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Re: Reverse interpolation?

Simon de Vet wrote:
> I have a little problem :)
> I have three sets of data - a list of altitudes (which are not evenly
> spaced, and are calculated with a long, confusing formula), an altitude
> I want to match, and a set of data that depends on altitude (ie: entry 1
> corresponds to altitude 1, entry 2 to altitude 2, etc...).
> Currently, I am calculating the differences between each entry in the
> altitude list and my known altitude, and using this to find the closest
> match, who's subscript I use in the data array. However, since there
> aren't very many values in my altitude list, the closest matches are
> often not very close at all, and the results become inaccurate.
> I want to get better results. Instead of finding that entry #5 is the
> closest match to my known altitude, and using #5 as the subscript in my
> data array, I'd like to find that, based on a linear interpolation,
> entry #5.32850 is an exact match, and using this value in an
> interpolation on my data array.
> I understand how to use a subscript to find an interpolated value. How
> would I go in the opposite direction, using a value to find a subscript?
> Thanks. I hope this isn't too confusing. I'm having a bad brane day.
> Simon

Hi Simon,

   I have dealt with similar problems (vertical regridding, to
give the kid a name), and I tend to resort on the spline routine
in these instances. Please find attached a piece of code that I
used to vertically regrid OH concentration fields. Just be
careful about end effects. For this application I overwrote the
topmostfour levels of the target grid with the values from the
top level of the old grid.


   FOR i=0,N_Elements(grid.lon)-1 DO BEGIN
         FOR j=0,N_Elements(grid.lat)-1 DO BEGIN
            oldx = reverse( reform( newoh[i,j,*,month-1] ) )
            oldy = reverse( reform( data.p ) )
            newy = reform( pgrid[i,j,*,month-1] )
            ;; Smooth spline interpolation onto new grid
            newxx = Spline( oldy, oldx, newy, 5. )
            ;; Enter new values in result field
            resultoh[i,j,*,month-1] =
[[ Dr. Martin Schultz   Max-Planck-Institut fuer Meteorologie   
[[                      Bundesstr. 55, 20146 Hamburg            
[[                      phone: +49 40 41173-308                 
[[                      fax:   +49 40 41173-298                 
[[ martin.schultz@dkrz.de