Algorithm 791: TSHEP2D: Cosine series Shephard method for bivariate interpolation of scattered data We describe a new algorithm for scattered data interpolation. It is based on a modified Shepard method similar to that of R. Renka’s Algorithm 660 [ibid. 14, No. 2, 149-150 (1988)] but uses 10-parameter cosine series nodal functions in place of quadratic polynomials. Also, the interpolant has continuous second partial derivatives. An accompanying survey article of the authors [ibid. 25, No. 1, 78-94 (1999; reviewed below)] presents test results that show the method to be more accurate than polynomial-based methods in terms of reproducing test functions with large variations and steep gradients