Algorithm to Calculate the area of a 2D polygon

I have two arrays, storing the x and y values of the polygon.

I wish to use these values to calculate the area of the polygon.

The polygon is a simple, concave 2d polygon.

Can someone possibly direct me to how I may achieve this.

Many Thanks.
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stats786Author Commented:
I tried implementing a quick test of what you suggested, but I don't get the values I expect. Can you spot what I'm doing wrong. Thanks.

import  java.lang.Math.*;

public class areaTest
      //Test Values
      //square - expected area = 8
      //static double[] polyX = { 1.0, 3.0, 3.0, 1.0, 1.0};
      //static double[] polyY = { 1.0, 1.0, 3.0, 3.0, 1.0};
      //triangle - expected area = 10.5
      static double[] polyX = { 1.0, 4.0, 1.0, 1.0};
      static double[] polyY = { 1.0, 4.0, 8.0, 1.0};
      public static void main (String args[])
            double test = calcArea(polyX, polyY);
            System.out.println( test );

      public static double calcArea(double xVals[],double yVals[])
            int i;
            double area = 0;
            double mag;
                  area = area + (xVals[i]*yVals[0])-(xVals[0]*yVals[i]);
            area = area/2;
            mag= Math.abs(area);

            return mag;


area = area + (xVals[i]*yVals[(i+1)%xVals.length])-(xVals[(i+1)%xVals.length]*yVals[i]);

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stats786Author Commented:
Thank you very much for that.

Is'nt the formula you've implemented using the following rhetoric:


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