# ncl_c_csvoro - Man Page

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/cssgrid/csshome.html

## Copyright

Copyright (C) 2000

University Corporation for Atmospheric Research

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