netcdfgroup] What attributes/dimensions are required to be set on a variable to make it georeferenced?

• To: netcdfgroup@xxxxxxxxxxxxxxxx
• Subject: [netcdfgroup] What attributes/dimensions are required to be set on a variable to make it georeferenced?
• From: James Adams <monocongo@xxxxxxxxx>
• Date: Fri, 27 Mar 2015 11:33:37 -0400

I have a variable that I've created in a file which I want to be able to
read as a georeferenced data set, i.e. using something like this with the
Java API:

        GridDataset gridDataset = GridDataset.open(netcdfFile);
        GeoGrid geoGrid = gridDataset.findGridByShortName(variableName);

Currently the NetCDF variable looks like this (from ncdump):

float available_water_capacity(time=1, lon=1385, lat=596);
  :_FillValue = -999.9f; // float
  :least_significant_digit = 3; // int
  :_ChunkSize = 1, 1385, 596; // int

I assume that I have not set either sufficient attributes and/or dimensions
on the data variable (available_water_capacity), so I'm fishing for ideas
as to how I can flesh this out when creating/writing the files in my Python
code.  The NetCDF file is currently created like so:

    # open the output file for writing, set its dimensions and variables
    output_dataset = netCDF4.Dataset(output_file, 'w')
    output_dataset.createDimension('time', None)
    output_dataset.createDimension('lon', len(input_lon_dimension))
    output_dataset.createDimension('lat', len(input_lat_dimension))

    # create lon dimension variable
    output_lon_variable = output_dataset.createVariable('lon','f4',('lon',))
    if 'units' in input_lon_variable.ncattrs():
        output_lon_variable.units = input_lon_variable.units
    output_lon_variable[0:len(input_lon_dimension):1] =
input_lon_variable[0:len(input_lon_dimension):1]

    # create lat dimension variable
    output_lat_variable = output_dataset.createVariable('lat','f4',('lat',))
    if 'units' in input_lat_variable.ncattrs():
        output_lat_variable.units = input_lat_variable.units
    output_lat_variable[0:len(input_lat_dimension):1] =
input_lat_variable[0:len(input_lat_dimension):1]

    # create a time variable with a single value
    output_time_variable =
output_dataset.createVariable('time','i4',('time',))
    output_time_variable.units = 'days since 1900-01-01 00:00:00'
    output_time_variable[0:1:1] = numpy.array([0])

    # create the soil constant variable
    output_soil_variable =
output_dataset.createVariable('available_water_capacity','f4',('time',
'lon', 'lat'),

 fill_value=fill_value,
                                                         zlib=True,

 least_significant_digit=3)

    attributeDictionary = { "Conventions":"CF-1.3" }
    output_dataset.setncatts(attributeDictionary)


The trouble is that when I read this NetCDF file in Java code which uses
the approach outlined above it does not find the variable as a grid in the
GridDataset, however I'm using this approach with other NetCDF variables
which are correctly georeferenced (they show up as Geo2D types in Panoply),
so I assume that I've failed to completely flesh out this variable so that
it'll be recognized as a grid. What more might I add in order to make this
variable discoverable as a georeferenced grid?

Thanks in advance if anyone has suggestions on this.

--James