# v.surf.idw.1grass man page

**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. 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 **npoints** 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 **npoints** 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).

## See Also

*g.region, r.surf.contour, r.surf.idw, r.surf.gauss, r.surf.fractal, r.surf.random, v.surf.rst*

## 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

*Last changed: $Date: 2014-12-27 23:51:00 +0100 (Sat, 27 Dec 2014) $*

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