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

Provided by: grass-doc_7.8.2-1build3_all

NAME

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

KEYWORDS

       vector, surface, interpolation, IDW

SYNOPSIS

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

   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

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name [required]
           Name of input vector map
           Or data source for direct OGR access

       layer=string
           Layer number or name
           Vector features can have category values in different layers. This  number  determines
           which layer to use. When used with direct OGR access this is the layer name.
           Default: 1

       column=name
           Name of attribute column with values to interpolate
           If  not given and input is 2D vector map then category values are used. If input is 3D
           vector map then z-coordinates are used.

       output=name [required]
           Name for output raster map

       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  vector 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.

       Values to interpolate are read from column option. If this option is not  given  than  the
       program  uses categories as values to interpolate or z-coordinates if the input vector map
       is 3D.

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.

Note that vector features without category in given layer are skipped.

If the user has a mask set, then interpolation is only done for those cells that fall
within the mask. The module has two separate modes of operation for selecting the vector
points that are used in the interpolation:

Simple, non-indexed mode (activated by -n flag)
When the -n flag is specified, all vector points in the input vector map are searched
through in order to find the npoints closest points to the centre of each cell in the
output raster map. This mode of operation can be slow in the case of a very large
number of vector points.

Default, indexed mode
By default (i.e. if -n flag is not specified), prior to the interpolation, input
vector points are indexed according to which output raster cell they fall into. This
means that only cells nearby the one being interpolated need to be searched to find
the npoints closest input points, and the module can run many times faster on dense
input maps. It should be noted that:

• Only vector points that lie within the current region are used in the
interpolation. If there are points outside the current region, this may have an
effect on the interpolated value of cells near the edges of the region, and this
effect will be more pronounced the fewer points there are. If you wish to also
include points outside the region in the interpolation, then either use the -n
flag, or set the region to a larger extent (covering all input points) and use a
mask to limit interpolation to a smaller area.

• If more than npoints points fall within a given cell then, rather than
interpolating, these points are aggregated by taking the mean. This avoids the
situation where some vector points can be discarded and not used in the
interpolation, for very dense input maps. Again, use the -n flag if you wish to
use only the npoints closest points to the cell centre under all circumstances.

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).

AUTHORS

       Michael Shapiro, U.S. Army Construction Engineering Research Laboratory
       Improved algorithm (indexes points according to cell and ignores  points  outside  current
       region) by Paul Kelly

SOURCE CODE

       Available at: v.surf.idw source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual