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NAME   -  Surface  interpolation  from vector point data by Inverse Distance Squared


       vector, interpolation

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

           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

           Verbose module output

           Quiet module output

           Name of input vector map

           Name for output raster map

           Layer number
           If set to 0, z coordinates are used (3D vector only)
           Default: 1

           Attribute table column with values to interpolate
           Required if layer > 0

           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  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.

       This program allows the user to use a GRASS vector point map file, rather  than  a  raster
       map layer, as input.


       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.

       To  read  and  interpolate  from the elevation co-ordinates as 3rd dimension of the vector
       geometry, use layer=0. In this case no column parameter has to be specified.

       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  count  points  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 count points 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).




       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: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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       © 2003-2013 GRASS Development Team