# Trying to find area under my plot

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Arya patwardhan 2022 年 11 月 28 日

I have plotted my data (two vectors of size 300x1 each) and it looks like this (see attachment):
I have tried to use trapz to get my area and get a value of around 46.
Looking at the graph, I can tell this area is wrong. Very roughly it should be around 12
So how do i get my area to equal to 12?
Thanks for any help
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Arya patwardhan 2022 年 11 月 28 日
I would also like to be able to find the postive and negative area's

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### 回答 (2 件)

Torsten 2022 年 11 月 28 日

To get the positive and negative areas separately, cut your closed curve at the two positions where it passes the x-axis and take the line connecting these two points on the x-axis to close the upper and lower curves.
n = 100;
I = linspace(0,1,n);
x = cos(2*pi*I);
y = sin(2*pi*I);
polyarea(x,y)
ans = 3.1395
pi
ans = 3.1416
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John D'Errico 2022 年 11 月 28 日
You don't give your data. You show only a plot. So make a polyshape of it. Then compute the area.
t = linspace(0,2*pi,50);
x = cos(t);
y = sin(t);
x(end) = [];
y(end) = [];
ps = polyshape(x,y);
area(ps)
ans = 3.1330
As you can see, this is just unit curcle, so the area would be pi, at least if we had more points.
plot(ps)
hold on
plot(x,y,'o')
axis equal
grid on
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Torsten 2022 年 11 月 29 日

I'd cover the area of interest by a fine enough mesh of squares and count them.
I think this will be more reliable than to use polyarea to the dataset that you included.
The other alternative is to work on your data so that you extract a sequence of points that don't overlap and repeat and that represent the upper and low part of your area, respectively.

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