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NGP

NGP

## Purpose

Interpolate an irregularly sampled field using Nearest Grid Point

## Explanation

This function interpolates irregularly gridded points to a
regular grid using Nearest Grid Point.

## Category

Mathematical functions, Interpolation

## Calling Sequence

Result = NGP, VALUE, POSX, NX[, POSY, NY, POSZ, NZ,
/AVERAGE, /WRAPAROUND, /NO_MESSAGE]

## Inputs

VALUE: Array of sample weights (field values). For e.g. a
temperature field this would be the temperature and the
keyword AVERAGE should be set. For e.g. a density field
this could be either the particle mass (AVERAGE should
not be set) or the density (AVERAGE should be set).
POSX: Array of X coordinates of field samples, unit indices: [0,NX>.
NX: Desired number of grid points in X-direction.

## Optional Inputs

POSY: Array of Y coordinates of field samples, unit indices: [0,NY>.
NY: Desired number of grid points in Y-direction.
POSZ: Array of Z coordinates of field samples, unit indices: [0,NZ>.
NZ: Desired number of grid points in Z-direction.

## Keyword Parameters

AVERAGE: Set this keyword if the nodes contain field samples
(e.g. a temperature field). The value at each grid
point will then be the average of all the samples
allocated to it. If this keyword is not set, the
value at each grid point will be the sum of all the
nodes allocated to it (e.g. for a density field from
a distribution of particles). (D=0).
WRAPAROUND: Set this keyword if the data is periodic and if you
want the first grid point to contain samples of both
sides of the volume (see below). (D=0).
NO_MESSAGE: Suppress informational messages.
Example of default NGP allocation: n0=4, *=gridpoint.
0 1 2 3 Index of gridpoints
* * * * Grid points
|---|---|---|---| Range allocated to gridpoints ([0.0,1.0> --> 0, etc.)
0 1 2 3 4 posx
Example of NGP allocation for WRAPAROUND: n0=4, *=gridpoint.
0 1 2 3 Index of gridpoints
* * * * Grid points
|---|---|---|---|-- Range allocated to gridpoints ([0.5,1.5> --> 1, etc.)
0 1 2 3 4=0 posx

## Outputs

Prints that a NGP interpolation is being performed of x
samples to y grid points, unless NO_MESSAGE is set.

## Restrictions

All input arrays must have the same dimensions.
Postition coordinates should be in `index units' of the
desired grid: POSX=[0,NX>, etc.

## Procedure

Nearest grid point is determined for each sample.
Samples are allocated to nearest grid points.
Grid point values are computed (sum or average of samples).

## Example

nx = 20
ny = 10
posx = randomu(s,1000)
posy = randomu(s,1000)
value = posx^2+posy^2
field = ngp(value,posx*nx,nx,posy*ny,ny,/average)
surface,field,/lego

## Notes

Use tsc.pro or cic.pro for a higher order interpolation schemes. A
standard reference for these interpolation methods is: R.W. Hockney
and J.W. Eastwood, Computer Simulations Using Particles (New York:
McGraw-Hill, 1981).

## Modification History

Written by Joop Schaye, Feb 1999.
Check for LONG overflow P. Riley/W. Landsman December 1999

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