v.surf.idw(1grass) GRASS GIS User's Manual v.surf.idw(1grass)
NAME
v.surf.idw - Provides surface interpolation from vector point data by
Inverse Distance Squared Weighting.
KEYWORDS
vector, surface, interpolation, IDW
SYNOPSIS
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]
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
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
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 ac-
cess 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 z-coordinates 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
DESCRIPTION
v.surf.idw fills a raster matrix with interpolated values generated
from a set of irregularly spaced vector data points using numerical ap-
proximation (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 in-
fluence 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
z-coordinates if the input vector map is 3D.
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 exe-
cute is related to the resolution of the current region, after an ini-
tial 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 interpo-
lation:
Simple, non-indexed mode (activated by -n flag)
When the -n flag is specified, all vector points in the input vec-
tor 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 inter-
polation, 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 interpola-
tion to a smaller area.
• If more than npoints points fall within a given cell then,
rather than interpolating, these points are aggregated by tak-
ing 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 in-
terpolated. The interpolation function peaks sharply over the given
data points for 0 < p < 1 and more smoothly for larger values. The de-
fault 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
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
AUTHORS
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
SOURCE CODE
Available at: v.surf.idw source code (history)
Accessed: unknown
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GRASS 7.8.7 v.surf.idw(1grass)
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