1v.surf.idw(1)               GRASS GIS User's Manual              v.surf.idw(1)
2
3
4

NAME

6       v.surf.idw   - Provides surface interpolation from vector point data by
7       Inverse Distance Squared Weighting.
8

KEYWORDS

10       vector, surface, interpolation, IDW
11

SYNOPSIS

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
19   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
25       --overwrite
26           Allow output files to overwrite existing files
27
28       --help
29           Print usage summary
30
31       --verbose
32           Verbose module output
33
34       --quiet
35           Quiet module output
36
37       --ui
38           Force launching GUI dialog
39
40   Parameters:
41       input=name [required]
42           Name of input vector map
43           Or data source for direct OGR access
44
45       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
52       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
57       output=name [required]
58           Name for output raster map
59
60       npoints=count
61           Number of interpolation points
62           Default: 12
63
64       power=float
65           Power parameter
66           Greater values assign greater influence to closer points
67           Default: 2.0
68

DESCRIPTION

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
80       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

NOTES

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
93       Note that vector features without category in given layer are skipped.
94
95       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
100       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
107       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
115           ·   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
125           ·   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
133       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
139       By setting npoints=1, the  module  can  be  used  to  calculate  raster
140       Voronoi diagrams (Thiessen polygons).
141

SEE ALSO

143         g.region,  r.surf.contour,  r.surf.idw, r.surf.gauss, r.surf.fractal,
144       r.surf.random, v.surf.rst
145
146       Overview: Interpolation and Resampling in GRASS GIS
147

AUTHORS

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

SOURCE CODE

154       Available at: v.surf.idw source code (history)
155
156       Main  index  | Vector index | Topics index | Keywords index | Graphical
157       index | Full index
158
159       © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
160
161
162
163GRASS 7.8.5                                                      v.surf.idw(1)
Impressum