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r.surf.idw(1grass) | GRASS GIS User's Manual | r.surf.idw(1grass) |

# NAME¶

**r.surf.idw** - Provides surface interpolation from
raster point data by Inverse Distance Squared Weighting.

# KEYWORDS¶

raster, surface, interpolation, IDW

# SYNOPSIS¶

**r.surf.idw**

**r.surf.idw --help**

**r.surf.idw** [-**e**] **input**=*name*
**output**=*name* [**npoints**=*integer*]
[--**overwrite**] [--**help**] [--**verbose**] [--**quiet**]
[--**ui**]

## Flags:¶

## Parameters:¶

**input**=*name***[required]**-

Name of input raster map **output**=*name***[required]**-

Name for output raster map **npoints**=*integer*-

Number of interpolation points

Default:*12*

# DESCRIPTION¶

*r.surf.idw* fills a grid cell (raster) matrix with
interpolated values generated from input raster data points. It uses a
numerical approximation technique based on distance squared weighting of the
values of nearest data points. The number of nearest data points used to
determined the interpolated value of a cell can be specified by the user
(default: 12 nearest data points).

If there is a current working mask, it applies to the output
raster map. Only those cells falling within the mask will be assigned
interpolated values. The search procedure for the selection of nearest
neighboring points will consider all input data, without regard to the mask.
The **-e** flag is the error analysis option that interpolates values
only for those cells of the input raster map which have non-zero values and
outputs the difference (see NOTES below).

The **npoints** parameter defines the number of nearest data
points used to determine the interpolated value of an output raster
cell.

# NOTES¶

*r.surf.idw* is a surface generation utility which uses
inverse distance squared weighting (as described in *Applied
Geostatistics* by E. H. Isaaks and R. M. Srivastava, Oxford University
Press, 1989) to assign interpolated values. The implementation includes a
customized data structure somewhat akin to a sparse matrix which enhances
the efficiency with which nearest data points are selected. For
latitude/longitude projections, distances are calculated from point to point
along a geodesic.

Unlike *r.surf.idw2* (addon), which processes all input data
points in each interpolation cycle, *r.surf.idw* attempts to minimize
the number of input data for which distances must be calculated. Execution
speed is therefore a function of the search effort, and does not increase
appreciably with the number of input data points.

*r.surf.idw* will generally outperform *r.surf.idw2*
except when the input data layer contains few non-zero data, i.e. when the
cost of the search exceeds the cost of the additional distance calculations
performed by *r.surf.idw2*. The relative performance of these utilities
will depend on the comparative speed of boolean, integer and floating point
operations on a particular platform.

Worst case search performance by *r.surf.idw* occurs when the
interpolated cell is located outside of the region in which input data are
distributed. It therefore behooves the user to employ a mask when geographic
region boundaries include large areas outside the general extent of the
input data.

The degree of smoothing produced by the interpolation will increase relative to the number of nearest data points considered. The utility may be used with regularly or irregularly spaced input data. However, the output result for the former may include unacceptable nonconformities in the surface pattern.

The **-e** flag option provides a standard surface-generation
error analysis facility. It produces an output raster map of the difference
of interpolated values minus input values for those cells whose input data
are non-zero. For each interpolation cycle, the known value of the cell
under consideration is ignored, and the remaining input values are used to
interpolate a result. The output raster map may be compared to the input
raster map to analyze the distribution of interpolation error. This
procedure may be helpful in choosing the number of nearest neighbors
considered for surface generation.

# KNOWN ISSUES¶

Module *r.surf.idw* works only for integer (CELL) raster
maps.

# SEE ALSO¶

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

Overview: Interpolation and Resampling in GRASS GIS

# AUTHOR¶

Greg Koerper

Global Climate Research Project

U.S. EPA Environmental Research Laboratory

200 S.W. 35th Street, JSB

Corvallis, OR 97333

# SOURCE CODE¶

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

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© 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual

GRASS 7.8.5 |