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

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