**-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

**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*

*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.

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.

- 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

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).

Overview: Interpolation and Resampling in GRASS GIS

Improved algorithm (indexes points according to cell and ignores points outside current region) by Paul Kelly

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

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