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


       vector, surface, interpolation, IDW

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

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

           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 vector map
           Or data source for direct OGR access

           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

           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

           Number of interpolation points
           Default: 12

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

DESCRIPTION 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.


       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. The module has two separate modes of operation for selecting the vector
       points that are used in the interpolation:

       Simple, non-indexed mode (activated by -n flag)
           When the -n flag is specified, all vector points in the input vector map are  searched
           through  in order to find the npoints closest points to the centre of each cell in the
           output raster map. This mode of operation can be slow in the  case  of  a  very  large
           number of vector points.

       Default, indexed mode
           By  default  (i.e.  if  -n  flag  is not specified), prior to the interpolation, input
           vector points are indexed according to which output raster cell they fall  into.  This
           means  that  only  cells nearby the one being interpolated need to be searched to find
           the npoints closest input points, and the module can run many times  faster  on  dense
           input maps. It should be noted that:

           ·   Only   vector  points  that  lie  within  the  current  region  are  used  in  the
               interpolation. If there are points outside the current region, this  may  have  an
               effect  on  the interpolated value of cells near the edges of the region, and this
               effect will be more pronounced the fewer points there are. If  you  wish  to  also
               include  points  outside  the  region in the interpolation, then either use the -n
               flag, or set the region to a larger extent (covering all input points) and  use  a
               mask to limit interpolation to a smaller area.

           ·   If  more  than  npoints  points  fall  within  a  given  cell  then,  rather  than
               interpolating, these points are aggregated by taking the  mean.  This  avoids  the
               situation  where  some  vector  points  can  be  discarded  and  not  used  in the
               interpolation, for very dense input maps. Again, use the -n flag if  you  wish  to
               use only the npoints closest points to the cell centre under all circumstances.

       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).



       Overview: Interpolation and Resampling in GRASS GIS


       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: 2016-08-03 13:50:26 +0200 (Wed, 03 Aug 2016) $


       Available at: source code (history)

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