# csa2s

csa2s calculates an approximating cubic spline for two-dimensional input data. If you want to weight the input data values, calculate derivatives, or handle data sparse areas specially, then you will need to use csa2xs.

Note: This function is only available in version 4.1.1 of NCL. If your site is licensed for version 4.1, then you can get version 4.1.1 for free. To get version 4.1.1 of NCAR Graphics software, please contact your site representative. If you don't know who your site representative is, then send email to ncarginf@ucar.edu or call (303) 497-1201.

## Synopsis

```    function csa2s(
xi[*]    : float,
yi[*]    : float,
zi[*]    : float,
knots[2] : integer
xo[*]    : float
yo[*]    : float
)

```

## Arguments

xi
A one-dimensional array of any size containing the X coordinates of the input data points.
yi
A one-dimensional array of the same size as xi containing the Y coordinates of the input data points.
zi
A one-dimensional array of the same size as xi and yi containing the functional values at the input data coordinates given by xi and yi. zi[k] is the input function value at (xi[k],yi[k]) for k=0 to dimsizes(xi)-1.
knots
The number of knots to be used in constructing the approximating surface. knots(0) and knots(1) must both be at least 4. The larger the value for knots, the closer the approximated surface will come to passing through the input function values.
xo
A one-dimensional array containing the X coordinates of the output surface.
yo
A one-dimensional array containing the Y coordinates of the output surface.

## Return value

csa2s returns a two-dimensional array containing the calculated functional values. The first dimension of the returned value has the same size as xo and the second dimension of the returned value has the same size as yo. If zo is the returned value, then zo(i,j) contains the functional value at coordinate (xo(i),yo(j)).

## Description

csa2s is in the csagrid package - a software package that implements a cubic spline approximation algorithm to fit a function to input data. The input for the approximation is a set of randomly-spaced data. These data may be one-dimensional, two-dimensional, or three-dimensional.

The general documentation for csagrid contains several complete examples for entries in the csagrid package.

## Example

```begin

;
;  Create the input arrays.
;
xmin = -1.
xmax =  1.
ymin = -1.
ymax =  1.

nx = 29
ny = 25

ndata = 1000
xi = new(ndata,float)
yi = new(ndata,float)
zi = new(ndata,float)

;
;  Generate input data using the function f(x,y) = y**2 - 0.5*y*x**2
;
do i=0,ndata-1
xi(i) = xmin + (xmax-xmin)*rand()/32767.
yi(i) = ymin + (ymax-ymin)*rand()/32767.
zi(i) = yi(i)*yi(i) - 0.5*xi(i)*xi(i)*yi(i)
end do

;
;  Set up the output grid.
;
xo = fspan(xmin,xmax,nx)
yo = fspan(ymin,ymax,ny)

knots = (/4,4/)
;
;  Calculate the approximated function values.
;
yo = csa2s(xi,yi,zi,knots,xo,yo)
end
```

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\$Revision: 1.3 \$ \$Date: 1999/03/18 22:38:59 \$