plotting griddata

2011 5 25
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Hey All, I was wondering if there was a way to modify the griddata program to include plotting. Given that I am using data points I know I will have to use meshgrid and was trying to figure out how it would be possible to include into the program. Any ideas? Mel

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Walter Roberson
Walter Roberson 2011-5-25
Why not just call meshgrid() after the griddata() call, as shown in the example in the documentation ?
Matt Fig
Matt Fig 2011-5-25
I think you mean MESH, not MESHGRID.
Melissa
Melissa 2011-5-25
As I am trying to implement in a GUI the command window will not be available. When I plug this code into each if statement (ie linear, cubic, nearest) I only receive a plot of the points and not of the generated surfaces. Any suggestions?
%Plotting Griddata
plot3(x,y,z,'o')
hold on
[X,Y]=meshgrid(xi,yi);
surf(X,Y,zi)
hold off

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Matt Fig
Matt Fig 2011-5-25

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From the doc:
x = rand(100,1)*4-2;
y = rand(100,1)*4-2;
z = x.*exp(-x.^2-y.^2);
ti = -2:.25:2;
[XI,YI] = meshgrid(ti,ti);
ZI = griddata(x,y,z,XI,YI);
mesh(XI,YI,ZI), hold
plot3(x,y,z,'o'), hold off
%
%
%
%
EDIT
So do this:
>> edit griddata % Once the file is up, do this:
ctrl+a,ctrl+c,ctrl+n,ctrl+v,ctrl+s % I recommend using mygriddata
Then look in the new file, and find the last switch statement of the main function. Immediately after that switch statement, put this:
mesh(xi,yi,zi)
hold on
plot3(x,y,z,'o')
hold off
Or whatever you want.

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Melissa
Melissa 2011-5-25
Thank you for responding. I understand how to plot in the command window using the examples but I wanted to put inside teh griddata code to plot automatically.
Walter Roberson
Walter Roberson 2011-5-25
Melissa, in my experience that is almost certainly the wrong approach to what you are doing.
Matt Fig
Matt Fig 2011-5-25
See my edit above...
Melissa
Melissa 2011-5-26
Again thank you so much for taking the time to help me I really appreciate it. I placed it after the switch function given the cases and it produced the error that Z must be a matrix and not a vector or scalar. I don't understand why this works in the command window and not the program itself.
Matt Fig
Matt Fig 2011-5-26
How did you call the new function? Because I called it like this:
x = rand(100,1)*4-2;
y = rand(100,1)*4-2;
z = x.*exp(-x.^2-y.^2);
ti = -2:.25:2;
[XI,YI] = meshgrid(ti,ti);
ZI = mygriddata(x,y,z,XI,YI); % Call new function, made as above
And it worked perfectly. Note that I called the new function.
Melissa
Melissa 2011-5-26
I think I am leaving out the meshgrid...hm I wonder if I can place that above the plotting code.
x =[-2 0 1 5 10 6]
y =[-3 7 4 0 3 -1]
z =[-12 7 8 8 4 0]
xi =0:1:10
yi =0:1:10
mygriddata(x,y,z,xi,yi)
Melissa
Melissa 2011-5-26
it works perfectly if I use meshgrid in the command window but why cant I use meshgrid within the program itself? I placed it right above the code you have given. Which I very much appreciate and am so thankful for you showing me where to place it.
Matt Fig
Matt Fig 2011-5-26
If you want to be able to call MYGRIDDATA as you did above, put this as the first line of the MYGRIDDATA:
[xi,yi] = meshgrid(xi,yi);
But notice that this will mess up your use of the function the other way... It will indeed be a custom function.
Melissa
Melissa 2011-5-26
life saver. thank you.
Walter Roberson
Walter Roberson 2011-5-26
I still say it was the wrong approach. If you were going to create a new function, why not create a function that contained the lines I showed and which called the unmodified griddata() ?
Matt Fig
Matt Fig 2011-5-26
That is how I would do it, Walter. But making a custom function from a built-in is o.k. too, I think. I have done so in the past. The thing to avoid is altering the MATLAB version.

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Walter Roberson
Walter Roberson 2011-5-25

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[X,Y] = meshgrid(xi,yi);
Z = griddata(x,y,z,X,Y);
mesh(X,Y,Z), hold
plot3(x,y,z,'o'), hold off

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Melissa
Melissa 2011-5-25
that only works in the command window.
Walter Roberson
Walter Roberson 2011-5-25
Perhaps we are misunderstanding each other, but there is no reason why the code I show above would not work in a script or function, including a callback function.
At most it might be necessary to be more careful about which axes to plot on to.

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