SURF2 - 2D interpolation for gridded data

       To use SURF2, load the NCAR Graphics library ngmath.

       Copyright (C) 2000
       University Corporation for Atmospheric Research

       The use of this Software is governed by a License Agreement.

UNIX                                               March 1998                                      SURF2(3NCARG)

       XX          (real,  input)  Contains  the  X  coordinate  of  a  point to be mapped onto the interpolated
                   surface.

       YY          (real, input) Contains the Y coordinate of  a  point  to  be  mapped  onto  the  interpolated
                   surface.

       M           (integer, input) The number of grid lines in the X direction. (M > 1)

       N           (integer, input) The number of grid lines in the Y direction. (N > 1)

       X           (real,  input)  An  array containing M X coordinates for grid lines in the X direction. These
                   values must be strictly increasing.

       Y           (real, input) An array containing N Y coordinates for grid lines in the  Y  direction.  These
                   values must be strictly increasing.

       Z           (real, input) An array containing M x N functional values at the grid points; Z(I,J) contains
                   the functional value at (X(I),Y(J)) for I=1,M and J=1,N.

       IZ          (integer, input) The row dimension of the matrix Z (IZ is greater than or equal to M).

       ZP          (real, input) An array of size M x N x 3.

       SIGMA       (real,  input)  Tension factor. Values near zero result in a cubic spline; large values (e.g.
                   50) result in nearly a polygonal line. A typical value is 1.

       SURF2 - 2D interpolation for gridded data

       SURF2 returns the interpolated value at the specified point T.

       surf1, fitgrid_params.

       Complete documentation for Fitgrid is available at URL
       http://ngwww.ucar.edu/ngdoc/ng/ngmath/fitgrid/fithome.html

       FUNCTION SURF2 (XX, YY, M, N, X, Y, Z, IZ, ZP, SIGMA)

       This  function  interpolates  a  surface  value at a specified coordinate using bi-splines under tension.
       SURF1 must be called before invoking SURF2. The desired interpolated value is returned as  the  value  of
       the function.