Hello

I am trying to determine the general formula that will allow me to interpolate a value "z" related to any point (x,y) where x1<x<x2 and y1<y<y2 from a surface map defined by 4 points z1(x1,y1),z2(x1,y2),z3(x2,y1) and z4(x2,y2).

I can do it in Excel for discrete values, and have tried using a kind of "weighted average" approach but I can't seem to nail this general formula.

Any help appreciated.

Refer to this Excel file for reference.

Thanks

SKB