Area 3D polygon on a plane

    static public double getSignedArea(List points, XYZ normal)
        int nPoints = points.Count;

        XYZ sum = XYZ.Zero;
        for (int index = 1; index < nPoints; index++)
            sum += points[index].CrossProduct(points[index - 1]);
        sum += points[0].CrossProduct(points[nPoints - 1]); 

        double area = normal.DotProduct(sum);
        return -0.5 * area;

To calculate the area of a 3D polygon on a plane. the normal is the plane’s normal.

Lastest update in March 2021, inital post in June 2020

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