QUADTERP

       Quadratic interpolation of X,Y vectors onto a new X grid

       Interpolate a function Y = f(X) at specified grid points using an
       average of two neighboring 3 point quadratic (Lagrangian) interpolants.
       Use LINTERP for linear interpolation

       QUADTERP, Xtab, Ytab, Xint, Yint, [ MISSING = ]

       Xtab - Vector (X TABle) containing the current independent variable
               Must be either monotonic increasing or decreasing
       Ytab - Vector (Y TABle) containing the dependent variable defined
               at each of the points of XTAB.
       Xint - Scalar or vector giving the values of X for which interpolated
               Y values are sought

       Yint - Interpolated value(s) of Y, same number of points as Xint

       MISSING - Scalar specifying Yint value(s) to be assigned, when Xint
               value(s) are outside of the range of Xtab.     Default is to
               truncate the out of range Yint value(s) to the nearest value
               of Ytab.   See the help for the INTERPOLATE function.

       3-point Lagrangian interpolation.  The average of the two quadratics
       derived from the four nearest  points is returned in YTAB.   A single
       quadratic is used near the end points.   VALUE_LOCATE is used
       to locate center point of the interpolation.

       QUADTERP provides one method of high-order interpolation.   The
           RSI interpol.pro function includes the following alternatives:
       interpol(/LSQUADRATIC) - least squares quadratic fit to a 4 pt
               neighborhood
       interpol(/QUADRATIC) - quadratic fit to a 3 pt neighborhood
       interpol(/SPLINE) - cubic spline fit to a 4 pt neighborhood
       Also, the IDL Astro function HERMITE fits a cubic polynomial and its
             derivative to the two nearest points.

       Unless MISSING keyword is set, points outside the range of Xtab in
       which valid quadratics can be computed are returned at the value
       of the nearest end point of Ytab (i.e. Ytab[0] and Ytab[NPTS-1] ).

       A spectrum has been defined using a wavelength vector WAVE and a
       flux vector FLUX.  Interpolate onto a new wavelength grid, e.g.
       IDL> wgrid = [1540.,1541.,1542.,1543.,1544.,1545.]
       IDL> quadterp, wave, flux, wgrid, fgrid
       FGRID will be a 5 element vector containing the quadratically
       interpolated values of FLUX at the wavelengths given in WGRID.

       ZPARCHECK

       31 October 1986 by B. Boothman, adapted from the IUE RDAF
       12 December 1988 J. Murthy, corrected error in Xint
       September 1992, W. Landsman, fixed problem with double precision
       August 1993, W. Landsman, added MISSING keyword
       June, 1995, W. Landsman, use single quadratic near end points
       Converted to IDL V5.0   W. Landsman   September 1997
       Fix occasional problem with integer X table,
       YINT is a scalar if XINT is a scalar   W. Landsman Dec 1999
       Use VALUE_LOCATE instead of TABINV W. Landsman  Feb. 2000

[Home Page] [Software, Documentation] [IDL Documentation] [Quick Reference] [Feedback]