NCL Website header
NCL Home > Documentation > Functions > Interpolation

natgrid_Wrap

Interpolates data from an unstructured (randomly-spaced) grid to a rectilinear grid using natural neighbor interpolation and retains metadata.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"

	function natgrid_Wrap (
		x  [*] : numeric,  
		y  [*] : numeric,  
		z      : numeric,  
		xo [*] : numeric,  
		yo [*] : numeric   
	)

	return_val  :  float or double

Arguments

x

An array of length npts containing the X coordinates of the input data points.

y

An array of length npts containing the Y coordinates of the input data points.

z

An array of any dimensionality (with the last dimension being npts) containing the functional values of the input data points. That is, z(...,i) is the value of the input function at coordinate (x(i),y(i)) for i=0,npts-1.

xo

A 1D array of any length (NumXOut) containing the X coordinates of the output data grid. The values in xo must be increasing, but need not be equally spaced.

yo

A 1D array of any length (NumYOut) containing the Y coordinates of the output data grid. The values in yo must be increasing, but need not be equally spaced.

Return value

An array of dimension N x NumXOut x NumYOut, where N represents all but the last (rightmost) dimension of z, containing the interpolated functional values at the points specified by xo and yo.

The output array will be of type double if any of the input is double, and float otherwise.

Description

This function performs interpolation from data on an unstructured grid to data on a rectilinear grid.

It is part of the Natgrid package, which implements a natural neighbor interpolation method. Much useful information is available at the above link, including the descriptions of many control parameters that can be modified to materially change the behavior of the package. (The functions nngetp and nnsetp are used to access these parameters.)

natgrid_Wrap accepts a set of randomly-positioned 2D point coordinates, together with function values at those coordinates, and returns a set of interpolated function values at coordinates on a user-specified rectangular grid. No missing values are allowed in the input data arrays.

natgrid_Wrap allows the input arrays to be of any numeric type, but it coerces them to type double and calls the same double-precision code as natgridd. It is important to know this when deciding which of the functions that retrieve aspects and slopes (nngetaspectd, nngetaspects, nngetsloped, and nngetslopes) to call.

Note: natgrid_Wrap is meant to be used by itself. It will not work in conjunction with the Natgrid routines named nnpnt[init|end][s|d] and nngetwts[d], which are used cooperatively in "specified-point" mode.

This function is identical to the built-in function natgrid except it retains metadata.

Examples

begin
  NumXOut = 21
  NumYOut = 21
  ISLIM = 6
  x = new((/ISLIM/),double)
  y = new((/ISLIM/),double)
  z = new((/ISLIM/),double)
  xi = new((/NumXOut/),double)
  yi = new((/NumYOut/),double)

  x = (/0.00, 1.00, 0.00, 1.00, 0.40, 0.75/)
  y = (/0.00, 0.00, 1.00, 1.00, 0.20, 0.65/)
  z = (/0.00, 0.00, 0.00, 0.00, 1.25, 0.80/)

  xc = 1./(NumXOut-1.) 
  xi = ispan(0,NumXOut-1,1) * xc

  yc = 1./(NumYOut-1.) 
  yi = ispan(0,NumYOut-1,1) * yc

  zi = natgrid_Wrap(x, y, z, xi, yi)
end

Errors

If an error code is returned by natgrid_Wrap, you can look up the meaning of the code in the Natgrid error table.