I have a contourf function constituted by x, y and z vectors (It is a large matrix). Then, I would like to find the x, y and z values corresponding to certain x and y vectors.

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Fedilberto Gonzalez 2021-11-22

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https://ww2.mathworks.cn/matlabcentral/answers/1593254-i-have-a-contourf-function-constituted-by-x-y-and-z-vectors-it-is-a-large-matrix-then-i-would-l

评论： Fedilberto Gonzalez 2021-11-23

I was plotting something with contourf. I created a beautiful picture with many x, y and z vectors. Then, I have certain x and y values which I would like to know their z value corresponding to the contourf.

For instance, below you can find the values used to create the contourf graph. However, I am not able to obtain the x ,y z related to the x and y points that I also attached.

Could you kindly help me please? I would be really grateful.

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Fedilberto Gonzalez 2021-11-22

x = contourf(:,1);

y = contourf(:,2);

z = contourf(:,3);

[X,Y] = meshgrid(x,y) ;

zg = griddata(x,y,z,X,Y) ;

[Cx, h] = contourf(X,Y,zg,15);

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Answer 1

Benjamin Kraus 2021-11-23

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I'm not sure I completely understand your question, but I think you are misusing meshgrid in your code example above.

Does this do what you want?

% Read the `contourf.txt` and `x and y points that I am interested in.txt` points into MATLAB
data = readmatrix('contourf.txt');
query = readmatrix('x and y points that I am interested in.txt');
x = data(:,1);
y = data(:,2);
z = data(:,3);
xq = query(:,1);
yq = query(:,2);
% Use griddate to determine the value at each of the query x/y points.
[Xq,Yq, zq] = griddata(x,y,z,xq,yq);
% If I understand correctly, the value of zq is what you are looking for.
% The rest of this code is just to visualize the results.
% Create X/Y grids based on the range of the input data.
n = 50;
[X,Y] = meshgrid(linspace(min(x),max(x),n), linspace(min(y),max(y),n));
% Sample the X/Y grid using griddate.
Z = griddata(x,y,z,X,Y);
% Draw the contour lines overlayed on a surface plot.
surf(X,Y,Z,'EdgeColor','none')
hold on
contour3(X,Y,Z,15,'EdgeColor','k')
% Overlay the original data points
plot3(x,y,z,'k.');
% Overlay the query data points.
plot3(xq,yq,zq,'ro');