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v.surf.idw(1)		    GRASS GIS User's Manual		 v.surf.idw(1)

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=nameA [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 ac-
	   cess	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=nameA [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 ap-
       proximation (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 in-
       fluence	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 exe-
       cute is related to the resolution of the	current	region,	after an  ini-
       tial 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 interpo-
       lation:

       Simple, non-indexed mode	(activated by -n flag)
	   When	 the -n	flag is	specified, all vector points in	the input vec-
	   tor 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	inter-
	   polation, 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:

	   o   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 interpola-
	       tion to a smaller area.

	   o   If more than npoints points fall	 within	 a  given  cell	 then,
	       rather  than interpolating, these points	are aggregated by tak-
	       ing 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 in-
       terpolated. The interpolation function peaks  sharply  over  the	 given
       data  points for	0 < p <	1 and more smoothly for	larger values. The de-
       fault 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

       Overview: Interpolation and Resampling in GRASS GIS

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

SOURCE CODE
       Available at: v.surf.idw	source code (history)

       Main  index  | Vector index | Topics index | Keywords index | Graphical
       index | Full index

       A(C) 2003-2020 GRASS Development	Team, GRASS GIS	7.8.4 Reference	Manual

GRASS 7.8.4							 v.surf.idw(1)

NAME | KEYWORDS | SYNOPSIS | DESCRIPTION | NOTES | SEE ALSO | AUTHORS | SOURCE CODE

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