NCL Website header
NCL Home > Documentation > Functions > Interpolation, Ngmath routines

csvoro

Determines Voronoi polygons for data randomly positioned on a sphere and returns vertices for the one surrounding a specified input point.

Prototype

	procedure csvoro (
		rlati [*] : numeric,  
		rloni [*] : numeric,  
		index     : integer,  
		cflag     : integer,  
		rlato [*] : numeric,  
		rlono [*] : numeric,  
		alen  [*] : numeric,  
		nca       : integer,  
		numv      : integer,  
		nv    [*] : integer   
	)

Arguments

rlati
rloni

Latitudes and longitudes of the input points, in degrees. The two arrays must be of the same size.

index

Index of the input coordinate for which you want to determine the Voronoi polygon (0 <= index < dimsizes(rlati)).

cflag

Flag indicating if this is the first call to csvoro to retrieve Voronoi polygons for this dataset (1=yes, 0=no). For a given dataset, calls after the first call are much faster than the first one.

rlato
rlono

(output)
Latitudes and longitudes, in degrees, of points to be used in defining the Voronoi polygons. The two arrays should be of the same size. All the returned points are the centers of circles circumscribed about Delaunay triangles formed from the input dataset. (Some of the Delaunay triangles may contain vertices which are "pseudo points" added to the original input dataset. This happens only when all of the original points fit in the same hemisphere, in which case three "pseudo points" are added in the opposite hemisphere.) The size of rlato and rlono should be two times the size of rlati and rloni.

alen

(output)
Array of circumradii, in degrees - the angles between circumcenters and their associated triangle vertices. The size of alen should be two times the size of rlati.

nca

(output)
The actual number of point coordinates returned in rlato and rlono. This number may be larger than the size of rlati if "pseudo points" were added to the original dataset.

numv

(output)
The number of vertices in the Voronoi polygon enclosing the coordinate (rlati(index),rloni(index)).

nv

(output)
An array containing numv indices for the Voronoi polygon enclosing the coordinate (rlati(index),rloni(index)). Indices returned in this array refer to the coordinates returned in rlato and rlono. For example, if the integer j is an element of the nv array, then (rlato(j),rlono(j)) is a vertex of the Voronoi polygon enclosing (rlati(index),rloni(index)). The indices in nv give the vertices of the Voronoi polygon in counter-clockwise order. nv should be the same size as rlati.

Description

csvoro is in the Cssgrid package - a software package that implements a tension spline interpolation algorithm to fit a function to input data randomly spaced on a unit sphere.

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

If missing values are detected in the input data, then those values are ignored when calculating the interpolating function.

There are various control parameters that you can set for this routine via the cssetp procedure. See this table for a description of the parameters.

See Also

cssgrid, cssgrid_Wrap, csstri, csvoro, css2c, csc2s, cssetp, csgetp

Examples

begin

;
;  Create input arrays.
;
  ndata = 100
  rlati = new(ndata,float)
  rloni = new(ndata,float)
;
  do i=0,ndata-1
    rlati(i) =  -90. + 180.*rand()/32767.
    rloni(i) = -180. + 360.*rand()/32767.
  end do

;
;  Obtain the Voronoi polygon for the first point in the input
;  dataset.  nv(0:numv-1) will contain integers to be used as
;  indices for rlato and rlono to determine the polygonal vertices.
;
  rlato = new(2*ndata,float)
  rlono = new(2*ndata,float)
  alen  = new(2*ndata,float)
  nca   = new(1,integer)
  numv  = new(1,integer)
  nv    = new(ndata,integer)
  csvoro(rlati,rloni,0,1,rlato,rlono,alen,nca,numv,nv)

end