.TH v.surf.idw 1grass "" "GRASS 6.4.4" "Grass User's Manual"
.SH NAME
\fI\fBv.surf.idw\fR\fR  - Surface interpolation from vector point data by Inverse Distance Squared Weighting.
.SH KEYWORDS
vector, interpolation
.SH SYNOPSIS
\fBv.surf.idw\fR
.br
\fBv.surf.idw help\fR
.br
\fBv.surf.idw\fR [\-\fBn\fR] \fBinput\fR=\fIname\fR \fBoutput\fR=\fIname\fR  [\fBlayer\fR=\fIinteger\fR]   [\fBcolumn\fR=\fIstring\fR]   [\fBnpoints\fR=\fIcount\fR]   [\fBpower\fR=\fIfloat\fR]   [\-\-\fBoverwrite\fR]  [\-\-\fBverbose\fR]  [\-\-\fBquiet\fR] 
.SS Flags:
.IP "\fB\-n\fR" 4m
.br
Don't index points by raster cell
.br
Slower but uses less memory and includes points from outside region in the interpolation
.IP "\fB\-\-overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB\-\-verbose\fR" 4m
.br
Verbose module output
.IP "\fB\-\-quiet\fR" 4m
.br
Quiet module output
.PP
.SS Parameters:
.IP "\fBinput\fR=\fIname\fR" 4m
.br
Name of input vector map
.IP "\fBoutput\fR=\fIname\fR" 4m
.br
Name for output raster map
.IP "\fBlayer\fR=\fIinteger\fR" 4m
.br
Layer number
.br
If set to 0, z coordinates are used (3D vector only)
.br
Default: \fI1\fR
.IP "\fBcolumn\fR=\fIstring\fR" 4m
.br
Attribute table column with values to interpolate
.br
Required if layer > 0
.IP "\fBnpoints\fR=\fIcount\fR" 4m
.br
Number of interpolation points
.br
Default: \fI12\fR
.IP "\fBpower\fR=\fIfloat\fR" 4m
.br
Power parameter; greater values assign greater influence to closer points
.br
Default: \fI2.0\fR
.PP
.SH DESCRIPTION
.PP
\fIv.surf.idw\fR 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.
.PP
This program allows the user to use a GRASS vector point map file,
rather than a raster map layer, as input.
.SH NOTES
.PP
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.
.PP
To read and interpolate from the elevation co-ordinates as 3rd dimension
of the vector geometry, use \fIlayer=0\fR. In this case no \fIcolumn\fR
parameter has to be specified.
.PP
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 \fI-n\fR flag may also be used to
achieve a similar result.
.PP
If more than \fIcount\fR 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 \fIcount\fR 
points closest to the centre of the cell will be interpolated).
.PP
The \fIpower=\fR 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 < \fIp\fR < 1 and more smoothly for
larger values. The default value for the power parameter is 2.  
.PP
By setting \fInpoints=1\fR, the module can be used 
to calculate raster Voronoi diagrams (Thiessen polygons).
.SH SEE ALSO
\fId.vect\fR
.br
\fIg.region\fR
.br
\fIr.surf.contour\fR
.br
\fIr.surf.idw\fR
.br
\fIr.surf.idw2\fR
.br
\fIr.surf.gauss\fR
.br
\fIr.surf.fractal\fR
.br
\fIr.surf.random\fR
.br
\fIv.surf.rst\fR
.SH AUTHOR
Michael Shapiro,  
U.S. Army Construction Engineering 
Research Laboratory
.br
Improved algorithm (indexes points according to cell and ignores
points outside current region) by Paul Kelly
.PP
\fILast changed: $Date: 2011-11-08 12:29:50 +0100 (Tue, 08 Nov 2011) $\fR
.PP
Full index
.PP
© 2003-2014 GRASS Development Team