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 1.622942 1.188229 1.304150 1.275165 0.000000 2.535847 1.856607 2.037733 1.992452 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