second derivatives from SPLIE2 natural spline assumed 0.000000 0.101434 0.074264 0.081509 0.079698 0.000000 0.405735 0.297057 0.326037 0.318791 0.000000 0.912905 0.668378 0.733586 0.717278 0.000000 1.622941 1.188230 1.304150 1.275165 0.000000 2.535847 1.856607 2.037733 1.992453 actual second derivatives 0.080000 0.080000 0.080000 0.080000 0.080000 0.320000 0.320000 0.320000 0.320000 0.320000 0.720000 0.720000 0.720000 0.720000 0.720000 1.280000 1.280000 1.280000 1.280000 1.280000 2.000000 2.000000 2.000000 2.000000 2.000000