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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]    [--over-
       write]  [--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=nameA [required]
	   Name	of input raster	map

       output=nameA [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 approxima-
       tion  technique	based  on  distance squared weighting of the values of
       nearest data points. The	number of nearest data points used  to	deter-
       mined  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 interpo-
       lated values. The search	procedure for the selection of nearest	neigh-
       boring 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 effi-
       ciency with which nearest data points are selected.  For	 latitude/lon-
       gitude  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	appre-
       ciably 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 interpo-
       lated cell is located outside of	the region in  which  input  data  are
       distributed.  It	therefore behooves the user to employ a	mask when geo-
       graphic region boundaries include large areas outside the  general  ex-
       tent 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  nonconformi-
       ties in the surface pattern.

       The  -e flag option provides a standard surface-generation error	analy-
       sis facility. It	produces an output raster map of the difference	of in-
       terpolated  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	neigh-
       bors 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

       Available at:	source code (history)

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       A(C) 2003-2020 GRASS Development	Team, GRASS GIS	7.8.5 Reference	Manual

GRASS 7.8.5


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