1r.surf.idw(1) Grass User's Manual r.surf.idw(1)
2
6 r.surf.idw - Surface interpolation utility for raster map layers.
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9 raster
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12 r.surf.idw
13 r.surf.idw help
14 r.surf.idw [-e] input=name output=name [npoints=integer] [--over‐
15 write]
16 Flags:
18 -e Output is the interpolation error
19 --overwrite
21 Parameters:
23 input=name
24 Name of input raster map
25 output=name
27 Name for output raster map
28 npoints=integer
30 Number of interpolation points Default: 12
31
33 r.surf.idw fills a grid cell (raster) matrix with interpolated values
34 generated from a set of input layer data points. It uses a numerical
35 approximation technique based on distance squared weighting of the val‐
36 ues of nearest data points. The number of nearest data points used to
37 determined the interpolated value of a cell can be specified by the
38 user (default: 12 nearest data points).
39 If there is a current working mask, it applies to the output raster
41 file. Only those cells falling within the mask will be assigned inter‐
42 polated values. The search procedure for the selection of nearest
43 neighboring points will consider all input data, without regard to the
44 mask. The -e flag is the error analysis option that interpolates val‐
45 ues only for those cells of the input raster map which have non-zero
46 values and outputs the difference (see NOTES below).
47 The npoints parameter defines the number of nearest data points used to
49 determine the interpolated value of an output raster cell.
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52 r.surf.idw is a surface generation utility which uses inverse distance
53 squared weighting (as described in Applied Geostatistics by E. H.
54 Isaaks and R. M. Srivastava, Oxford University Press, 1989) to assign
55 interpolated values. The implementation includes a customized data
56 structure somewhat akin to a sparse matrix which enhances the effi‐
57 ciency with which nearest data points are selected. For latitude/lon‐
58 gitude projections, distances are calculated from point to point along
59 a geodesic.
60 Unlike r.surf.idw2, which processes all input data points in each
62 interpolation cycle, r.surf.idw attempts to minimize the number of
63 input data for which distances must be calculated. Execution speed is
64 therefore a function of the search effort, and does not increase appre‐
65 ciably with the number of input data points.
66 r.surf.idw will generally outperform <A
68 HREF="r.surf.idw2.html">r.surf.idw2 except when the input data layer
69 contains few non-zero data, i.e. when the cost of the search exceeds
70 the cost of the additional distance calculations performed by <A
71 HREF="r.surf.idw2.html">r.surf.idw2. The relative performance of these
72 utilities will depend on the comparative speed of boolean, integer and
73 floating point operations on a particular platform.
74 Worst case search performance by r.surf.idw occurs when the interpo‐
76 lated cell is located outside of the region in which input data are
77 distributed. It therefore behooves the user to employ a mask when geo‐
78 graphic region boundaries include large areas outside the general
79 extent of the input data.
80 The degree of smoothing produced by the interpolation will increase
82 relative to the number of nearest data points considered. The utility
83 may be used with regularly or irregularly spaced input data. However,
84 the output result for the former may include unacceptable nonconformi‐
85 ties in the surface pattern.
86 The -e flag option provides a standard surface-generation error analy‐
88 sis facility. It produces an output raster map of the difference of
89 interpolated values minus input values for those cells whose input data
90 are non-zero. For each interpolation cycle, the known value of the cell
91 under consideration is ignored, and the remaining input values are used
92 to interpolate a result. The output raster map may be compared to the
93 input raster map to analyze the distribution of interpolation error.
94 This procedure may be helpful in choosing the number of nearest neigh‐
95 bors considered for surface generation.
96
98 r.surf.contour, r.surf.idw2, r.surf.gauss, r.surf.fractal, r.surf.ran‐
99 dom, v.surf.idw, v.surf.rst
100
102 Greg Koerper
103 Global Climate Research Project
104 U.S. EPA Environmental Research Laboratory
105 200 S.W. 35th Street, JSB
106 Corvallis, OR 97333
107 Last changed: $Date: 2006/04/13 19:01:38 $
109 Full index
111GRASS 6.2.2 r.surf.idw(1)