NAME¶
v.surf.idw - Surface interpolation from vector point data by
Inverse Distance Squared Weighting.
KEYWORDS¶
vector, interpolation
SYNOPSIS¶
v.surf.idw
v.surf.idw help
v.surf.idw [-
n]
input=
name output=
name
[
layer=
integer] [
column=
string]
[
npoints=
count] [
power=
float] [--
overwrite]
[--
verbose] [--
quiet]
Flags:¶
- -n
-
Don't index points by raster cell
Slower but uses less memory and includes points from outside region in the
interpolation
- --overwrite
-
Allow output files to overwrite existing files
- --verbose
-
Verbose module output
- --quiet
-
Quiet module output
Parameters:¶
- input=name
-
Name of input vector map
- output=name
-
Name for output raster map
- layer=integer
-
Layer number
If set to 0, z coordinates are used (3D vector only)
Default: 1
- column=string
-
Attribute table column with values to interpolate
Required if layer > 0
- npoints=count
-
Number of interpolation points
Default: 12
- power=float
-
Power parameter; greater values assign greater influence to closer points
Default: 2.0
DESCRIPTION¶
v.surf.idw fills a raster matrix with interpolated values generated from
a set of irregularly spaced data points using numerical approximation
(weighted averaging) techniques. The interpolated value of a cell is
determined by values of nearby data points and the distance of the cell from
those input points. In comparison with other methods, numerical approximation
allows representation of more complex surfaces (particularly those with
anomalous features), restricts the spatial influence of any errors, and
generates the interpolated surface from the data points.
This program allows the user to use a GRASS vector point map file, rather than a
raster map layer, as input.
NOTES¶
The amount of memory used by this program is related to the number of vector
points in the current region. If the vector point map is very dense (i.e.,
contains many data points), the program may not be able to get all the memory
it needs from the system. The time required to execute is related to the
resolution of the current region, after an initial delay determined by the
time taken to read the input vector points map.
To read and interpolate from the elevation co-ordinates as 3rd dimension of the
vector geometry, use
layer=0. In this case no
column parameter
has to be specified.
If the user has a mask set, then interpolation is only done for those cells that
fall within the mask. However, all vector points in the current region are
used even if they fall outside the mask. Vector points outside the current
region are not used in the interpolation. A larger region may be set and a
mask used to limit interpolation to a smaller area if it is desired to use
vector points from outside the region in the interpolation. The
-n flag
may also be used to achieve a similar result.
If more than
count points fall into one target raster cell, the mean of
all the site values will determine the cell value (unless the -n flag is
specified, in which case only the
count points closest to the centre of
the cell will be interpolated).
The
power= parameter defines an exponential distance weight. Greater
values assign greater influence to values closer to the point to be
interpolated. The interpolation function peaks sharply over the given data
points for 0 <
p < 1 and more smoothly for larger values. The
default value for the power parameter is 2.
By setting
npoints=1, the module can be used to calculate raster Voronoi
diagrams (Thiessen polygons).
SEE ALSO¶
d.vect
g.region
r.surf.contour
r.surf.idw
r.surf.idw2
r.surf.gauss
r.surf.fractal
r.surf.random
v.surf.rst
AUTHOR¶
Michael Shapiro, U.S. Army Construction Engineering Research Laboratory
Improved algorithm (indexes points according to cell and ignores points outside
current region) by Paul Kelly
Last changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $
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© 2003-2014 GRASS Development Team