CURVD - derivatives for 1D functions.

       To use CURVD, 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                                      CURVD(3NCARG)

       T           (real, input) The abscissa for which an interpolated function value is desired.

       N           (integer, input) The number of input data values. (N > 1)

       X           (real, input) An array containing the abscissae for the input function.

       Y           (real,  input)  An  array  containing  the functional values of the input data -- Y(K) is the
                   functional value at X(K) for K=1,N.

       YP          (real, input) Contains values for the second derivative (as calculated by CURV1).

       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.

       CURVD - derivatives for 1D functions.

       CURVD returns the derivative value of the interpolated curve at the specified point T.

       curv1, fitgrid_params.

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

       FUNCTION CURVD (T, N, X, Y, YP, SIGMA)

       This  function  calculates a derivative at a specified point using a spline under tension.  CURV1 must be
       called prior to calling CURVD, and the values thus obtained used as input to CURVD. The derivative  value
       is returned.