Regridding coordinates with Python-Xarray

user_7763326 0
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I've a NetCDF file in which the variables are stored in 0 to 360-degrees longitude. I would like to convert it to -180 to 180 degrees. This should be a rather straightforward task but for some reason I can't seem to make some of the examples given in the tutorial work out.

ds = xr.open_dataset(file_)   
>ds
<xarray.Dataset>
Dimensions:  (lev: 1, lon: 720, time: 1460)
Coordinates:
* lon      (lon) float64 0.0 0.5 1.0 1.5 2.0 2.5 ... -2.5 -2.0 -1.5 -1.0 -0.5
* lev      (lev) float32 1.0
* time     (time) datetime64[ns] 2001-01-01 ... 2001-12-31T18:00:00
Data variables:
 V        (time, lev, lon) float32 13.281297 11.417505 ... -19.312767

I try using the help of Dataset.assign_coord

ds.V.assign_coords(lon=((ds.V.lon + 180) % 360 - 180)) 
#gives me a new array with lon -180 to 180
ds['V'] = ds.V.assign_coords(lon=((ds.V.lon + 180) % 360 - 180))
# didn't modify the V for some reason?

So, assign_coords worked but setting the variable back to Dataset doesn't work. After many tries, I figured to directly modify the coordinates "lon" because they're linked to the Datavariable "V" via dictionary.

ds.coords['lon'] = (ds.coords['lon'] + 180) % 360 - 180
#solves the problem!

Second Problem I encountered is in sorting my data variable according to the above-modified longitudes. I tried

 ds['V'] = ds.V.sortby(ds.lon)
 >ds.V 

 # the array is not sorted according to -180 to 180 values

But when I sort the dataset and assign it, it works. 

ds = ds.sortby(ds.lon) # now my dataset is sorted to -180 to 180 degrees lon

It would be very helpful for my understanding of xarrays if someone can point out why my first approach for both problems are not working?

python-xarray netcdf python
4 Answers

I apologise for the one-liner but this is exactly how I have solved this issue: d = d.assign_coords(longitude=(((d.longitude + 180) % 360) - 180)).sortby('longitude') you should work at Dataset level and not at DataArray.

user_2496570

There's one principle that explains why both of your initial approaches didn't work. In a Dataset, variables have values along coordinates. The coordinates have a separate existence in the Dataset from the variables. You may have three variables U, V, and W which all vary along some coordinate longitude within the dataset. On their own, it's fine for U and V to have their longitude values in different orders, but within the dataset they must have the same ordering.

When you assign a variable to a dataset where the dataset already has the coordinate of the variable, xarray will automatically re-order that variable to have the same ordering as the dataset. It will also do nice things like add nan values wherever the variable does not have values for a given coordinate in the dataset.

Here's an example where I've made a Dataset and DataArray that both have a longitude coordinate, but in reversed directions. When I assign the DataArray to the Dataset, the coordinate is automatically reversed.

In[17]: ds
Out[17]: 
<xarray.Dataset>
Dimensions:    (longitude: 10)
Coordinates:
  * longitude  (longitude) float64 360.0 320.0 280.0 240.0 200.0 160.0 120.0 ...
Data variables:
    *empty*

In [18]: da
Out[18]: 
<xarray.DataArray (longitude: 10)>
array([ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.])
Coordinates:
  * longitude  (longitude) float64 0.0 40.0 80.0 120.0 160.0 200.0 240.0 ...

In [19]: ds['v'] = da

In [20]: ds['v']
Out[20]: 
<xarray.DataArray 'v' (longitude: 10)>
array([ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.])
Coordinates:
  * longitude  (longitude) float64 360.0 320.0 280.0 240.0 200.0 160.0 120.0 ...

Here's a similar example where it adds nan automatically:

In [27]: ds
Out[27]: 
<xarray.Dataset>
Dimensions:    (longitude: 10)
Coordinates:
  * longitude  (longitude) float64 360.0 320.0 280.0 240.0 200.0 160.0 120.0 ...
Data variables:
    *empty*

In [28]: da
Out[28]: 
<xarray.DataArray (longitude: 3)>
array([ 0.,  0.,  0.])
Coordinates:
  * longitude  (longitude) float64 0.0 40.0 80.0

In [29]: ds['v'] = da

In [30]: ds['v']
Out[30]: 
<xarray.DataArray 'v' (longitude: 10)>
array([ nan,  nan,  nan,  nan,  nan,  nan,  nan,   0.,   0.,   0.])
Coordinates:
  * longitude  (longitude) float64 360.0 320.0 280.0 240.0 200.0 160.0 120.0 ...
user_53225740
user_60682940

cdo is OK and quick for those problems, Like :

cdo sellonlatbox,-180,180,-90,90 a.nc b.nc

a.nc is your data, and b.nc is the result you want.

user_135934630
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