the polyarea function gives the wrong area

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Hi, I have a question with the polyarea function

x = [0 3 3 0 0 1 1 2 2 1]; y = [0 0 3 3 0 1 2 2 1 1];

With this curve the value it gives is 8, which is correct

However I made a very similar curve

x = [0 0.4258 0.4258 0 0.009 0.4168 0.4168 0.009 0.009]; y = [0 0 0.1807 0.1807 0 0.009 0.009 0.1717 0.1717 0.009];

and the value it gives is polyarea (x, y) -> 0.1433. And the value that should give is 0.010593

someone knows why ?

[madhan ravi]

The later dimensions are not the same

[KSSV]

YOu have not copied entire x. To use polyarea your order of the coordinates should follow a order..if not you will get wrong area. Give the entire x. In this case x is 1X9 and y is 1X10. So, we cannot apply polyarea.

[Sergio arias fernandez]

Sorry, it's true

These are the vectors x, y

x = [0 0.4258 0.4258 0 0.009 0.4168 0.4168 0.009 0.009]; y = [0 0 0.1807 0.1807 0 0.009 0.009 0.1717 0.1717 0.009]; polyarea (x, y)

the result should be 0.010593

Thank you

Answer 1

[Bruno Luong]

I think MATLAB result is right, your expectation is wrong.

Check area

>> 0.1807*0.4258-(0.1807-2*0.009)*(0.4258-2*0.009)

ans =

       0.010593000000000

>>

which is the same than MATLAB

[Sergio arias fernandez]

x = [0 0.4258 0.4258 0 0 0.009 0.4168 0.4168 0.009 0.009]; y = [0 0 0.1807 0.1807 0 0.009 0.009 0.1717 0.1717 0.009]; polyarea(x,y)

ans=0.1433

[Bruno Luong]

x = [0 0.4258 0.4258 0      0 0.009 0.009  0.4168 0.4168 0.009];
y = [0 0      0.1807 0.1807 0 0.009 0.1717 0.1717 0.009  0.009]; 
polyarea(x,y)

ans =

   0.010593000000000

PS: You enter the points in wrong order for the inner rectangle

[Sergio arias fernandez]

Thank you very much Bruno

why this order is ok

and this order in wrong?

[Bruno Luong]

Yes the order of the second plot is wrong; MATLAB adds (instead of subtracts) the areas of the two rectangles if they run in the same direction (anti-clockwise for both here)

[Sergio arias fernandez]

thanks a lot!

Answer 2

[Sergio arias fernandez]

What do you mean?

That the coordinate vectors are of different dimension? They are vectors of dimension 10 in both cases

Thank you