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

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