Error in interp2: The grid vectors do not define a grid of points that match the given values

I have an map z of size [512,721] and two corresponding axes x and y with 512 and 721 elements respectively. I want to interpolate the map to 1024 by 1024 points and am using interp2 for this.

I generate new axes xi and yi as follows:

x = 1:512; y = 1:721;
nx = 512; ny = 721; ni = 1024;
xi = 1:nx/ni:ni; yi = 1:ny/ni:ny;

I know that xi and yi have only 1023 elements now put at the moment I am struggling with the error I get when running interp2. So here is a short minimal example:

z = rand(n1,n2);    %original 2D-map
zi = interp2(x,y,z,xi,yi);

This throws the error:

Error using griddedInterpolant
The grid vectors do not define a grid of points that match the given
values.

The same happens if I run the code with meshgrids instead of vectors:

[xm,ym] = meshgrid(x,y);
[xmi,ymi] = meshgrid(xi,yi);
zmi = interp2(xm,ym,z,xmi,ymi);

I also tried switching x and y (and xi and yi) in interp2, in which case the code runs without an error, however zmi is then a vector with 1023 elements:

zmiflipped = interp2(y,x,z,yi,xi);
all(size(zmiflipped) == [   1,1023])

Can someone help me understand the error and explain to me how I can fix it? I am using MATLAB R2013a

[Torsten]

x,y and z must be vectors of the same length (in your case 512*721). z(i) is the function value in point (x(i),y(i)) (i=1,...,512*721).

This is not the case in your example.

Best wishes

Torsten.

[Till]

Thanks for your reply Torsten.

Sorry if this is obvious but do you mean that i need to transpose xm and ym when I want to insert them into the interpolation with meshgrids? Because in this case I get the new error:

Data is in MESHGRID format, NDGRID format is required.
Convert your data as follows:
 X = X'; Y = Y'; V = V'; F = griddedInterpolant(X,Y,V)

which to me seems to indicate that transposing would not solve this issue.

Answer 1

[Andrei Bobrov]

x = 1:512; 
y = 1:721;
nx = 512; 
ny = 721; 
ni = 1024;
xi = linspace(1,nx,ni); 
yi = linspace(1,ny,ni);

[X,Y] = ndgrid(x,y);
z = rand(nx,ny);
F = griddedInterpolant(X,Y,z,'cubic');
[XI,YI] = ndgrid(xi,yi);
zi = F(XI,YI);

Plese read about griddedInterpolant.

[Till]

Thank you Andrei, do you know why this works with griddedInterpolant but not with interp2?

[Andrei Bobrov]

Please read Mathematics->Functionality being removed or changed