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

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

10       raster, surface, interpolation, IDW
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SYNOPSIS

13       r.surf.idw
14       r.surf.idw --help
15       r.surf.idw [-e] input=name  output=name   [npoints=integer]    [--over‐
16       write]  [--help]  [--verbose]  [--quiet]  [--ui]
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18   Flags:
19       -e
20           Output is the interpolation error
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22       --overwrite
23           Allow output files to overwrite existing files
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25       --help
26           Print usage summary
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28       --verbose
29           Verbose module output
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31       --quiet
32           Quiet module output
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34       --ui
35           Force launching GUI dialog
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37   Parameters:
38       input=name [required]
39           Name of input raster map
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41       output=name [required]
42           Name for output raster map
43
44       npoints=integer
45           Number of interpolation points
46           Default: 12
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DESCRIPTION

49       r.surf.idw  fills  a grid cell (raster) matrix with interpolated values
50       generated from input raster data points. It uses a numerical approxima‐
51       tion  technique  based  on  distance squared weighting of the values of
52       nearest data points. The number of nearest data points used  to  deter‐
53       mined  the  interpolated  value  of a cell can be specified by the user
54       (default: 12 nearest data points).
55
56       If there is a current working mask, it applies  to  the  output  raster
57       map. Only those cells falling within the mask will be assigned interpo‐
58       lated values. The search procedure for the selection of nearest  neigh‐
59       boring points will consider all input data, without regard to the mask.
60       The -e flag is the error analysis option that interpolates values  only
61       for  those cells of the input raster map which have non-zero values and
62       outputs the difference (see NOTES below).
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64       The npoints parameter defines the number of nearest data points used to
65       determine the interpolated value of an output raster cell.
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NOTES

68       r.surf.idw  is a surface generation utility which uses inverse distance
69       squared weighting (as described  in  Applied  Geostatistics  by  E.  H.
70       Isaaks  and R. M.  Srivastava, Oxford University Press, 1989) to assign
71       interpolated values. The  implementation  includes  a  customized  data
72       structure  somewhat  akin  to  a sparse matrix which enhances the effi‐
73       ciency with which nearest data points are selected.  For  latitude/lon‐
74       gitude  projections, distances are calculated from point to point along
75       a geodesic.
76
77       Unlike r.surf.idw2 (addon), which processes all input  data  points  in
78       each interpolation cycle, r.surf.idw attempts to minimize the number of
79       input data for which distances must be calculated. Execution  speed  is
80       therefore a function of the search effort, and does not increase appre‐
81       ciably with the number of input data points.
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83       r.surf.idw will generally outperform r.surf.idw2 except when the  input
84       data layer contains few non-zero data, i.e. when the cost of the search
85       exceeds the cost of the additional distance calculations  performed  by
86       r.surf.idw2. The relative performance of these utilities will depend on
87       the comparative speed of boolean, integer and floating point operations
88       on a particular platform.
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90       Worst  case  search  performance by r.surf.idw occurs when the interpo‐
91       lated cell is located outside of the region in  which  input  data  are
92       distributed.  It therefore behooves the user to employ a mask when geo‐
93       graphic region boundaries  include  large  areas  outside  the  general
94       extent of the input data.
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96       The  degree  of  smoothing  produced by the interpolation will increase
97       relative to the number of nearest data points considered.  The  utility
98       may  be used with regularly or irregularly spaced input data.  However,
99       the output result for the former may include unacceptable  nonconformi‐
100       ties in the surface pattern.
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102       The  -e flag option provides a standard surface-generation error analy‐
103       sis facility. It produces an output raster map  of  the  difference  of
104       interpolated values minus input values for those cells whose input data
105       are non-zero. For each interpolation cycle, the known value of the cell
106       under consideration is ignored, and the remaining input values are used
107       to interpolate a result. The output raster map may be compared  to  the
108       input  raster  map  to analyze the distribution of interpolation error.
109       This procedure may be helpful in choosing the number of nearest  neigh‐
110       bors considered for surface generation.
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KNOWN ISSUES

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

116           r.surf.contour,    r.surf.gauss,   r.surf.fractal,   r.surf.random,
117       v.surf.idw, v.surf.rst
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119       Overview: Interpolation and Resampling in GRASS GIS
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AUTHOR

122       Greg Koerper
123       Global Climate Research Project
124       U.S. EPA Environmental Research Laboratory
125       200 S.W. 35th Street, JSB
126       Corvallis, OR 97333
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SOURCE CODE

129       Available at: r.surf.idw source code (history)
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131       Main index | Raster index | Topics index | Keywords index  |  Graphical
132       index | Full index
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134       © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
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138GRASS 7.8.5                                                      r.surf.idw(1)
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