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.
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).
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