# ncl_c_csvoro man page

c_csvoro — calculate Voronoi polygons for points on a sphere.

## Function Prototype

```
void c_csvoro(int, float [], float [],
int, int, float [], float [], float [],
int *, int *, int [], int *);
```

## Synopsis

```
void *c_csvoro (n, rlat, rlon, ni, nf, plat, plon, rc,
nca, numv, nv, ier);
```

## Description

- n
- The number of input data points, n > 2.
- rlat
- An array containing the latitudes of the input data, expressed in degrees. The first three points must not be collinear (lie on a common great circle).
- rlon
- An array containing the longitudes of the input data, expressed in degrees.
- ni
- The index of the input coordinate for which you want to determine the Voronoi polygon (0 <= ni < n).
- nf
- Flag indicating if this is the first call to c_csvoro to retrieve Voronoi polygons for this dataset (1=yes, 0=no). Calls subsequent to the first call for a given dataset are much faster than the first call.
- plat
- The latitudes for the vertices of the Voronoi polygons. These are circumcenters of circles passing through the Delaunay triangles. If a coordinate is a boundary point, then the circle may pass through certain "pseudo points" that have been added to the original dataset in order to complete the Voronoi polygon.
- plon
- The longitudes for the vertices of the Voronoi polygons.
- rc
- Array containing circumradii (arc lengths in degrees of the angle between a circumcenter and its associated triangle vertices).
- nca
- *nca is the actual number of circumcenters returned in plat and plon. This number may be larger than n if the input dataset has boundary points, since certain "pseudo points" may have been added to the original dataset in order to complete the Voronoi polygon set.
- numv
- *numv is the number of vertices in the Voronoi polygon enclosing the coordinate (rlat[ni],rlon[ni]).
- nv
- An array containing numv indices for the Voronoi polygon enclosing the coordinate (rlat[ni],rlon[ni]). The indices returned in this array refer to the coordinates returned in plat and plon. For example, if the integer "j" is an element of the nv array, then (plat[j],plon[j]) is a vertex of the Voronoi polygon enclosing (rlat[ni],rlon[ni]). The indices in nv list out the vertices of the Voronoi polygon in counter-clockwise order.
- ier
- 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_csvoro is called if you want to determine the Voronoi polygons for data randomly positioned on a sphere. Each call to c_csvoro calculates the vertices for the Voronoi polygon surrounding a specified input point.

## Access

To use c_csvoro, load the NCAR Graphics library ngmath.

## See Also

css_overview, c_cssgrid, cssgrid_errors

Complete documentation for Cssgrid 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.

## Info

NCAR GRAPHICS May 2000