v.surf.idw  Provides surface interpolation from vector point data
by Inverse Distance Squared Weighting.
vector, surface, interpolation, IDW
v.surf.idw
v.surf.idw help
v.surf.idw [
n]
input=
name
[
layer=
string] [
column=
name]
output=
name [
npoints=
count]
[
power=
float] [
overwrite] [
help]
[
verbose] [
quiet] [
ui]
 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
 help

Print usage summary
 verbose

Verbose module output
 quiet

Quiet module output
 ui

Force launching GUI dialog
 input=name [required]

Name of input vector map
Or data source for direct OGR access
 layer=string

Layer number or name
Vector features can have category values in different layers. This number
determines which layer to use. When used with direct OGR access this is
the layer name.
Default: 1
 column=name

Name of attribute column with values to interpolate
If not given and input is 2D vector map then category values are used. If
input is 3D vector map then zcoordinates are used.
 output=name [required]

Name for output raster map
 npoints=count

Number of interpolation points
Default: 12
 power=float

Power parameter
Greater values assign greater influence to closer points
Default: 2.0
v.surf.idw fills a raster matrix with interpolated values generated from
a set of irregularly spaced vector 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.
Values to interpolate are read from
column option. If this option is not
given than the program uses
categories as values to interpolate or
zcoordinates if the input vector map is 3D.
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.
Note that vector features without category in given
layer are
skipped.
If the user has a mask set, then interpolation is only done for those cells that
fall within the mask. The module has two separate modes of operation for
selecting the vector points that are used in the interpolation:
 Simple, nonindexed mode (activated by n flag)

When the n flag is specified, all vector points in the input vector
map are searched through in order to find the npoints closest
points to the centre of each cell in the output raster map. This mode of
operation can be slow in the case of a very large number of vector
points.
 Default, indexed mode

By default (i.e. if n flag is not specified), prior to the
interpolation, input vector points are indexed according to which output
raster cell they fall into. This means that only cells nearby the one
being interpolated need to be searched to find the npoints closest
input points, and the module can run many times faster on dense input
maps. It should be noted that:
 •
 Only vector points that lie within the current region are used in the
interpolation. If there are points outside the current region, this may
have an effect on the interpolated value of cells near the edges of the
region, and this effect will be more pronounced the fewer points there
are. If you wish to also include points outside the region in the
interpolation, then either use the n flag, or set the region to a
larger extent (covering all input points) and use a mask to limit
interpolation to a smaller area.
 •
 If more than npoints points fall within a given cell then, rather
than interpolating, these points are aggregated by taking the mean. This
avoids the situation where some vector points can be discarded and not
used in the interpolation, for very dense input maps. Again, use the
n flag if you wish to use only the npoints closest points
to the cell centre under all circumstances.
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).
g.region, r.surf.contour, r.surf.idw,
r.surf.gauss, r.surf.fractal, r.surf.random,
v.surf.rst
Overview: Interpolation and Resampling in GRASS GIS
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: 20160803 13:50:26 +0200 (Wed, 03 Aug 2016) $
Available at: v.surf.idw source code (history)
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