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 sites 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 sites file.
If the user has a mask set, then interpolation is only done
for those cells that fall within the mask. However, all
sites in the current region are used even
if they fall outside the mask. Sites 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 sites from outside the region in the
interpolation.
If more than count sites fall into one target raster cell,
the mean of all the site values will determine the cell value (unless
the -n flag is specifed, in which case only the count
sites closest to the centre of the cell will be interpolated).
The module can be used to calculate raster Voronoi diagrams (Thiessen polygons)
when setting npoints=1.
SEE ALSO
d.sites
g.region
r.mask
r.surf.contour
r.surf.idw
r.surf.idw2
r.surf.gauss
r.surf.fractal
r.surf.random
v.surf.rst
parser
AUTHOR
Michael Shapiro,
U.S. Army Construction Engineering
Research Laboratory
Improved algorithm (indexes sites according to cell and ignores
sites outside current region) by Paul Kelly [valid for 5.7?]
Last changed: $Date$