Re: [Matplotlib-users] Strange behavior with basemap.interp() and masked arrays

[Jeff W. <jef...@no...>]

> Goodman, Alexander (398J-Affiliate) 
> <mailto:ale...@jp...>
> June 24, 2013 4:30 PM
> Just something I thought I should add to this, but would the following 
> code seem like a reasonable workaround? Basically, since 
> map_coordinates() cannot handle mask arrays, I thought that perhaps 
> passing in the actual mask itself to interp() with order=3 may produce 
> close to reasonable results, eg:
>
> dataout = interp(datain, xin, yin, xout, yout, masked=True, order=3)
> maskin = datain.mask.astype(int)
> maskout = interp(datain.mask, xin, yin, xout, yout, order=3)
> dataout.mask = dataout.mask | maskout
>
> Thanks,
> Alex
>
>
> ------------------------------------------------------------------------
Alex:  The basemap.interp docstring includes a note describing a trick I 
often use with masked arrays:

Note

If datain is a masked array and order=1 (bilinear interpolation) is 
used, elements of dataout will be masked if any of the four surrounding 
points in datain are masked. To avoid this, do the interpolation in two 
passes, first with order=1 (producing dataout1), then with order=0 
(producing dataout2). Then replace all the masked values in dataout1 
with the corresponding elements in dataout2 (using numpy.where). This 
effectively uses nearest neighbor interpolation if any of the four 
surrounding points in datain are masked, and bilinear interpolation 
otherwise.


I suppose the same trick might work with order=3, but I have never tried it.

-Jeff