Re: Regridding from low to high resolution ?

Hi All,

Thanks for the advice - the NCL solution sounds harder than CDO so I'll try
CDO first...

Best,

Mad.

On Thu, Apr 19, 2012 at 5:20 AM, Daran Rife
<Daran.Rife@gl-garradhassan.com>wrote:

> Hi Noel,
>
> As usual, Dennis is right about this.
>
> Interpolation and/or simple averaging are not appropriate for
> categorical
> data. Consider Dennis' example of various land classifications (desert,
> forest, glacier, etc.). Even if we assigned a numerical value to these
> categories (e.g., desert = 1, forest = 2, glacier = 3) it would be
> meaning-
> less to compute average values. For example, what would 1.5 represent?
>
> Granted, we could choose the data categories to avoid large gaps between
>
> adjacent category indices, and if "nearby" indices represent similar or
> nearby categories, then a (weighted) average may be a good choice,
> because
> it may be "close" to what an ideal regridded value should represent.
>
> The solution Dennis proposed will probably work well, so long as the
> source grid and destination grid are not greatly dissimilar. However, in
> the case where the grid increments are "very different" (say coarsening
> your source data by 50%), then you need to determine the "dominant"
> category is within each of your destination grid cells. The most
> straight
> forward way to do this is to calculate the most frequently occurring
> category (i.e., the mode) in each new grid cell.
>
> It would be fairly straight forward to code this in NCL, though it might
> be a bit slow. As an alternative, Climate Data Operators has a built-in
> function called remaplaf for regridding categorical data. Provided your
> input data are in one of the supported data formats (netCDF, HDF, GRIB
> 1/2), CDO might be a good option.
>
> https://code.zmaw.de/embedded/cdo/1.5.4/cdo.html#x1-4880002.12.1
> https://code.zmaw.de/boards/1/topics/925
>
> I hope this helps.
>
> Daran
>
> --
>
> Date: Wed, 18 Apr 2012 11:21:37 -0600
> From: Dennis Shea <shea@ucar.edu>
> Subject: Re: Regridding from low to high resolution ?
> To: Noel Aloysius <noel.aloysius@gmail.com>, Madeleine Patterson
> <madeleine.patterson77@gmail.com>
> Cc: ncl-talk <ncl-talk@ucar.edu>
> Message-ID: <4F8EF821.4050609@ucar.edu>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> For categorical data, bilinear interpolation is likely not
> appropriate. Consider vegland_1 at
>
> http://www.ncl.ucar.edu/Applications/classification.shtml
>
> If the target grid point is in the middle of 4 points with
> categories 16, 10, 1, 2 then bilinear interpolation would
> yield 29/4 -> 7.25 -> 7. It is highly unlikely that this
> would be the case.
>
> Though slow, I would suggest a 2-step procedure
>
> [1]
> http://www.ncl.ucar.edu/Document/Functions/Built-in/grid2triple.shtml
>
> to convert the source grid to a triplet.
>
> [2] then
>
> http://www.ncl.ucar.edu/Document/Functions/Built-in/triple2grid.shtml
>
>
> ===
> untested ...
>
>
> undef("regrid_cat")
> function regrid_cat (clat[*], clon[*], cgrid[*][*] \ ; categorical grid
> ,tlat[*], tlon[*]) ; target grid
>
> begin
> d = grid2triple (clon,clat,cgrid) ; d(3,ld)
> cnew = triple2grid(d(0,:), d(1,:), d(2,:), tlon, tlat, False)
>
> copy_VarAtts(cgrid, cnew) ; contributed.ncl
> cnew!0 = "lat"
> cnew!1 = "lon"
> cnew&lat = tlat
> cnew&lon = tlon
>
> return( cnew )
> end
>
>
> On 4/18/12 9:17 AM, Noel Aloysius wrote:
> > You can use CDO tools to do this, somthing like
> >
> > cdo remapbil,1x1grid_target.txt infile.nc <http://infile.nc>
> > outfile.nc <http://outfile.nc>
> >
> > where remapbil does bilinear interpolation (and remapbic is for
> > bicubic), 1x1grid_target.txt is the user input file with grid size
> > information.
> >
> > Noel
> >
> >
> > On Wed, Apr 18, 2012 at 11:08 AM, Madeleine Patterson
> > <madeleine.patterson77@gmail.com
> > <mailto:madeleine.patterson77@gmail.com>> wrote:
> >
> > Hi All,
> >
> > I was wondering if there is a low-to-high-resolution function for
> > regridding a global map of a categorical variable?
> >
> > I found all the regridding scripts but they all employ
> > interpolation, whereas I just want to 'pixelate' a 2x2.5 deg map
> to
> > a 0.5 x 0.5 degree map...
> >
> > Does NCL have a function which will do this?
> >
> > Thanks
> >
> > M
> >
>
>
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Received on Fri Apr 20 12:55:32 2012