The interp() function returns an interpolated value of the function at the value of its argument. The functional relation is previously initialized using the fitting mode command spline. The interpolation is obtained from cubic splines. Natural (i.e., the second derivative of the interpolating function at either or both the first and last point of the original data equal zero) cubic spline or specific first derivatives at the extreme points of the original data set are specified while initializing the process using spline command.
spline