MissionPlanner.GridUI.calcpolygonarea C# (CSharp) Method

        double calcpolygonarea(List<PointLatLngAlt> polygon)
        {
            // should be a closed polygon
            // coords are in lat long
            // need utm to calc area

            if (polygon.Count == 0)
            {
                CustomMessageBox.Show("Please define a polygon!");
                return 0;
            }

            // close the polygon
            if (polygon[0] != polygon[polygon.Count - 1])
                polygon.Add(polygon[0]); // make a full loop

            CoordinateTransformationFactory ctfac = new CoordinateTransformationFactory();

            GeographicCoordinateSystem wgs84 = GeographicCoordinateSystem.WGS84;

            int utmzone = (int)((polygon[0].Lng - -186.0) / 6.0);

            IProjectedCoordinateSystem utm = ProjectedCoordinateSystem.WGS84_UTM(utmzone, polygon[0].Lat < 0 ? false : true);

            ICoordinateTransformation trans = ctfac.CreateFromCoordinateSystems(wgs84, utm);

            double prod1 = 0;
            double prod2 = 0;

            for (int a = 0; a < (polygon.Count - 1); a++)
            {
                double[] pll1 = { polygon[a].Lng, polygon[a].Lat };
                double[] pll2 = { polygon[a + 1].Lng, polygon[a + 1].Lat };

                double[] p1 = trans.MathTransform.Transform(pll1);
                double[] p2 = trans.MathTransform.Transform(pll2);

                prod1 += p1[0] * p2[1];
                prod2 += p1[1] * p2[0];
            }

            double answer = (prod1 - prod2) / 2;

            if (polygon[0] == polygon[polygon.Count - 1])
                polygon.RemoveAt(polygon.Count - 1); // unmake a full loop

            return Math.Abs(answer);
        }