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NAME   -  Provides  surface interpolation from raster point data by Inverse Distance
       Squared Weighting.


       raster, surface, interpolation, IDW

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

           Output is the interpolation error

           Allow output files to overwrite existing files

           Print usage summary

           Verbose module output

           Quiet module output

           Force launching GUI dialog

       input=name [required]
           Name of input raster map

       output=name [required]
           Name for output raster map

           Number of interpolation points
           Default: 12

DESCRIPTION fills a grid cell (raster) matrix with interpolated values generated from input
       raster 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 (addon), 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.


       Module works only for integer (CELL) raster maps.


       Overview: Interpolation and Resampling in GRASS GIS


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

       Last changed: $Date: 2017-01-05 08:56:15 +0100 (Thu, 05 Jan 2017) $


       Available at: source code (history)

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