Interpolates data from an unstructured (randomly-spaced) grid to a rectilinear grid using natural neighbor interpolation.
function natgrid ( x [*] : numeric, y [*] : numeric, z : numeric, xo [*] : numeric, yo [*] : numeric ) return_val : float or double
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.
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.
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 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.
As of NCL version 4.2.0.a033 Natgrid culls duplicate points. If any of the x/y/z triples are duplicates, they will automatically be culled. Informational messages are printed indicating which points have been culled. There is an upper limit on the number of such messages, the default being ten. This limit can be adjusted using the control parameter "mdm"; setting "mdm" to zero suppress all such messages. If two x/y coordinate pairs are duplicates, but the z values are distinct, then a fatal error is issued. The user needs to figure out how to handle such data.
natgrid 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.
natgrid can be used to interpolate at a list of specified points. To do this one must first enter "specified-point mode" by calling nnpntinit, then calling nnpnt to do the interpolation, and finally calling nnpntend to exit "specified-point mode."
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(x, y, z, xi, yi) end
If an error code is returned by natgrid, you can look up the meaning of the code in the Natgrid error table.