Tom, have a look at:

library(raster)
?resample
?projectRaster

Robert

On Thu, Jul 5, 2012 at 10:03 AM, Tom Roche <Tom_Roche@pobox.com> wrote:

>
> I'd appreciate advice regarding tools and methods for transforming
> values "contained" in a netCDF spatial grids in one {projection,
> resolution} to another, or pointers to other resources to consult.
> Apologies if the following is too verbose (I'm hoping this is a common
> usecase) or uses the wrong vocabulary (please correct where needed),
> but here's what I mean:
>
> I'm a student attempting to work on N2O using a particular atmospheric
> model (CMAQ). My group want to take N2O emissions from various sources
> in the continental US (CONUS) and then "blow them around" (and do some
> chemistry and photolysis) with CMAQ. We have other CMAQ inputs for our
> usecase on a 12 km x 12 km Lambert conformal conic (LCC) grid, so we
> need to get our N2O emissions on the same grid (i.e., "regrid" them).
> However, most of my emissions will be coming from global inventories
> (EDGAR, GEIA, GFED) which are gridded lat-lon at differing resolutions
> (e.g., 0.1°x0.1°, 1°x1°).
>
> I know how to reproject grid *points* (corners, centers) from lat-lon
> to 12-km LCC using R package=M3
>
> http://cran.r-project.org/web/packages/M3/
>
> What I don't understand yet (algorithmically, anyway--mental picture
> below) is how to reallocate gridded *values* from any one given
> {projection, resolution} tuple to another. E.g.:
>
> EDGAR 4.2 gives the mass emitted from each 0.1°x0.1° lat-lon gridcell
> over the globe. At 40° N (a middle latitude of CONUS), 0.1°x0.1° is
> approx 11 km N-S x 8.5 km E-W. I need to transform the values from
> those gridcells into those for a 12x12 km LCC CMAQ grid (which of
> course will deviate slightly from those exact dimensions--we have the
> exact grid centers and corners) over CONUS, such that, over the same
> area, the sum of the masses in the CMAQ grid matches that of the EDGAR
> grid.
>
> In the general case, each transformed/CMAQ gridcell will intersect/
> overlap parts of 4 source/EDGAR gridcells, so the emissions captured
> within the CMAQ gridcell will be the sum of the 4 products (for each
> of the overlappings)
>
> overlap area *
> total emissions from overlapped EDGAR gridcell /
> total area of overlapped EDGAR gridcell
>
> Calculating that for a large grid sounds like work, but also sounds
> like work someone has previously needed to do. (This doesn't sound to
> me like a unique task--am I missing something?) As a coder, I'm more
> than willing to reuse :-) but am wondering where to look.
>
> Your coding recommendations are appreciated, as would be pointers to
> helpful resources. Thanks in advance, and feel free to forward,
> Tom Roche <Tom_Roche@pobox.com>
>
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