NAME

       c_ftsurf - 2D tension spline interpolation of rectangular data

FUNCTION PROTOTYPE

       float *c_ftsurf (int, int, float *, float *, float *,
                        int, int, float *, float *, int *);

SYNOPSIS

       float *c_ftsurf (mi, ni, xi, yi, zi, mo, no, xo, yo, ier);

DESCRIPTION

       mi          The number of grid lines in the X direction. (mi > 1)

       ni          The number of grid lines in the Y direction. (ni > 1)

       xi          Pointer  to  an  array  containing  m  X  coordinates  for grid lines in the X
                   direction.

       yi          Pointer to an array containing n  Y  coordinates  for  grid  lines  in  the  Y
                   direction.

       zi          Pointer  to  m  x  n floats which are the functional values at the grid points
                   defined by xi and yi.

       mo          The number of output values in the X direction.

       no          The number of output values in the Y direction.

       ier         The value *ier is an error flag as per:

                     = 0 -- no error.
                     = 1 -- if n is less than 2 or m is less than 2.
                     = 2 -- if X or Y values are not strictly increasing.

RETURN VALUE

       c_ftsurf returns a  pointer  to  an  array  containing  mo  x  no  floats  which  are  the
       interpolated values on the grid specified by the arrays xo and yo.

USAGE

       This  procedure  calculates an interpolatory surface passing through a rectangular grid of
       function values. The surface computed is a tensor product of splines under tension.

       c_ftsurf is called after all of the desired values for control parameters  have  been  set
       using  the procedures c_ftseti, c_ftsetr, c_ftsetc, c_ftsetfa. The control parameters that
       apply to c_ftsurf are: sig, zx1, zxm, zy1, zyn, z11, zm1, z1n, zmn, df1,  df2,  df3,  df4,
       df5, df6, df7, df8

       The  value for the parameter sig specifies the 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. (the default).

       zx1  is  an array containing n X-partial derivatives of the function along the line xi[0],
       that is zx1[j] is  the  X-partial  derivative  at  point  (x[0],y[j])  for  j=0,n-1.  This
       parameter  may  be defaulted by setting the value for df1 appropriately. The default is to
       compute zx1 internally. Values for zx1 can be set using the procedure c_ftsetfa.

       zxm is an array containing n X-partial derivatives of the function along the line xi[m-1],
       that  is  zxm[j]  is  the  X-partial derivative at point (xi[m-1],yi[j]) for j=0,n-1. This
       parameter may be defaulted by setting the value for df2 appropriately. The default  is  to
       compute zx2 internally. Values for zxm can be set using the procedure c_ftsetfa.

       zy1  is  an array containing m Y-partial derivatives of the function along the line yi[0],
       that is zy1[j] is  the  Y-partial  derivative  at  point  (x[i],y[0])  for  i=0,m-1.  This
       parameter  may  be defaulted by setting the value for df3 appropriately. The default is to
       compute zy1 internally. Values for zy1 can be set using the procedure c_ftsetfa.

       zyn is an array containing m Y-partial derivatives of the function along the line yi[n-1],
       that  is  zyn[j]  is  the  Y-partial  derivative  at point (x[i],y[n-1]) for i=0,m-1. This
       parameter may be defaulted by setting the value for df4 appropriately. The default  is  to
       compute zyn internally. Values for zyn can be set using the procedure c_ftsetfa.

       z11,  zm1,  z1n,  zmn  specify X-Y-partial derivatives of the function at the four corners
       (xi[0],yi[0]), (xi[m-1],yi[0]), (xi[0],yi[n-1]), (xi[m-1],yi[n-1]), These  parameters  may
       be  defaulted  by setting the values for df5, df6, df7, df8, appropriately. The default is
       to compute z11, zm1, z1n, zmn internally.

ACCESS

       To use c_ftsurf, load the NCAR Graphics library ngmath.

SEE ALSO

       fitgrid_params, c_ftseti, c_ftsetr, c_ftsetc.

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

COPYRIGHT

       Copyright (C) 2000
       University Corporation for Atmospheric Research

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