MATLAB: I’m trying to find the area and the center of a polygon

data analysispolygon

I'm trying to find the area and the center of a polygon, made of a set of coordinates, x & y Here is the code I have so far, the results it's giving me are way off, can anyone see the problem?
%Load the polygon points file
load Drawing1.txt;
x = Drawing1(:,1);
y = Drawing1(:,2);
%Calculate the number of rows
Nrows = numel(textread('Drawing1.txt','%1c%*[^\n]'));
%Set initial values
A = 0;
Cx = 0;
Cy = 0;
for k=0:(Nrows-1);
    if k==0
        %End point is the same as point 0
        i=Nrows;
        j=1;
    else
        %Otherwise use the current point
        i=k;
        j=k+1;
    end
    %Calculate centre points of polygons
    A=A+(((x(i)*y(j))-(x(j)*y(i)))*cosd((y(i)+y(j))/2));
    Cx=Cx+((x(i)+x(j))*((x(i)*y(j))-(x(j)*y(i))));
    Cy=Cy+((y(i)+y(j))*((x(i)*y(j))-(x(j)*y(i))));
end
%Apply terms outside summation
A=0.5*A
Cx = (1/(6*A))*Cx
Cy = (1/(6*A))*Cy
Thanks!

Best Answer

No reason to reinvent the wheel:
doc polyarea
And here is a function I wrote for centroid calculation:
function [x_bar, y_bar, As] = xycentroid(x,y)
%Function calculates centroid and area of a list of xy points
%SCd 11/24/2010
%



%
%Input Arguments:
%   -x: vector of x coordinates (can be row or column vector)
%   -y: vector of y coordinates (can be row or column vector)
%
%Output Arguments:
%   -x_bar: x location of centroid
%   -y_bar: y location of centroid
%   -A: area of polygon
%
      %Error checking:
      assert(nargin==2,'This function expects 2 and only 2 input arguments');
      assert(all(size(x(:))==size(y(:))),'Input arguments: x & y are expected to be the same length');
      x = x(:);
      y = y(:);
      %Engine:
      A = x(1:end-1).*y(2:end)-x(2:end).*y(1:end-1);
      As = sum(A)/2;
      x_bar = (sum((x(2:end)+x(1:end-1)).*A)*1/6)/As;
      y_bar = (sum((y(2:end)+y(1:end-1)).*A)*1/6)/As;
end
I guess I didn't know about polyarea when I wrote this !
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