r.surf.idw(1grass) GRASS GIS User's Manual r.surf.idw(1grass)
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] [--over-
write] [--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 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
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 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 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 appre-
ciably 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 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.
KNOWN ISSUES
Module r.surf.idw works only for integer (CELL) raster maps.
SEE ALSO
r.surf.contour, r.surf.gauss, r.surf.fractal, r.surf.random,
v.surf.idw, v.surf.rst
Overview: Interpolation and Resampling in GRASS GIS
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: unknown
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GRASS 7.8.7 r.surf.idw(1grass)
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