1v.surf.idw(1) GRASS GIS User's Manual v.surf.idw(1)
2
6 v.surf.idw - Provides surface interpolation from vector point data by
7 Inverse Distance Squared Weighting.
8
10 vector, surface, interpolation, IDW
11
13 v.surf.idw
14 v.surf.idw --help
15 v.surf.idw [-n] input=name [layer=string] [column=name] output=name
16 [npoints=count] [power=float] [--overwrite] [--help] [--verbose]
17 [--quiet] [--ui]
18 Flags:
20 -n
21 Don’t index points by raster cell
22 Slower but uses less memory and includes points from outside region
23 in the interpolation
24 --overwrite
26 Allow output files to overwrite existing files
27 --help
29 Print usage summary
30 --verbose
32 Verbose module output
33 --quiet
35 Quiet module output
36 --ui
38 Force launching GUI dialog
39 Parameters:
41 input=name [required]
42 Name of input vector map
43 Or data source for direct OGR access
44 layer=string
46 Layer number or name
47 Vector features can have category values in different layers. This
48 number determines which layer to use. When used with direct OGR
49 access this is the layer name.
50 Default: 1
51 column=name
53 Name of attribute column with values to interpolate
54 If not given and input is 2D vector map then category values are
55 used. If input is 3D vector map then z-coordinates are used.
56 output=name [required]
58 Name for output raster map
59 npoints=count
61 Number of interpolation points
62 Default: 12
63 power=float
65 Power parameter
66 Greater values assign greater influence to closer points
67 Default: 2.0
68
70 v.surf.idw fills a raster matrix with interpolated values generated
71 from a set of irregularly spaced vector data points using numerical
72 approximation (weighted averaging) techniques. The interpolated value
73 of a cell is determined by values of nearby data points and the dis‐
74 tance of the cell from those input points. In comparison with other
75 methods, numerical approximation allows representation of more complex
76 surfaces (particularly those with anomalous features), restricts the
77 spatial influence of any errors, and generates the interpolated surface
78 from the data points.
79 Values to interpolate are read from column option. If this option is
81 not given than the program uses categories as values to interpolate or
82 z-coordinates if the input vector map is 3D.
83
85 The amount of memory used by this program is related to the number of
86 vector points in the current region. If the vector point map is very
87 dense (i.e., contains many data points), the program may not be able to
88 get all the memory it needs from the system. The time required to exe‐
89 cute is related to the resolution of the current region, after an ini‐
90 tial delay determined by the time taken to read the input vector points
91 map.
92 Note that vector features without category in given layer are skipped.
94 If the user has a mask set, then interpolation is only done for those
96 cells that fall within the mask. The module has two separate modes of
97 operation for selecting the vector points that are used in the interpo‐
98 lation:
99 Simple, non-indexed mode (activated by -n flag)
101 When the -n flag is specified, all vector points in the input vec‐
102 tor map are searched through in order to find the npoints closest
103 points to the centre of each cell in the output raster map. This
104 mode of operation can be slow in the case of a very large number of
105 vector points.
106 Default, indexed mode
108 By default (i.e. if -n flag is not specified), prior to the inter‐
109 polation, input vector points are indexed according to which output
110 raster cell they fall into. This means that only cells nearby the
111 one being interpolated need to be searched to find the npoints
112 closest input points, and the module can run many times faster on
113 dense input maps. It should be noted that:
114 · Only vector points that lie within the current region are used
116 in the interpolation. If there are points outside the current
117 region, this may have an effect on the interpolated value of
118 cells near the edges of the region, and this effect will be
119 more pronounced the fewer points there are. If you wish to also
120 include points outside the region in the interpolation, then
121 either use the -n flag, or set the region to a larger extent
122 (covering all input points) and use a mask to limit interpola‐
123 tion to a smaller area.
124 · If more than npoints points fall within a given cell then,
126 rather than interpolating, these points are aggregated by tak‐
127 ing the mean. This avoids the situation where some vector
128 points can be discarded and not used in the interpolation, for
129 very dense input maps. Again, use the -n flag if you wish to
130 use only the npoints closest points to the cell centre under
131 all circumstances.
132 The power parameter defines an exponential distance weight. Greater
134 values assign greater influence to values closer to the point to be
135 interpolated. The interpolation function peaks sharply over the given
136 data points for 0 < p < 1 and more smoothly for larger values. The
137 default value for the power parameter is 2.
138 By setting npoints=1, the module can be used to calculate raster
140 Voronoi diagrams (Thiessen polygons).
141
143 g.region, r.surf.contour, r.surf.idw, r.surf.gauss, r.surf.fractal,
144 r.surf.random, v.surf.rst
145 Overview: Interpolation and Resampling in GRASS GIS
147
149 Michael Shapiro, U.S. Army Construction Engineering Research Laboratory
150 Improved algorithm (indexes points according to cell and ignores points
151 outside current region) by Paul Kelly
152
154 Available at: v.surf.idw source code (history)
155 Main index | Vector index | Topics index | Keywords index | Graphical
157 index | Full index
158 © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
160GRASS 7.8.5 v.surf.idw(1)