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NAME

       v.surf.idw   -  Provides  surface interpolation from vector point data by Inverse Distance
       Squared Weighting.

KEYWORDS

       vector, surface, interpolation, IDW

SYNOPSIS

       v.surf.idw
       v.surf.idw --help
       v.surf.idw [-n] input=name  [layer=string]   [column=name]   output=name   [npoints=count]
       [power=float]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -n
           Don’t index points by raster cell
           Slower  but  uses  less  memory  and  includes  points  from  outside  region  in  the
           interpolation

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name [required]
           Name of input vector map
           Or data source for direct OGR access

       layer=string
           Layer number or name
           Vector features can have category values in different layers. This  number  determines
           which layer to use. When used with direct OGR access this is the layer name.
           Default: 1

       column=name
           Name of attribute column with values to interpolate
           If  not given and input is 2D vector map then category values are used. If input is 3D
           vector map then z-coordinates are used.

       output=name [required]
           Name for output raster map

       npoints=count
           Number of interpolation points
           Default: 12

       power=float
           Power parameter
           Greater values assign greater influence to closer points
           Default: 2.0

DESCRIPTION

       v.surf.idw fills a raster  matrix  with  interpolated  values  generated  from  a  set  of
       irregularly  spaced  vector data points using numerical approximation (weighted averaging)
       techniques. The interpolated value of a cell is determined by values of nearby data points
       and  the  distance of the cell from those input points.  In comparison with other methods,
       numerical approximation allows representation of more complex surfaces (particularly those
       with anomalous features), restricts the spatial influence of any errors, and generates the
       interpolated surface from the data points.

       Values to interpolate are read from column option. If this option is not  given  than  the
       program  uses categories as values to interpolate or z-coordinates if the input vector map
       is 3D.

NOTES

       The amount of memory used by this program is related to the number of vector points in the
       current  region.  If the vector point map is very dense (i.e., contains many data points),
       the program may not be able to get all the memory it needs  from  the  system.   The  time
       required  to  execute is related to the resolution of the current region, after an initial
       delay determined by the time taken to read the input vector points map.

       Note that vector features without category in given layer are skipped.

       If the user has a mask set, then interpolation is only done  for  those  cells  that  fall
       within  the  mask.  However, all vector points in the current region are used even if they
       fall outside the mask. Vector points outside the  current  region  are  not  used  in  the
       interpolation.  A  larger  region  may  be set and a mask used to limit interpolation to a
       smaller area if it is desired to  use  vector  points  from  outside  the  region  in  the
       interpolation. The -n flag may also be used to achieve a similar result.

       If  more  than  npoints  fall into one target raster cell, the mean of all the site values
       will determine the cell value (unless the -n flag is specified, in  which  case  only  the
       npoints closest to the centre of the cell will be interpolated).

       The power parameter defines an exponential distance weight.  Greater values assign greater
       influence to values closer to the point to be  interpolated.  The  interpolation  function
       peaks  sharply  over  the  given  data  points  for 0 < p < 1 and more smoothly for larger
       values. The default value for the power parameter is 2.

       By setting npoints=1, the  module  can  be  used  to  calculate  raster  Voronoi  diagrams
       (Thiessen polygons).

SEE ALSO

          g.region,  r.surf.contour,  r.surf.idw,  r.surf.gauss,  r.surf.fractal,  r.surf.random,
       v.surf.rst

AUTHORS

       Michael Shapiro, U.S. Army Construction Engineering Research Laboratory
       Improved algorithm (indexes points according to cell and ignores  points  outside  current
       region) by Paul Kelly

       Last changed: $Date: 2014-12-27 23:51:00 +0100 (Sat, 27 Dec 2014) $

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