A rational quadratic spline function [Sarfraz, M. (1993), Monotonocity preserving interpolation with tension control using quadratic by linear functions, Journal of Scientific Research, Vol. 22 (1), 1-12 ] has been extended to a rational bi-quadratic spline function. Simple data dependent constraints are derived on the free parameters in the description of rational bi-quadratic spline function to preserve the shape of 3D data. The developed scheme is economical to compute and visually pleasant, it also gives the extra freedom to the user to refine the shape of the surface as desired. Further data dependent constraints are also derived on the free parameters to preserve the shape of data that lies above the plane.