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NGP

NGP

Name


      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.
      Position 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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