Ubuntu Manpage: r.surf.idw - Provides surface interpolation from raster point data by Inverse Distance Squared

NAME

       r.surf.idw   -  Provides  surface  interpolation  from  raster  point  data  by  Inverse Distance Squared
       Weighting.

KEYWORDS

       raster, surface, interpolation, IDW

SYNOPSIS

       r.surf.idw
       r.surf.idw --help
       r.surf.idw  [-e]  input=name  output=name   [npoints=integer]    [--overwrite]    [--help]    [--verbose]
       [--quiet]  [--ui]

   Flags:
       -e
           Output is the interpolation error

       --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 raster map

       output=name [required]
           Name for output raster map

       npoints=integer
           Number of interpolation points
           Default: 12

DESCRIPTION

       r.surf.idw  fills  a  grid cell (raster) matrix with interpolated values generated from input raster data
       points. It uses a numerical approximation technique based on distance squared weighting of the values  of
       nearest  data  points.  The  number of nearest data points used to determined 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  interpolated  values.  The  search  procedure for the selection of nearest
       neighboring 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

r.surf.idw 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 efficiency with which nearest data points are selected. For latitude/longitude
projections, distances are calculated from point to point along a geodesic.

Unlike r.surf.idw2 (addon), which processes all input data points in each interpolation cycle, r.surf.idw
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 appreciably with the number of input
data points.

r.surf.idw will generally outperform r.surf.idw2 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 r.surf.idw2. 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 r.surf.idw occurs when the interpolated cell is located outside of the
region in which input data are distributed. It therefore behooves the user to employ a mask when
geographic region boundaries include large areas outside the general extent 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 nonconformities in the surface
pattern.

Surface-generation error analysis
The -e flag option provides a standard surface-generation error analysis facility. It produces an output
raster map of the difference of interpolated 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 neighbors considered for surface generation.

KNOWN ISSUES

       Module r.surf.idw works only for integer (CELL) raster maps.

AUTHOR

       Greg Koerper
       Global Climate Research Project
       U.S. EPA Environmental Research Laboratory
       200 S.W. 35th Street, JSB
       Corvallis, OR 97333

SOURCE CODE

       Available at: r.surf.idw source code (history)

       Accessed: Friday Apr 04 01:19:40 2025

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       © 2003-2025 GRASS Development Team, GRASS GIS 8.4.1 Reference Manual

GRASS 8.4.1                                                                                   r.surf.idw(1grass)