CODEWITHSUNDEEP
netcdfpython-xarray
sfluck
sfluck

Reputation: 375

create netCDF file with xarray, define variable data types

I created a netCDF-File with xarray for inputting topography height info, surface type info etc. into a numerical weather prediction model. I managed to create a file, however, the model requires the different variables to be different data types.

My dataset bolund_static looks as follows:

<xarray.Dataset>
Dimensions:          (x: 800, y: 200)

Coordinates:
  * y                (y) float64 1.0 3.0 5.0 7.0 9.0 ... 393.0 395.0 397.0 399.0
  * x                (x) float64 1.0 3.0 5.0 ... 1.595e+03 1.597e+03 1.599e+03
Data variables:
    zt               (y, x) float64 1.0 1.0 1.0 1.0 1.0 ... 1.0 1.0 1.0 1.0 1.0
    vegetation_type  (y, x) float64 -127.0 -127.0 -127.0 ... -127.0 -127.0
    water_type       (y, x) float64 3.0 3.0 3.0 3.0 3.0 ... 3.0 3.0 3.0 3.0 3.0
    soil_type        (y, x) float64 -127.0 -127.0 -127.0 ... -127.0 -127.0
    pavement_type    (y, x) float64 -127.0 -127.0 -127.0 ... -127.0 -127.0
Attributes:
    version:         1
    origin_z:        0.0
    origin_y:        694682.098
    origin_x:        6177441.825
    origin_lat:      12.098271
    origin_lon:      55.70364
    rotation_angle:  0.0
    palm_version:    6.0
    origin_time:     2019-04-01 12:00:00 +01

Saving this array with bolund_static.to_netcdf(), it saves all variables as double datatypes. This info I got by making an ncdump of the created netcdf-file.

netcdf bolund_static {
dimensions:
    y = 200 ;
    x = 800 ;
variables:
    double y(y) ;
        y:_FillValue = NaN ;
    double x(x) ;
        x:_FillValue = NaN ;
    double zt(y, x) ;
        zt:_FillValue = NaN ;
    double vegetation_type(y, x) ;
        vegetation_type:_FillValue = NaN ;
    double water_type(y, x) ;
        water_type:_FillValue = NaN ;
    double soil_type(y, x) ;
        soil_type:_FillValue = NaN ;
    double pavement_type(y, x) ;
        pavement_type:_FillValue = NaN ;

// global attributes:
        :version = 1 ;
        :origin_z = 0. ;
        :origin_y = 694682.098 ;
        :origin_x = 6177441.825 ;
        :origin_lat = 12.098271 ;
        :origin_lon = 55.70364 ;
        :rotation_angle = 0. ;
        :palm_version = 6. ;
        :origin_time = "2019-04-01 12:00:00 +01" ;
data: <...>

I would need vegetation_type, water_type, soil_type and pavement_type to be of type NC_BYTE instead of NC_DOUBLE after exporting, and x,y,zt as NC_FLOAT. How would I go about changing these datatypes? Is that possible from within the xarray/Python environment?

Upvotes: 3

Views: 2554

Answers (1)

Light_B
Light_B

Reputation: 1800

While saving your dataset you can make use of encoding parameter which takes input as a dictionary.

bolund_static.to_netcdf(filename.nc, encoding={'var1':{'dtype':'int8'}, 'var2':{'dtype':'int8'}})
# replace var1, var2 with your desired variable names
# for NC_float save dtype as int32, for NC_Byte save as int8

You can also modify other attributes of the variable such as _FillValue, etc. Another important note is that xarray automatically saves the time unit from the nearest possible starting time by default. If you want to change that then you can do that in the same way as well

bolund_static.to_netcdf(filename.nc, encoding={'var1':{'dtype':'int8'},\
'var2':{'dtype':'int8'}, 'origin_time':{'units':'seconds/hours since a 2000-01-01 00:00:00'}})

Upvotes: 5

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