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triple2grid_Wrap

Places unstructured (randomly-spaced) data onto the nearest locations of a rectilinear grid and retains metadata.

Prototype

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

	function triple2grid_Wrap (
		x      [*] : numeric,  
		y      [*] : numeric,  
		data       : numeric,  
		xgrid  [*] : numeric,  
		ygrid  [*] : numeric,  
		option [1] : logical   
	)

	return_val [dimsizes(ygrid)][dimsizes(xgrid)] :  float or double

Arguments

x
y

One-dimensional arrays of the same length containing the coordinates associated with the data values. For geophysical variables, x and y correspond to longitude and latitude respectively.

data

A multi-dimensional array, whose rightmost dimension is the same length as x and y, containing the values associated with the x and y coordinates. Missing values, indicated via data@_FillValue, may be present but will be ignored.

xgrid

A one-dimensional array of length M containing the x coordinates associated with the returned two-dimensional grid. For geophysical variables, these are longitudes. The coordinates' values must be monotonically increasing.

ygrid

A one-dimensional array of length N containing the y coordinates associated with the returned two-dimensional grid. For geophysical variables, these are latitudes. The coordinates' values must be monotonically increasing.

option

If option=False, this function will operate under default mode. That is, values located outside the grid domain specified by xgrid and ygrid will be ignored, and values within the grid domain must be "surrounded" by four grid points. The grid point is assigned to the nearest of the four grid points.

If option=True, then this variable may have associated with it the attributes domain, method and/or distmx.

Currently, only the default options are implemented for the domain and method attributes.

option@method

An integer value equal to 1 [default] or zero. option@method = 1 means to use the great circle distance formula for distance calculations. Currently, the default is the only option.

option@domain

A float value > 0 (default is 0.0). If present, the larger this factor the wider the spatial domain allowed to influence grid boundary points. Typically, option@domain is 1.0 or 2.0. If option@domain <= 0.0, then values located outside the grid domain specified by xgrid and ygrid will not be used (default behavior). Currently, the default is the only option.

option@distmx

Setting option@distmx allows the user to specify a search radius (km) beyond which observations are not considered for nearest neighbor. Only applicable when option@method = 1. The default distmx=1e20 (km) means that every grid point will have a nearest neighbor.

NOTE: prior to Version 4.3.1 there was an option@strict attribute. This is now ignored.

Return value

The return array will be K x N x M, where K represents the leftmost dimensions of data. It will be of type double if any of the input is double, and float otherwise.

Description

This function puts unstructured data (randomly-spaced) onto the nearest locations of a rectilinear grid.

This function does not perform interpolation; rather, each individual data point is assigned to the nearest grid point. It is possible that upon return, grid will contain grid points set to data@_FillValue if no x(n),y(n) are nearby.

Examples

Example 1

Assume xlon, ylat and zVal are one-dimensional (1D) arrays of length KPTS. Let lon and lat be 1D arrays specifying grid locations. Then:

   grid = triple2grid_Wrap(xlon,ylat,zVal, lon,lat, False)

will use default behavior and return a 2D array of size N x M.

Example 2

Same as Example 1 but observations within 275 km of a grid point will be considered as a "nearest neighbor". The great circle distance formula will be used to calculate distances.