Problem in using interp2

Hello,

I want to interpolate my latitude longitude values. They are in the form of matrix.

Lat = 406x270;

Lon = 406X270;

I have read docs about it but still can't figure out that how to do interpolation. Vq = interp2(X,Y,V,Xq,Yq). How to use V here? What is meant by 'Xq and Yq contain the coordinates of the query points'?

I have another variable M (2030x1354). So, I want Lat Lon to become:

Lat1 = 2030x1354;

Lon1 = 2030x1354;

So, should I use M instead of V? Then what about Xq and Yq?

After going through some questions I tried using meshgrid for it. But it gave me following error;

Error using repmat
Requested 109620x109620 (44.8GB) array exceeds maximum array size preference. Creation of arrays greater than this limit may take a long time
and cause MATLAB to become unresponsive.

Lat Lon data is attached. Is there anyother way to do interpolation? Thank you.

Answer 1

[KSSV]

Vq = interp2(X,Y,V,Xq,Yq) ; 

In the above V is the value at (X,Y). You need to have that value.It seems you have only Lon and Lat here.

% Demo
[X,Y,Z] = peaks(100) ;    % Z is here V 
% Do interpolation 
x0 = min(X(:)) ; x1 = max(X(:)) ; 
y0 = min(Y(:)) ; y1 = max(Y(:)) ; 
x = linspace(x0,x1,50) ; 
y = linspace(y0,y1,50) ; 
[Xq,Yq] = meshgrid(x,y) ; 
Zq = interp2(X,Y,Z,Xq,Yq) ; 

So get your V i.,e. Z i.e. respective Lon, LAt value.

[IMC]

My M variable is 2030x1354. And I want Lat Lon to be of same size.

So, can I use M in place of V?

[KSSV]

Yes your M is V.

[IMC]

For Xq and Yq, using meshgrid gives this error:

[Xq,Yq] = meshgrid(lat,lon)

Error using repmat
Requested 109620x109620 (44.8GB) array exceeds maximum array size preference. Creation of arrays greater than this limit may take a long time
and cause MATLAB to become unresponsive.

Error in meshgrid (line 61)
        xx = repmat(xrow,size(ycol));

[KSSV]

It seems you are using meshgrid on a matrix. Show us your whole code.

[IMC]

@KSSV, here is the code;

M = hdfread(hdf_filename, 'Optical_Properties'); % 2030X1354 double
lat = hdfread(hdf_filename, 'Latitude'); % 406x270 double
lon = hdfread(hdf_filename, 'Longitude'); % 406x270 double

[Xq,Yq] = meshgrid(lat,lon) ; 
Zq = interp2(lat,lon,M,Xq,Yq) ; 

Error:
Error using repmat
Requested 109620x109620 (44.8GB) array exceeds maximum array size preference. Creation of arrays greater than this limit may take a long time
and cause MATLAB to become unresponsive.

Error in meshgrid (line 61)
        xx = repmat(xrow,size(ycol));

Error in Untitled12 (line 75)
[Xq,Yq] = meshgrid(lat,lon) ;

[KSSV]

M = hdfread(hdf_filename, 'Optical_Properties'); % 2030X1354 double
lat = hdfread(hdf_filename, 'Latitude'); % 406x270 double
lon = hdfread(hdf_filename, 'Longitude'); % 406x270 double

[m,n] = size(M) ; 
x0 = min(lon(:)) ; x1 = max(lon(:)) ; 
y0 = min(lat(:)) ; y1 = max(lat(:)) ; 
x = linspace(x0,x1,m) ; 
y = linspace(y0,y1,n) ; 
[X,Y] = meshgrid(x,y) ; 
Z = interp2(X,Y,M',lon,lat)' ; 

pcolor(lon,lat,Z') ;
shading interp ;
colorbar

[IMC]

@KSSV, why the output 'Z' is 270x406 double?

[KSSV]

Because Xq, Yq is of that size.

[IMC]

@KSSV My purpose of doing interpolation is that I have a text file. Text file contains 2 column vectors (having lat and lon values). So,

lat_txt = 3798x1 double;

lon_txt = 3798x1 double;

I want to use these lat_txt and lon_txt to match with values of lat and lon (from hdf file). And for all the lat and lon values which are equal, extract the value of M (from hdf file).

Sorry, I really don't understand how the output 'Z (270X406)' will do here.

[KSSV]

Then attach the text file.

[IMC]

I've attached the text file.