How to extract x and y position of contour line?

In old matlab versions, I could extract x and y points along a contour line as shown below. This is incredibly useful in my work. Now in R2015 the approach doesn't work (hh.XData and hh.YData are the x and y values of the data you're contouring, rather than the x,y position of the contour line). In more recent versions of contour, how can I extract x,y points along a contour line? If I can't do it via contour, is there another way?

z = peaks; % Load in a dataset that all matlab users have
x = 1:size(peaks, 2); y = 1:size(peaks, 1);
figure;
pcolor(x, y, z); % Plot the data
shading flat;
hold on;
[cc, hh] = contour('v6', x, y, z, [0 10^5], 'k'); % Overlay contour line (will plot 3 continuous contours)
xPointsOfContour = get(hh(1), 'xdata'); % Get points along the first contour
yPointsOfContour = get(hh(1), 'ydata');
plot(xPointsOfContour, yPointsOfContour, 'w.'); % Demo that points on contour line were extracted

Answer 1

[K E]

Just realized that the contour matrix cc now seems to contain x and y location data,

pcolor(x, y, z); % Plot the data
hold on;
shading flat
[cc, hh] = contour(x, y, z, [0 10^5], 'k'); % Overlay contour line
hold on;
cc(cc<1) = NaN;
h = plot(cc(1,:), cc(2, :), 'w.');

[Todd]

But still I have similar problem, I can't access the individual curves that USED to be in contour - children in r2013. What do I have to now manually extract them??

[Tanziha Mahjabin]

Hi,

I was trying to extract the time and depth points for 24 deg cemcius coutour line.I tried same process:

[td,c] = contour(x,-y,temp_grid,[24,24], 'k'); % where x is time in 7 minute interval, y is depth.

But the output time interval went haphazard. Can I keep same time interval while extracting?

[Adam Danz]

@Tanziha Mahjabin could you provide a minimal working example?

[Tanziha Mahjabin]

I interpolated the data after the extraction for 5 min timestep...all good. Thank you for your reply though.

[Tanziha Mahjabin]

Hi, If I have 24 deg C temperature contour in a transect. How can I calculate the velocity points of that specific line?

[Adam Danz]

What does velocity have to do with temperature? Are you asking how to compute the rate of temperature change at any point along the contour plot?

This is a completely different question than the topic in this thread. I encourage you to ask a new question.

[Ayush Tiwari]

what does 'w.' do here ?

[Adam Danz]

w = white

'.' = point markers

w. = white point markers

Answer 2

[Yuhang Luo]

https://ww2.mathworks.cn/matlabcentral/fileexchange/38863-extract-contour-data-from-contour-matrix-c this function might work.

[Adam Danz]

Also see

https://www.mathworks.com/matlabcentral/fileexchange/74010-getcontourlinecoordinates

[cm, h] = contour(. . .);
contourTable = getContourLineCoordinates(cm);
% --or--
contourTable = getContourLineCoordinates(h);

%     contourTable =
%      Level      Group        X             Y     
%     ________    _____    __________    __________
%      -5.8504      1       -0.040982        -1.625
%      -5.8504      1       -0.027916         -1.75
%      -5.8504      1               0       -1.7823
%      -5.8504      1        0.045032          -1.5
%     etc....

Answer 3

[John L]

Plot a single contour, then plot(hh.ContourMatrix(1,2:end),hh.ContourMatrix(2,2:end))

[Lars K.]

hi john,

with your answer you explained something very important using contour. I struggled and lost a lot of time, while i was trying to find intersections of two contour lines.

Plotting both contour matrix as lines and execute

[x,y,~,~]=intersection(contour1(1,:),contour1(2,:),contour2(1,:),contour2(2,:),true)

(btw thanks a lot to Douglas Schwarz) the intersection script found 6 intersections (4 wrong ones). Plotting both contour lines with markers i saw these other data points the first time.

The first column of the contour matrix M contains the value of the contour line and does not belong to the xy data points. With Johns little but important hint using

[x,y,~,~]=intersection(contour1(1,2:end),contour1(2,2:end),contour2(1,2:end),contour2(2,2:end),true)

the problem was solved and i got my two correct intersections.

[KFoster]

The first column of the contour matrix M denotes what contour level and how many vertices are included in that particular line; similar columns are found throughout the M matrix between the coordinates for each line. It's in the matlab documentation here: https://www.mathworks.com/help/matlab/ref/contour.html#mw_6bc1813f-47cf-4c44-a454-f937ea210ab6