Page 115 - Excel for Scientists and Engineers: Numerical Methods

92 EXCEL: NUMERICAL METHODS

Figure 5-18. Cubic interpolation in a two-way table.
The shaded cells are the ones used in the interpolation.
(folder 'Chapter 05 Interpolation', workbook 'Interpolation 11', module ' Cubic Interpolation 2-Way')

We'll use the InterpC function to perform the interpolation. Figure 5-19
shows the z values, interpolated at y = 63% using the four bracketing y values, for
the four bracketing x values. The formula in cell M8 is

=InterpC(63%,$E$3:$H$3,E8:H8)

Figure 5-19. First steps in cubic interpolation in a two-way table.
(folder 'Chapter 05 Interpolation', workbook 'Interpolation II', module ' Cubic Interpolation 2-Way')

Then, in this one-way table, we use the formula

=InterpC(L15,$L$8:$L$Il ,$M$8:$M$11)
in cell MI 5 to obtain the final interpolated result, as shown in Figure 5-20.