Query: setting the variable name for a grid_mapping

[martinjuckes]

Hello David,

the script you gave me works fine now, and I've been able to modify it to adjust the names of variables in the output netcdf file by using either nc_set_variable or nc_set_dimension, with one exception: there is a grid_mapping variable which is generated with the name rotated_latitude_longitude and carries the datum: I can't find a construct which corresponds to this variable. Is there a way of changing the name-in-file of the grid_mapping variable?

[davidhassell]

Hi Martin,

There is indeed no construct in the field, because this grid_mapping netCDF variable was generated internally by the cfdm.write function. Therefore, at the present time there is no way to set its netCDF variable name.

Perhaps the best way to do this is to introduce a new nc_set_datum_variable method for the CoordinateReference object, with corresponding del, get, has methods.

You should be able to work around it by not setting the datum on the vertical CRS, and instead introducing a horizontal CRS object that contains just the datum. However, I tried this and it didn't look quite right - I'll look into this...

[martinjuckes]

Hi David,

there is no urgency about this. Your suggestion of a nc_set_datum_variable sounds like the right way forward, and I'm happy to wait for that rather than explore work-arounds.

[davidhassell]

Thanks - I'll implement that in v1.7.5. (By the way, the workaround does work correctly, afterall)

[martinjuckes]

Great

[davidhassell]

Fixed in version 1.7.5 in PyPi. The Datum object itself has the nc_set_variable method:

# Create the datum for the coordinate reference constructs
datum = cfdm.Datum(parameters={'earth_radius': 6371007.})

# Set the netCDF name for a grid mapping variable that might be created from this datum
datum.nc_set_variable('my_name')

# Create the coordinate conversion for the vertical coordinate
# reference construct
coordinate_conversion_v = cfdm.CoordinateConversion(
         parameters={'standard_name': 'atmosphere_hybrid_height_coordinate',
                     'computed_standard_name': 'altitude'},
         domain_ancillaries={'a': domain_anc_A,
                             'b': domain_anc_B,
                             'orog': domain_anc_OROG})

# Create the vertical coordinate reference construct
vertical_crs = cfdm.CoordinateReference(
                 datum=datum,
                 coordinate_conversion=coordinate_conversion_v,
                 coordinates=[dim_Z])

# Set the coordinate reference construct
tas.set_construct(vertical_crs)

(as well as del, get and has methods).

[martinjuckes]

Are you sure that works? I've tried it .. the datum.nc_set_variable('my_name') appears to work, but it doesn't change the name of the grid_mapping variable in the file.

[davidhassell]

Hi Martin, could you send me your script, so I may try it? Thanks

…

On Wed, 15 May 2019 at 16:23, Martin ***@***.***> wrote: Are you sure that works? I've tried it .. the datum.nc_set_variable('my_name') appears to work, but it doesn't change the name of the grid_mapping variable in the file. — You are receiving this because you modified the open/close state. Reply to this email directly, view it on GitHub <#3?email_source=notifications&email_token=AB6AA4FNBJQ3B5PU7DSIYILPVQTHPA5CNFSM4HNAWZF2YY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGODVPAQAQ#issuecomment-492701698>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AB6AA4DPEFI7AU2KTS7RIK3PVQTHPANCNFSM4HNAWZFQ> .

-- David Hassell National Centre for Atmospheric Science Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading RG6 6BB Tel: +44 118 3785183 http://www.met.reading.ac.uk/

[martinjuckes]

Hi David,

The script I have is here: test_write.py, with the output it prints and the resulting NetCDF file.

I've included the line datum.nc_set_variable('my_datum_name') in the python script, but the resulting variable is still rotated_latitude_longitude.

[davidhassell]

Thanks - I'll have a look.

[davidhassell]

Hi Martin,

The reason that the my_datum_name name wasn't used is because cfdm spotted that the datum is identical to that used in the existing grid mapping variable called rotated_latitude_longitude, and so creating another grid mapping variable was unecessary.

If you take the horizontal CRS out (#tas.set_construct(horizontal_crs)) and rerun you get:

$ ncdump delme.nc
<snip>
	char my_datum_name ;
		my_datum_name:earth_radius = 6371007. ;
		my_datum_name:grid_mapping_name = "latitude_longitude" ;
<snip>

Which is correct, I think.

Thanks,
David

[martinjuckes]

Yes, thanks .. that is working now.

I also tried using horizontal_crs.datum.nc_set_variable( 'my_datum_name' ), but this is also ignored: it looks as though the variable name of the datum object is being ignored for some reasons when there is a grid_mapping present.

[davidhassell]

That's as expected. A datum in CF-netCDF can not exist outside of a grid mapping variable, so the name of the grid mapping variable, if it exists as a data model construct, takes precedence.

If a horizontal coordinate reference construct does not exist when a vertical coordinate reference construct has a datum, then a grid_mapping variable has to be "made up" by cfdm.write to store said datum. It is only in this case that the datum's netCDF variable name gets used.

[martinjuckes]

I understand it exists ... but I didn't expect that the CF data model would prescribe a NetCDF variable name ... that appears to be outside the letter and the spirit of the convention. It should be possible to change that name.

[davidhassell]

The issue here pivots on whether or not you want to write a second grid_mapping variable containing the vertical datum mapping that duplicates part of the first grid_mapping that describes the horizontal crs . I have assumed not , but ....

[martinjuckes]

My issue is that I want to change the name of the datum variable in the file. I don't want a 2nd one.

[davidhassell]

I think that this has been resolved for some time, in that you can (now) set the netCDF variable name of a grid mapping variable:

>>> import cfdm
>>> f = cfdm.example_field(7)
>>> print(f)
Field: eastward_wind (ncvar%ua)
-------------------------------
Data            : eastward_wind(time(3), air_pressure(1), grid_latitude(4), grid_longitude(5)) m s-1
Cell methods    : time(3): mean
Dimension coords: time(3) = [1979-05-01 12:00:00, 1979-05-02 12:00:00, 1979-05-03 12:00:00] gregorian
                : air_pressure(1) = [850.0] hPa
                : grid_latitude(4) = [0.44, ..., -0.88] degrees
                : grid_longitude(5) = [-1.18, ..., 0.58] degrees
Auxiliary coords: latitude(grid_latitude(4), grid_longitude(5)) = [[52.4243, ..., 51.1163]] degrees_north
                : longitude(grid_latitude(4), grid_longitude(5)) = [[8.0648, ..., 10.9238]] degrees_east
Coord references: grid_mapping_name:rotated_latitude_longitude

>>> c = f.construct('grid_mapping_name:rotated_latitude_longitude')
>>> c.nc_set_variable('new_name')
>>> cfdm.write(f, 'new_file.nc')

Very happy to reopen this if needed.

Thanks,
David