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Re: [ferret_users] regridding etopo5



Hi David,
You could also do another regridding operation from etopo5, using linear or averaging to regrid to your coarser grid.

Ansley


David Wang wrote:
Hey Ansley,

Thanks very much for your response. I don't think I have time working on an external function right now. maybe some time in the future, then I will contact you.
This was to generate the bathymetry file for an ocean model. But since currently I actually don't need a very accurate representation of the sea floor and the coastlines, I may still use my present crude one derived from Levitus.

Cheers,
-d.

On 5/30/06, Ansley Manke <Ansley.B.Manke@noaa.gov> wrote:
Hi David,
I think this would require an external function. You'd pass in the data
on the finer grid as one argument, and a second argument would be some
dummy variable on the desired output grid.  Then the function would need
to determine which grid points from the fine grid lie within the grid
boxes of the output data, and compute the median value, assigning it to
the "result" of the function, on the coarser grid.

There would be some things to decide. Would you weight input data
according to what portion of the input grid cell lies within the output
grid cell?  That is, if you had the fine grid and the output grid lining
up like this, with data at the o's and grid cell edges at the |'s,

|---o---|---o---|---o---|---o---|---o---|
    |----------O----------|----------O----------|----------O----------|

then, would data at the first o in the fine data, which lies right on
the edge of an output grid cell, be weighted the same as the next two
data points which lie entirely within the output grid cell? or would its
contribution to the value of the median be smaller?  (and similar
considerations due to working in 2 dimensions.)

Another consideration would be whether to apply the cosine latitude
correction when working on a latitude-longitude grid.

If you (or anyone!) would like to work on this, I can give you more
ideas and help you get started.

Ansley

David Wang wrote:

> Hello Ferreters,
>
> I'm thinking of regridding etopo5 bathymetry data onto a much coarser
> grid (say 0.5x0.5) in such a way that the regridded value at each new
> grid point is chosen to be the median of all etopo5 grid points within
> each new grid box. Are there any tricks for doing this in FERRET?
>
> Thanks,
> - d.



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