Find z value out of x and y values from a countour in command window

Hello guys i have data of x,y,and z value of a countour.

How to make z value appear by knowing x and y value in command window? Instead of clicking b in workspace.

[Adam Danz]

How do you have the x,y,z data organized (I can't tell from the image you shared)?

If you know x and y, all you need to do is find their indices to and use them to look up z, assuming the data are organzed efficiently.

[agung pratama]

I'm using the surf plot, I clicked b in workspace to see the x,y,z values. I forgot how we can check z value by comment in command window

Answer 1

[Star Strider]

Clicking on the contours will only show you the level of the contour. To see the value labels of the contours use ShowText:

contour(X,Y,Z,'ShowText','on')

.

[agung pratama]

Thanks, buy I only want to know 1 z value in certain x,y values over and over after I change the contour.

[Star Strider]

My pleasure!

Either plot many contours to get finer resolution, or use a mesh or surf plot, and perhaps a text object to display the coordinates you want. You would need to define the coordiantes programatically, or perhaps use inputdlg to enter the coordinates.

[agung pratama]

I'm using surf plot. I can see z value after clicking b in workspace and search z value that I want. But Ijust knew that we can also find the z value bg type a comment in command window, somehow I forget the comment. I already trying to remember like you can see in the picture.

[Star Strider]

My idea was to do exactly that, and display the results in a text object on the surf plot.

You would need to write the code to get the data from your matrix (or interpolate it to specific coordinates, depending on what you want to do). Using inputdlg was the way I suggested to choose the coordinates.

EDIT — (5 Oct 2020 at 20:20)

A relatively easy way of finding and displaying the ‘Z’ values for any set of (x,y) coordinates is to use the griddedinterpolant function. Since your data appear to exist as a matrix, this would likely be be straightforward to implement.

Using an example from the documentation and my own additions to it:

[x,y] = ndgrid(-5:0.8:5);
z = sin(x.^2 + y.^2) ./ (x.^2 + y.^2);

F = griddedInterpolant(x,y,z);
xq =  2;                                                    % Input From ‘inputdlg’
yq = -3;                                                    % Input From ‘inputdlg’
vq = F(xq,yq);

figure
surf(x,y,z)
hold on
plot3(xq, yq, vq, '^r', 'MarkerFaceColor','r')
hold off
text(xq, yq, vq, sprintf('(%.2f, %.2f, %.3f)\n\\downarrow', xq, yq, vq), 'HorizontalAlignment','center', 'VerticalAlignment','bottom', 'Color','r')

producing —

.

Answer 2

[Adam Danz]

A surf plot is not a contour plot. That was a little confusing. Thanks for clearing that up.

Use the handle to your surface object and to get the XData, YData, and ZData properties.

[x,y,z] = sphere(20); 
h = surf(x,y,z,'FaceColor','interp'); 
x = h.XData;
y = h.YData;
z = h.ZData;

If you didn't save the object handle you can get it from the current axis using,

h = findobj(gca, 'type', 'surface');

Once you have the x,y,z data you can locate the nearest (x,y) values and use those indices to get the corresponding z values. Due to floating point roundoff error, unless your x and y values have very low precision, you'll need to find the nearest values to your target values. There may be more than 1 z-value corresponding to the (x,y) indices. Here's an example continued from above,

%  Plot surf object
[x,y,z] = sphere(20); 
h = surf(x,y,z,'FaceColor','interp','FaceAlpha',0.2,'EdgeAlpha',.3); 
x = h.XData;
y = h.YData;
z = h.ZData;
axis equal
hold on
view([-25.8, 11.7])

% Identify the (x,y) targets 
target = [-0.25, -0.18164]; % [x,y]
% Define a limit of precision
closeEnough = 0.001;
% Find the corresponding z values
dist = pdist2([x(:),y(:)], target); 
idx = dist <= closeEnough; 

% Plot results
plot3(x(idx),y(idx),z(idx), 'm*', 'MarkerSize', 10, 'LineWidth', 2)

As you can see, given a single (x,y) coordinate, there are two (or more) z coordinates.