# ncl_curvs - Man Page

calculate values for a smoothing spline

## Synopsis

`CALL CURVS (N, X, Y, D, ISW, S, EPS, YS, YSP, SIGMA, TEMP, IER) `

This subroutine calculates certain values that are used by CURV2 in order  to compute an interpolatory smoothing spline under tension through a  sequence of functional values. The actual computation of the interpolated  values must be done using CURV2.

Three parameters are used to control the degree of smoothness -- D, S, and EPS.

`The parameter D is a value indicating the degree of confidence in the  accuracy of the input function values -- it should be an approximation of  the standard deviation of error. Effectively the value of D controls how  close the smoothed curve comes to the input data points. If D is small  then the interpolated curve will pass close to the input data. The larger  the value of D, the more freedom the smooth curve has in how close it comes to the input data values. `

S is a more subtle global smoothing parameter. S must be non-negative.  For small values of S, the interpolated curve approximates the tension  spline and for larger values of S, the curve is smoother. A reasonable  value for S is REAL(N).

`EPS controls the precision to which S is interpreted; EPS must be  between 0. and 1.  inclusive. A reasonable value for EPS is SQRT(2./REAL(N)). `

## Description

N

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

X

(integer, input) An array containing the abscissae for the input function.  These must be increasing values.

Y

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

D

(integer, input) A user-specified value containing the observed weights. D may be either an array or a scalar, depending on the value of ISW (as described below).

ISW

(integer, input) A switch for interpreting the value of D. If ISW=0, then D  is an array of length N (D contains an individual error estimate for each input data value); if ISW=1, then D is a scalar that serves as an error estimate for every single data item.

S

(integer, input) Contains the value for smoothing. S must be non-negative. Larger values for S yield greater smoothing. A reasonable value is REAL(N).

EPS

(integer, input) Contains a tolerance value for the relative precision to  which S should be interpreted. EPS must be between 0. and 1. inclusive. A reasonable value is SQRT(2./REAL(N)).

YS

(integer, input) An array of length N.

YSP

(integer, input) An array of length N.

SIGMA

(integer, 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.

TEMP

(integer, input) Scratch space.

IER

(integer, output) An error return value.  If IER is returned as 0, then no errors were detected.

= 1 if N is less than 2.
= 2 if S is negative.
= 3 if EPS is negative or greater than 1.
= 4 if X values are not strictly increasing.
= 5 if D is negative.

## Access

To use CURVS, load the NCAR Graphics library ngmath.

## See Also

curv2, fitgrid_params.

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

## Info

March 1998 UNIX NCAR GRAPHICS