Area between a curve and a baseline

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Joe Abraham
Joe Abraham on 13 Nov 2012
I want to create a base line (red line) from the plot and calculate area between the base line and curve (blue line).Curve is a vector. Baseline is drawn manually using code below. [x y]=ginput(2); line(x,y);
Any help/ suggestions is highly appreciated.
Thanks

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Accepted Answer

Andrei Bobrov
Andrei Bobrov on 13 Nov 2012
Edited: Andrei Bobrov on 13 Nov 2012
data - your array < 2 x N >, data(1,:) - abscissa; data(2,:) - ordinate.
plot(data(1,:),data(2,:));
[x y]=ginput(2);
t = x(1) <= data(1,:) & x(2) >= data(1,:);
X = data(1,t);
out = trapz(X,diff([interp1(x,y,X);data(2,t);]))
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Joe Abraham
Joe Abraham on 28 Nov 2012
Sorry for delayed reply. I got the following error when computed ur code.
??? Attempted to access xx(:,1); index out of bounds because size(xx)=[3,0].
Error in ==> specgraph.areaseries.refresh at 82 v = [xx(:,k) yy(:,k)];
Error in ==> graph2d.series.schema>LdoDirtyAction at 108 refresh(h);
Warning: Error occurred while evaluating listener callback. > In area at 100 In DSC at 27

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More Answers (1)

Teja Muppirala
Teja Muppirala on 13 Nov 2012
You could try something along the lines of this:
% Just making some data
x0 = 0:0.001:5;
y0 = sin(x0);
plot(x0,y0);
grid on; set(gca,'layer','top');
% Get your two points
[x y]=ginput(2); line(x,y);
% Break up the line segment into a sufficiently fine grid
N = 10001;
x_fine = linspace(x(1),x(2),N);
dx = x_fine(2)-x_fine(1);
% Interpolate the values at the grid points
top = interp1(x0,y0,x_fine); % Top of the area
bottom = interp1(x,y,x_fine); % Bottom of the area
% Get the area using TRAPZ and show it
area = dx*trapz(top-bottom)
patch([x_fine fliplr(x_fine)], [top fliplr(bottom)],'r');
Just replace the sine with whatever data you are using (the example above uses row vectors, so if your data is in a column instead, you'll probably have to make some minor modifications)
  1 Comment
Joe Abraham
Joe Abraham on 28 Nov 2012
Hi. Thanks for the help. I get the following error when tried executing your code. Do you have any idea what went wrong?
??? Error using ==> interp1 at 259 The values of X should be distinct.
Error in ==> DSC at 28 top = interp1(T1,HbyC,x_fine); % Top of the area

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