Method adapted from [url=https://math.stackexchange.com/questions/2010758/how-do-i-fit-a-paraboloid-surface-to-nine-points-and-find-the-minimum]https://math.stackexchange.com/questions/2010758/how-do-i-fit-a-paraboloid-surface-to-nine-points-and-find-the-minimum[/url]