DESCRIPTION

v.random randomly generates vector points within the current region using the selected random number generator.

v.random can generate also 3D vector points or write random value to attribute table. Point height range or attribute value range is controlled by specifying zmin and zmax values. Both z values are included in range (zmin <= z <= zmax). Generated random attribute value type can be controlled by column data type. Use INTEGER column type for integers and DOUBLE PRECISION for floating point numbers. Integer values are calculated by rounding random floating point number.

To produce repeatable results a random seed can be set using the option seed.

Restriction to vector areas

If an input vector map with areas is specified, the location of random points is restricted to the selected areas. By default, the requested number of points are distributed across all areas.

If the -a flag is given, the requested number of points is generated for each individual area. For example, if 20 points should be generated and the input map has 100 individual areas, 2000 points will be generated in total.

EXAMPLES

All examples use the North Carolina sample dataset.

Generate 20 random points with binary attribute (only 0 or 1): 

v.random output=binary_random n=20 zmin=0 zmax=1 column='binary INTEGER'
Generate 20 random 3d points with height attribute [0:20] and specific random seed: 
v.random seed=501 output=height_random n=20 zmin=0 zmax=1 column=height
Get 20 random samples from raster map: 
g.region -p rast=elevation
v.random output=random_samples n=20
v.db.addtable map=random_samples layer=1 columns='cat INTEGER, sample DOUBLE PRECISION'
v.what.rast vector=random_samples raster=elevation@PERMANENT layer=1 column=sample
Generate 20 random points and sample attribute data from geology (vector) map: 
g.region -p vect=geology
v.random output=random_samples n=20
v.db.addtable map=random_samples layer=1 columns='cat integer, geology varchar(100)'
v.what.vect vector=random_samples layer=1 column=geology qvector=geology@PERMANENT qlayer=1 qcolumn=label
Generate 20 random points in forested areas 
g.region -p rast=landclass96
r.to.vect -v input=landclass96 output=landclass96 type=area
v.random input=landclass96 output=random_samples n=20 where="label = 'forest'"
Generate 20 random points in each forest patch 
g.region -p rast=landclass96
r.to.vect -v input=landclass96 output=landclass96 type=area
v.random input=landclass96 output=random_samples n=20 where="label = 'forest'" -a

SEE ALSO

UNIX man pages for rand(3) and drand48(3).

g.region, r.random, v.perturb, v.sample v.what.rast v.what.vect

BUGS

The RNG used by v.perturb should probably be added to this program.

AUTHOR

James Darrell McCauley <darrell@mccauley-usa.com>,
when he was at: Agricultural Engineering Purdue University

Last changed: $Date$