how to plot 2D contour lines from the data file

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Kona Veera Ganesh
Kona Veera Ganesh on 27 May 2021
Commented: Adam Danz on 18 Jan 2022 at 21:24
I tried to plot the 2D contour from my data file (which has three columns ,such as X,Y and intensity value) , I had plotted from scatter command it is filling ,but we need line contours,how to build it,thanks inadvance.

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Answers (2)

Benjamin Großmann
Benjamin Großmann on 27 May 2021
Edited: Benjamin Großmann on 27 May 2021
contour(x, y, z) gives you the contour plot, z is the intensity value
To create matrices of proper size, look at the scatteredInterpolant object. I do not recommend using approaches with unique since you have noisy data.:
% Define the object
F = scatteredInterpolant(x,y,v);
% !! Please have a close look at interpolation method and extrapolation
% method
% Create a proper mesh
xm = [min(x):max(diff(x)):max(x)];
ym = [min(y):max(diff(y)):max(y)];
[Xm,Ym] = meshgrid(xm, ym);
% Interpolate Z values
Z = F(Xm,Ym);
% create contour plot
Adam Danz
Adam Danz on 27 May 2021
FWIW I think the contour family of functions should accept the syntax you described as long as x and y are defined according to a grid but for now, we're stuck with the requirement to reshape the data.

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Adam Danz
Adam Danz on 27 May 2021
Edited: Adam Danz on 27 May 2021
Contour plots require z to be a matrix. contour(Z)
You can define the x and y values, too, and they can be vectors or matrices contour(X,Y,Z)
The important thing is that each element of z is defined in x and y.
Working with gridded data
If you're working with three vectors x,y,intensity, I assume you've got duplicates in x and y that define a grid similar to the example below in which case you just need to reshape the intensity data.
% Create nx3 matrix of [x,y,intensity] values
xyz(:,1) = repelem(1:10,1,10)';
xyz(:,2) = repmat(1:10,1,10)';
xyz(:,3) = reshape(magic(10),100,1)
xyz = 100×3
1 1 92 1 2 98 1 3 4 1 4 85 1 5 86 1 6 17 1 7 23 1 8 79 1 9 10 1 10 11
% Reshape the intensity vector into a matrix
[xUnq,~,xIdx] = unique(xyz(:,1));
[yUnq,~,yIdx] = unique(xyz(:,2));
zMat = nan(numel(yUnq),numel(xUnq));
zIdx = sub2ind(size(zMat),yIdx,xIdx);
zMat(zIdx) = xyz(:,3);
% Plot contour
Working with non-gridded data
After providing your data, it turns out that your x and y values do not form a grid. This means you must design a grid and average all of the intensity values within the grid.
This demo creates a 50x50 grid but you can design the grid differently by specifying the x and y grid edges or by changing the number of bins (see contour documentation).
Load data
filename = unzip('');
data = readmatrix('210.txt');
data(1:20,:) % show the first 20 rows
ans = 20×3
-433.0130 -198.0000 -0.0002 -433.0130 -199.0000 -0.0001 -432.1470 -197.5000 0.0002 -432.1470 -198.5000 -0.0000 -432.1470 -199.5000 -0.0001 -431.2810 -198.0000 -0.0001 -431.2810 -199.0000 -0.0002 -430.4150 -197.5000 0.0002 -430.4150 -198.5000 -0.0001 -430.4150 -199.5000 -0.0001
Create the 2D grid
nbins = 50; % Number of bins for x and y
[xBinNum, xEdges] = discretize(data(:,1),nbins);
[yBinNum, yEdges] = discretize(data(:,2),nbins);
[unqXYbins, ~, zBinNum] = unique([xBinNum(:), yBinNum(:)],'rows');
Average the z values within each bin of the 2D grid
xyz is a matrix containing the x and y bin numbers and the averaged z values within each bin. Missing data will be ignored. Missing data are undefined intensities for some combinations of x and y bins.
zBinMean = splitapply(@(x)mean(x,'omitnan'),data(:,3), zBinNum);
xyz = [unqXYbins, zBinMean];
Reshape the z values into a matrix
Rows and columns of the matrix are defined by the bin edges.
zMat = nan(nbins,nbins);
zIdx = sub2ind(size(zMat),xyz(:,2),xyz(:,1));
zMat(zIdx) = xyz(:,3);
Plot the contour map and compare it to the raw data shown as a 3D scatter plot
The x and y values will be the bin centers xBinCnt, yBinCnt
xBinCnt = xEdges(2:end) - diff(xEdges);
yBinCnt = yEdges(2:end) - diff(yEdges);
contour(xBinCnt, yBinCnt, zMat)
grid on
title('binned averages')
axis square
scatter3(data(:,1), data(:,2), data(:,3), 30, data(:,3), 'filled')
grid on
view(-56, 5)
title('raw data')
Plot the contour again but zoom into relevant section
contour(xBinCnt, yBinCnt, zMat)
xlim([-40 22])
ylim([-10 2])
title('zoomed in')
grid on
axis square
As you can see, your data are mostly flat along the XY plane at z=0 except for a column of data at about (0,0). That explains the relatively flat contour plot except for the activity around (0,0).

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