I am working on some energy data in radiation
I have to do 2D spline interpolation on a tabular data

My questions are

This is expained well in NR but i can't figure out one thing
i can see where w(i,k) is coming from

It seems like is a tabulated values or saved table, but i still can see it. I magine that i could be some kind of a interpolant or linear weighted function but not sure about that. Can some one explain to be.

My second question is, if the pair of point on which to do the interpolation lies beyond the rectangular area, how can one extrapolate using spline


Thanks very much


Here is the link to the Reference Page

My problem is on Page 119

http://lib-www.lanl.gov/numerical/bookcpdf/c3-3.pdf