Ubuntu Manpage: v.surf.idw - Surface interpolation from vector point data by Inverse Distance Squared Weighting.

NAME

       v.surf.idw  - Surface interpolation from vector point data by Inverse Distance Squared Weighting.

KEYWORDS

       vector, interpolation

SYNOPSIS

       v.surf.idw
       v.surf.idw help
       v.surf.idw    [-n]    input=name   output=name    [layer=integer]     [column=string]     [npoints=count]
       [power=float]   [--overwrite]  [--verbose]  [--quiet]

   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

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=name
           Name of input vector map

       output=name
           Name for output raster map

       layer=integer
           Layer number
           If set to 0, z coordinates are used (3D vector only)
           Default: 1

       column=string
           Attribute table column with values to interpolate
           Required if layer > 0

       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 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 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.

To read and interpolate from the elevation co-ordinates as 3rd dimension of the vector geometry, use
layer=0. In this case no column parameter has to be specified.

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 -n flag may also be used to achieve a similar result.

If more than count 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 count points closest to
the centre of the cell will be interpolated).

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).

AUTHOR