# calculate area of boundary of set of 2d-data point

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ha ha on 7 Feb 2018
Commented: Niklas Kurz on 15 Jul 2020
Example: I wanna calculate the area of the boundary of set 2d-data as below illustration: I use the code(polyarea) as follows:
clear;clc;
x=[1;5;5;1;2];
y=[5;5;1;1;3];
plot(x,y,'.')
k = boundary(x,y);% generate boundary of data points
hold on;
plot(x(k),y(k));
A = polyarea(x,y);%calculate the area of boundary
But the result is: 12 <--- wrong???
How can I fix it?

Roger Stafford on 7 Feb 2018
I would advise you to "close" the polygon so that the first point is the same as the last point. Otherwise, Matlab may be a bit confused as to how the polygon is defined.
x=[1;5;5;1;2;1];
y=[5;5;1;1;3;5];
Niklas Kurz on 15 Jul 2020
I like you! You allowed me a second, highly valuable perspective on this issue. Even posted on my date of birth. So it was just destined to be.

Mehdi Mosafer on 1 Nov 2018
There is a simpler way; just get the 2nd output of the function "boundary:"
[k,A] = boundary(x,y)
Here, "A" is the area.

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