# ncl_c_csa2s man page

c_csa2s — cubic spline approximation, simple entry for two-dimensional input, gridded output

## Function Prototype

```
float *c_csa2s(int, float [], float [], float [], int [],
int, int, float [], float [], int *);
```

## Synopsis

```
int c_csa2s(int n, float xi[], float yi[], float zi[], int knots[2],
int no, int mo, float xo[], float yo[], int *ier);
```

## Description

- n
- (integer,input) The number of input data points. It must be that n is greater than 3 and, depending on the size of knots below, n may have to be larger.
- xi
- (real, input) An array dimensioned for n containing the X coordinate values for the input function.
- yi
- (real, input) An array dimensioned for n containing the Y coordinate values for the input function.
- zi
- (real, input) An array containing the functional values of the input function -- zi[k] is the functional value at (xi[k], yi[k]) for k=0,n-1.
- knots
- (integer, input) The number of knots to be used in each coordinate direction in constructing the approximation spline. knots[0] and knots[1] must be at least 4. The larger the value for knots, the closer the approximated curve will come to passing through the input function values.
- no
- (integer, input) The number of X coordinate values to be calculated for the output surface.
- mo
- (integer, input) The number of Y coordinate values to be calculated for the output surface.
- xo
- (real, input) An array dimensioned for no containing the X coordinates of the output grid.
- yo
- (real, output) An array dimensioned for mo containing the Y coordinates of the output grid.
- ier
- (pointer to integer, output) An error return value. If *ier is returned as 0, then no errors were detected. If *ier is non-zero, then refer to the error list in the error table for details.

## Usage

c_csa2s is called to find an approximating cubic spline surface 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 c_csa2xs.

c_csa2s returns a pointer to a linear array of data that is the approximated grid stored in row-major order. That is, if out is declared as

`float *out;`

and we set:

`out = c_csa2s(n, x, y, z, knots, no, mo, xo, yo, &ier);`

then out[i*mo+j] is the approximated function value at coordinate point (xo[i], yo[j]) for 0 <= i < no and 0 <= j < mo. The space for out is allocated internal to c_csa2s and is no * mo floats in size.

## Access

To use c_csa2s, load the NCAR Graphics library ngmath.

## See Also

csagrid, c_csa2xs, c_csa2ls, c_csa2lxs

Complete documentation for Csagrid is available at URL

http://ngwww.ucar.edu/ngdoc/ng/ngmath/c…

## Copyright

Copyright (C) 2000

University Corporation for Atmospheric Research

The use of this Software is governed by a License Agreement.