Coarsen xarray DataArray with weighted mean

user_3826115 0
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Let's say I have some DataArray:

da = xr.DataArray(
    data=np.random.random((25,25)),
    dims=["x", "y"],
    coords=dict(
        x=np.arange(25),
        y=np.arange(25),
    ),
)

I want to downsample this array to 5x5 chunks. I can do this with the coarsen function:

da_coarse = da.coarsen(x=5,y=5).mean()

As I understand it, this is basically splitting the DataArray into 5x5 "chunks" and averaging each chunk into one value. However, what I want to do is take a weighted average of this 5x5 group, so the center points are weighted more heavily in the final mean than the edge points.

I can create a gaussian kernel with weights like this:

def gkern(kernlen=21, sig=3):
    import scipy.stats as st
    
    """Returns a 2D Gaussian kernel."""
    x = np.linspace(-(kernlen/2)/sig, (kernlen/2)/sig, kernlen+1)
    kern1d = np.diff(st.norm.cdf(x))
    kern2d = np.outer(kern1d, kern1d)
    return kern2d/kern2d.sum()

window = gkern(5)

Where window is now a 5x5 array with the desired weights for each point. However, I am unsure how to implement this window/kernel when doing the averaging in the coarsen function.

What is the best way to do this?

python-xarray python
1 Answers

One way to do this is through DataArrayCoarsen.construct, which allows you to more easily operate on individual windows at a time:

weights = xr.DataArray(gkern(5), dims=["x_window", "y_window"])
windowed_da = da.coarsen(x=5, y=5).construct(
    x=("x_coarse", "x_window"),
    y=("y_coarse", "y_window")
)
coarsened = (weights * windowed_da).sum(["x_window", "y_window"]) / weights.sum()

windowed_da is the original DataArray, but reshaped into individual windows of the size specified in the coarsen step.

user_38255330
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