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From: Sal on 10 Jul 2010 20:07
Hello everyone,
I am trying to write a function that gets certain data values from netcdf file and then, using 2D interpolation, calculates the value of data in between the coordinates. The syntax I am using is as follows;

ZI = interp2(X,Y,Z,XI,YI)


data_set =
-5.1695
-5.6024
-5.3846
-5.8743
(data_set is 4x1 and contains the data obtained from netcdf file)

x_set = [0,25,60,80];

y_set = [0,25,60,80];
(both x_set and y_set are 1x4)

data1 = interp2(x_set,y_set, data_set, 17.728, 12.664)
%17.728 is distance along x
%12.664 is distance along y

At first, I had trouble synchronising the lengths of x and y_sets, and data_set arrays. i kept getting error messages of "The lengths of the X and Y vectors must match Z."
This was due to the fact that intially, I had x_set and y_set equal to [0:20:80] 5x1 arrays which are not of the same length as data_set. (4x1)

However, when i changed x_set and y_set to the ones as shown above, i started getting the error: Subscript indices must either be real positive integers or logicals.

I must be making a mistake somewhere but can't seem to figure out?? should i instead try ID interpolation? any ideas/pointers are most appreciated.

thank you
Salman
From: us on 10 Jul 2010 20:24
"Sal " <salman.hafeez(a)mail.mcgill.ca> wrote in message <i1b1v9$618$1(a)fred.mathworks.com>...
> Hello everyone,
> I am trying to write a function that gets certain data values from netcdf file and then, using 2D interpolation, calculates the value of data in between the coordinates. The syntax I am using is as follows;
>
> ZI = interp2(X,Y,Z,XI,YI)
>
>
> data_set =
> -5.1695
> -5.6024
> -5.3846
> -5.8743
> (data_set is 4x1 and contains the data obtained from netcdf file)
>
> x_set = [0,25,60,80];
>
> y_set = [0,25,60,80];
> (both x_set and y_set are 1x4)
>
> data1 = interp2(x_set,y_set, data_set, 17.728, 12.664)
> %17.728 is distance along x
> %12.664 is distance along y
>
> At first, I had trouble synchronising the lengths of x and y_sets, and data_set arrays. i kept getting error messages of "The lengths of the X and Y vectors must match Z."
> This was due to the fact that intially, I had x_set and y_set equal to [0:20:80] 5x1 arrays which are not of the same length as data_set. (4x1)
>
> However, when i changed x_set and y_set to the ones as shown above, i started getting the error: Subscript indices must either be real positive integers or logicals.
>
> I must be making a mistake somewhere but can't seem to figure out?? should i instead try ID interpolation? any ideas/pointers are most appreciated.
>
> thank you
> Salman

a hint:
- your Z, ie, DATA_SET, must be 2D...

help interp2; % <- look at it carefully(!)...

us
From: Sal on 10 Jul 2010 21:15
Hi us,
Thank you for the tip..i changed data_set to 2x2 and made x_set and y_set into a 1x2 array, and it worked.

However, whenever I change the order of values stored in the data_set array, i get a different interpolated value. (however, the difference isn't very large, ranging from -5.31 to -5.41 and -5.59 )

Each of the values stored in data_set are actually data values, obtained on set of specific coordinates on a grid within the netcdf file. I am using 2D interpolation to find values for set of points that fall within the boundary of specified coordinates.

(42,25) = -5.6024 (43,25) = -5.8743
(42,24) = -5.1695 (43,24) = -5.3846

For this case, I was trying to find the data value at (42.728, 24.664) and the scale used for grid is 1:80

My main concern right now is how should i store the data values (obtained from netcdf file, for specific coordinates) within the data_set array? should it be as follows

data array = [ (42,25) (43,25)
(42,24) (43,24) ]

or some other way??
Please let me know

Salman
From: Bruno Luong on 11 Jul 2010 08:32
"Sal " <salman.hafeez(a)mail.mcgill.ca> wrote in message <i1b5va$4ar$1(a)fred.mathworks.com>...

>
> My main concern right now is how should i store the data values (obtained from netcdf file, for specific coordinates) within the data_set array? should it be as follows
>
> data array = [ (42,25) (43,25)
> (42,24) (43,24) ]
>
> or some other way??
> Please let me know

The INTERP2(x,y,Z, ...) assumes the Z data is generated meshgrid-like, i.e., first dimension corresponds to y, second dimension to x. This is written in the doc. Of course you could also swap x/y when calling INTERP2.

Bruno
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