NAME¶
CURVS - 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
COPYRIGHT¶
Copyright (C) 2000
University Corporation for Atmospheric Research
The use of this Software is governed by a License Agreement.