void UpdateArea()
{
double xmin, xmax, ymin, ymax;
List<Point> px, py;
px = Points.OrderBy (p => p.X).ToList();
py = Points.OrderBy (p => p.X).ToList();
xmin = px[0].X;
xmax = px[px.Count-1].X;
ymin = py[0].Y;
ymax = py[py.Count-1].Y;
TopLeft = new Point (xmin, ymin);
TopRight = new Point (xmax, ymin);
BottomLeft = new Point (xmin, ymax);
BottomRight = new Point (xmax, ymax);
}