How to interpolate gridded matrix to a non-gridded one?

Hi all,

I'm struggling with an interpolation issue and I would really appreaciate some ideas on this. I have two matrixes, A and B, with A being non-gridded while B is gridded. I want to interpolate B for A grid points. I tried to build an interpolant function for A and then assess B through it, but I can't overcome the non-uniformity of A. Any thoughts that may help would be really appreaciated.

Thanks in advance

Joao

Answer 1

[Stephen23]

Because your data points in B are gridded you can simply use any of the Gridded Data Interpolation tools. The fact that your sample points A are not gridded is totally irrelevant. Here is a simple example, adapted from the interp2 help:

% gridded data points:
[X,Y] = meshgrid(-3:3);
V = peaks(X,Y);
% non-gridded sample points:
Xq = 6*rand(1,1000)-3;
Yq = 6*rand(1,1000)-3;
%
Vq = interp2(X,Y,V,Xq,Yq);
%
scatter3(Xq,Yq,Vq);

[Joao]

Thanks for your reply Stephen. I've tried that, but like that am I not interpolating A to B? I need to interpolate B to A. My goal is for every point of A to have the equivalent of B in order to be able to compute some statistics.

[Stephen23]

@Joao: you wrote in your question that you "want to interpolate B for A grid points", and that "A being non-gridded while B is gridded".

This is what I showed in my answer. X and Y are gridded (like your B), and Xq and Yq are non-gridded (like your A). I showed how to interpolate the gridded data at the non-gridded points, exactly as you requested: " I want to interpolate B for A grid points"

If you actually want to do the opposite, i.e. interpolate non-gridded data points A at gridded points B, then use griddata. Once again the locations of the output sample points is totally irrelevant. This example taken straight from the help:

% non-gridded data:
xy = -2.5 + 5*gallery('uniformdata',[200 2],0);
x = xy(:,1);
y = xy(:,2);
v = x.*exp(-x.^2-y.^2);
% grid points:
[xq,yq] = meshgrid(-2:.2:2, -2:.2:2);
% interpolate:
vq = griddata(x,y,v,xq,yq);
% plot
figure
mesh(xq,yq,vq);
hold on
plot3(x,y,v,'o');

[Joao]

Hi Stephen,

Sorry for the confusion. You're right the first time. I've manage to solve the problem using TriScatteredInterp. When using interp2 has you suggested I got an error saying that my X and Y need to be generated using meshgrid. Thanks for your help!

João