Source: https://www.researchgate.net/profile/Mohamed_Mourad_Lafifi/post/Can_you_offer_a_solution_to_fitting_a_bivariate_normal_surface_preferably_in_Matlab/attachment/5a5537424cde266d58817fad/AS:581005217144832@1515533846288/download/Statistics+Toolbox+for+Use+with+Matlab.pdf