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# csa2lx

Calculates an approximating cubic spline for two-dimensional input data.

## Prototype

```	function csa2lx (
xi     [*] : numeric,
yi     [*] : numeric,
zi         : numeric,
wts        : numeric,
knots   : integer,
smth    : numeric,
nderiv  : integer,
xo     [*] : numeric,
yo     [*] : numeric
)

return_val  :  float or double
```

## Arguments

xi

A 1-dimensional array of length nxi containing the X coordinates of the input data points.

yi

A 1-dimensional array of length nxi containing the Y coordinates of the input data points.

zi

An array of any dimensionality (last dimension must be nxi) containing the functional values at the input data coordinates given by xi and yi. That is, zi(...,i) is the input function value at (xi(i),yi(i)) for i=0 to nxi-1.

wts

A scalar or an array of length nxi containing weights for the zi values at the input xi values. If wts is an array, then wts(k) is a weight for the value of zi(...,k) for k=0,nxi-1. If you do not desire to weight the input zi values, then set wts equal to -1. The weights in the wts array are relative and may be set to any non-negative value. When csa2lx is called, the weights are summed and the individual weights are normalized so that the weight sum is unity.

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.

smth

A parameter that controls extrapolation into sparse data regions. If smth is zero, then nothing special is done in sparse data regions. A good first choice for smth is 1.

nderiv

Specifies whether you want functional values (nderiv=0), first derivative values (nderiv=1), or second derivative values (nderiv=2) in each of the two coordinate directions.

xo

A one-dimensional array of length nxo containing the X coordinates of a list of points where functional values are desired. xo can be a single point.

yo

A one-dimensional array of the same length as xo containing the Y coordinates of a list of points where functional values are desired. yo can be a single point.

## Return value

An array containing the calculated functional values. The array will be the same size as zi, except that the last dimension will be of length nxo. If zo is the returned value, then zo(...,i) contains the functional value at coordinate (xo(i),yo(i)) for i=0,nxo-1.

The array is double if any of the input values is double; otherwise, it is float.

## Description

This function is part of 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, which may be one-dimensional, two-dimensional, or three-dimensional. The general documentation for Csagrid contains several complete examples.

The following three two-dimensional functions all do the same thing, differing only in the type of the input and output arrays: csa2 (generic input/output); csa2s (single input/output); csa2d (double input/output).

If you want to weight the input data values, calculate derivatives, or handle sparse data areas specially, you should instead use one of these "expanded" functions (note the "x" following the "2" in the name): csa2x (generic input/output); csa2xs (single input/output); csa2xd (double input/output).

If you want to compute function values at a specified list of coordinate positions, rather than at coordinate positions forming a rectangular grid, you should use one of these six "list form" functions (note the "l" following the "2" in the name): csa2l; csa2ls; csa2ld; csa2lx; csa2lxs; csa2lxd.

## Examples

```begin

;
;  Create the input arrays.
;
xmin = -1.4
xmax =  1.4
ymin = -1.2
ymax =  1.2

ndata = 500
xi = new(ndata,float)
yi = new(ndata,float)
zi = new(ndata,float)

do i=0,ndata-1
xi(i) = xmin + (xmax-xmin)*rand()/32767.
yi(i) = ymin + (ymax-ymin)*rand()/32767.
zi(i) = xi(i) + yi(i)
t1 = 1.0/((fabs(xi(i)-0.1))^2.75 + fabs(yi(i))^2.75 + 0.09)
t2 = 1.0/((fabs(xi(i)+0.1))^2.75 + fabs(yi(i))^2.75 + 0.09)
zi(i) = 0.3*(zi(i)+t1-t2)
end do

;
;  Find an approximated value for the second order mixed partial at
;  a single point.
;
knots = (/10,10/)
xo = 0.
yo = 0.
knots = 4
wts = -1.
smth = 0.
nderiv = (/1,1/)
zo = csa2lx(xi,yi,zi,wts,knots,smth,nderiv,xo,yo)

end
```