v.perturb - GRASS manual
Author: GRASS Development Team
Description: v.perturb: Random location perturbations of vector points.
Keywords: vector, geometry, statistics, random, point pattern, level1

## NAME

v.perturb - Random location perturbations of vector points.

## KEYWORDS

vector, geometry, statistics, random, point pattern, level1

## SYNOPSIS

v.perturb
v.perturb --help
v.perturb [-sb] input=name [layer=string] output=name [distribution=string] parameters=float[,float,...] [minimum=float] [seed=integer] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

### Flags:

-s
  Generate random seed (result is non-deterministic)
-b
  Do not build topology
  Advantageous when handling a large number of points
--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 ('-1' for all layers)
  A single vector map can be connected to multiple database tables. This number determines which table to use. When used with direct OGR access this is the layer name.
  Default: -1
output=name [required]
  Name for output vector map
distribution=string
  Distribution of perturbation
  Options: uniform, normal
  Default: uniform
parameters=float[,float,...] [required]
  Parameter(s) of distribution
  If the distribution is uniform, only one parameter, the maximum, is needed. For a normal distribution, two parameters, the mean and standard deviation, are required.
minimum=float
  Minimum deviation in map units
  Default: 0.0
seed=integer
  Seed value for the random number generator
  Using the same seed ensures identical results, while a randomly generated seed produces different outcomes in each run.

## DESCRIPTION

v.perturb reads a vector map of points and writes the same points but perturbs the eastings and northings by adding either a uniform or normal delta value. Perturbation means that a variating spatial deviation is added to the coordinates.

## NOTES

The uniform distribution is always centered about zero. The associated parameter is constrained to be positive and specifies the maximum of the distribution; the minimum is the negation of that parameter. Do perturb into a ring around the center, the minimum parameter can be used.

Usually, the mean (first parameter) of the normal distribution is zero (i.e., the distribution is centered at zero). The standard deviation (second parameter) is naturally constrained to be positive.

Output vector points are not guaranteed to be contained within the current geographic region.

## EXAMPLES

### Random, uniformly distributed selection

To create a random, uniformly distributed selection of possible new points with a radius of 100,000 map units, use the following command:

    v.perturb input=comm_colleges output=uniform_perturb parameters=100000

Your map should look similar to this figure:
Figure: Map showing the actual community college points and uniformly random chosen points.

### Normal distributed selection

For a normal distribution with a mean of 5000 and standard deviation of 2000, use the following command:

    v.perturb input=comm_colleges output=normal_perturb distribution=normal parameters=5000,2000

Figure: Map showing the actual community college points and normally random chosen and colored points. Notice that each point is closer to the original point.

### Normal distributed selection with a minimum value

In order to include a minimum value of 500, use the following command:

    v.perturb input=comm_colleges output=min_perturb distribution=normal parameters=100000,1000 minimum=500

## SEE ALSO

v.random, v.univar

## AUTHORS

James Darrell McCauley
when he was at: Agricultural Engineering Purdue University

Random number generators originally written in FORTRAN by Wes Peterson and translated to C using f2c.

## SOURCE CODE

Available at: v.perturb source code (history)

Latest change: Sunday Jul 27 05:53:30 2025 in commit: da409580ef15011f3bac379c76d41101c9e36347

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