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NAME  - Surface interpolation utility for raster map.


       raster, interpolation

SYNOPSIS help  [-e]  input=name  output=name   [npoints=integer]   [--overwrite]  [--verbose]

           Output is the interpolation error

           Allow output files to overwrite existing files

           Verbose module output

           Quiet module output

           Name of input raster map

           Name for output raster map

           Number of interpolation points
           Default: 12

DESCRIPTION fills a grid cell (raster) matrix with interpolated values generated from a set
       of  input layer data points. It uses a numerical approximation technique based on distance
       squared weighting of the values of nearest data points. The number of nearest data  points
       used to determined the interpolated value of a cell can be specified by the user (default:
       12 nearest data points).

       If there is a current working mask, it applies to the output raster map. Only those  cells
       falling within the mask will be assigned interpolated values. The search procedure for the
       selection of nearest neighboring points will consider all input data,  without  regard  to
       the  mask.   The  -e  flag  is the error analysis option that interpolates values only for
       those cells of the input raster map which have non-zero values and outputs the  difference
       (see NOTES below).

       The  npoints  parameter  defines  the  number of nearest data points used to determine the
       interpolated value of an output raster cell.

NOTES is a surface generation utility which uses inverse distance  squared  weighting
       (as  described  in  Applied  Geostatistics  by  E. H. Isaaks and R. M.  Srivastava, Oxford
       University Press, 1989) to assign  interpolated  values.  The  implementation  includes  a
       customized  data  structure somewhat akin to a sparse matrix which enhances the efficiency
       with  which  nearest  data  points  are  selected.   For  latitude/longitude  projections,
       distances are calculated from point to point along a geodesic.

       Unlike,  which  processes  all input data points in each interpolation cycle, attempts to minimize the number of input  data  for  which  distances  must  be
       calculated.  Execution  speed  is  therefore a function of the search effort, and does not
       increase appreciably with the number of input data points. will generally outperform except when the input data layer contains
       few  non-zero  data,  i.e.  when the cost of the search exceeds the cost of the additional
       distance  calculations  performed  by  The  relative  performance  of  these
       utilities  will  depend  on  the  comparative speed of boolean, integer and floating point
       operations on a particular platform.

       Worst case search performance by occurs when the interpolated cell  is  located
       outside  of the region in which input data are distributed. It therefore behooves the user
       to employ a mask when geographic region boundaries include large areas outside the general
       extent of the input data.

       The degree of smoothing produced by the interpolation will increase relative to the number
       of nearest data points considered.  The utility may be used with regularly or  irregularly
       spaced  input  data.   However,  the output result for the former may include unacceptable
       nonconformities in the surface pattern.

       The -e flag option provides a standard  surface-generation  error  analysis  facility.  It
       produces  an output raster map of the difference of interpolated values minus input values
       for those cells whose input data are non-zero. For each  interpolation  cycle,  the  known
       value  of the cell under consideration is ignored, and the remaining input values are used
       to interpolate a result. The output raster map may be compared to the input raster map  to
       analyze  the  distribution  of  interpolation  error.   This  procedure  may be helpful in
       choosing the number of nearest neighbors considered for surface generation.

SEE ALSO,,,,,,


       Greg Koerper
       Global Climate Research Project
       U.S. EPA Environmental Research Laboratory
       200 S.W. 35th Street, JSB
       Corvallis, OR 97333

       Last changed: $Date: 2011-11-08 01:42:51 -0800 (Tue, 08 Nov 2011) $

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