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

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Till
Till 2015-7-23
评论： Andrei Bobrov 2015-7-23
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
  2 个评论
Torsten
Torsten 2015-7-23
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
Till 2015-7-23
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.

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采纳的回答

Andrei Bobrov
Andrei Bobrov 2015-7-23
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.

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