[R] Resampling a grid to coarsen its resolution

[Sharpie]

Steve Murray-2 wrote:
> 
> Dear all,
> 
> I have a grid (data frame) dataset at 0.5 x 0.5 degrees spatial resolution
> (720 columns x 360 rows; regular spacing) and wish to coarsen this to a
> resolution of 2.5 x 2.5 degrees. A simple calculation which takes the mean
> of a block of points to form the regridded values would do the trick.
> Values which should be excluded from the calculation are -9999 (unless all
> points within a block are -9999, in which case -9999 should be returned as
> the 'new' cell).
> 
> How would I go about achieving this in R?
> 
> Any help or guidelines would be very much appreciated.
> 
> Many thanks,
> 
> Steve
> 

Check out interp.surface.grid() in the fields package.  I would also
recommend replacing your -9999 values with NAs  so they don't introduce
distortion on the edge of your grid during the interpolation. 
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