Re: [ferret_users] regridding etopo5

[Ansley Manke <ansley.b.manke@xxxxxxxx>]

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.