bilinear/cubic convolution

[Darrell McCauley]

Hello all. I would like to use bilinear interpolation or cubic
convolution interpolation to do some cross validation when
interpolating site data to raster format. My site data are scattered
and their locations will not necessarily line up with cell centers.
I'm trying to do a little better than just picking the nearest cell to
estimate an error.

My question is: is this capability already available in GRASS?

Just thought I'd check before I spend too much time on this. I can
envision a shell-script solution, but I'll probably create another
GRASS command (in C) unless someone stops me.

This operation is sort of like r.resample (it uses nearest cell and is
lacking the other two methods), except I only want a few estimates at
precise, non-gridded locations.

--Darrell

P.S. if this isn't already implemented, what should the 
     new command be called? s.sample? The I/O would be
     s.sample [-qclBC] input=name rast=name [z=value] [output=name]
     where input/output are sites. Output is difference
     between interpolated value from cells and input-site.
     z is a scaling factor to multiply cell values by (e.g., 
     0.001 if cell values are 1000 time the actual z value)
     -q quiet; -c use category label instead of category;
     -l use description if sites format is "N|E|#n desc";
     -B use bilinear method; -C use cubic convolution method
     (default method is nearest neighbor)