## DESCRIPTION

*r.surf.idw2* 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. It is the most appropriate method to
apply to most spatial data.
The **npoints** parameter defines the number of points to use for
interpolation. The default is to use the 12 nearest points when
interpolating the value for a particular cell.
## NOTES

The amount of memory used by this program is related to the
number of non-zero data values in the input map layer. If
the input raster map layer is very dense (i.e., contains
many non-zero data points), the program may not be able to
get all the memory it needs from the system. The time
required to execute increases with the number of input data
points.
If the user has a mask set, then interpolation is only done
for those cells that fall within the mask. However, all
non-zero data points in the input layer are used even if
they fall outside the mask.

This program does not work with latitude/longitude data
bases. Another surface generation program, named
*r.surf.idw*,
should be used with latitude/longitude data bases.

The user should refer to the manual entries for

*r.surf.idw*

*r.surf.contour*

*v.surf.rst*

to
compare this surface generation program with others available in GRASS.

## SEE ALSO

*r.surf.contour*,
*r.surf.idw*,
*r.surf.gauss*,
*r.surf.fractal*,
*r.surf.random*,
*r.surf.idw2*,
*v.surf.rst*
## AUTHOR

Michael Shapiro, U.S.Army Construction Engineering Research Laboratory
*Last changed: $Date$*