GRASS GIS manual: r.random

Note: A new GRASS GIS stable version has been released: GRASS GIS 7.4, available here.
Updated manual page: here

NAME

r.random - Creates a raster map layer and vector point map containing randomly located points.

KEYWORDS

raster, sampling, vector, random

SYNOPSIS

r.random

r.random --help

r.random [-zidb] input=name [cover=name] npoints=number[%] [raster=name] [vector=name] [--overwrite] [--help] [--verbose] [--quiet] [--ui]

Flags:

-z: Generate points also for NULL category
-i: Report information about input raster and exit
-d: Generate vector points as 3D points
-b: Do not build topology in points mode
--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 raster map
cover=name: Name of cover raster map
npoints=number[%] [required]: The number of points to allocate
raster=name: Name for output raster map
vector=name: Name for output vector map

DESCRIPTION
NOTES
EXAMPLES
KNOWN ISSUES
SEE ALSO
AUTHOR

DESCRIPTION

The program r.random allows the user to create a raster map layer and/or a vector points map containing coordinates of points whose locations have been randomly determined. The program locates these randomly generated vector points (sites) within the current geographic region and mask (if any), on non-NULL category value data areas within a user-specified raster map layer. If the user sets the -z flag, points will be randomly generated across all cells (even those with NULL values).

The category values and corresponding category names already associated with the random point locations in the input map layer are assigned to these points in the raster_output map layer. If the -z is specified, then a unique entry is made for the value used where the input was NULL. This value is at least 1 less than the smallest value in the input raster and is given a medium gray color.

NOTES

If a cover raster map is specified and the cover map contains NULL (no data) values, these points are suppressed in the resulting vector_output or raster_output map.

The vector_output file created by r.random contains vector points that represent the center points of the randomly generated cells. A value attribute contains the cell value of the input raster (or the assigned value when -z is used).
If a cover map is additionally specified, a second column covervalue is populated with raster values from the cover map.

If the user sets the -b flag, vector points are written without topology to minimize the required resources. This is suitable input to v.surf.rst and other vector modules.

The user may specify the quantity of random locations to be generated either as a positive integer (e.g., 10), or as a percentage of the raster map layer's cells (e.g., 10%, or 3.05%). The number of cells considered for the percentage reflects whether or not the -z flag was given. Options are 0-100; percentages less than one percent may be stated as decimals.

Flag -i prints the raster map's name and location, the total number of cells under the current region settings, and the number of NULL valued cells under the current region settings. Then module exits without doing anything. Useful for deciding on the number of sites to have r.random create. WARNING: this feature may be removed in future. Use g.region and r.report instead.

g.region -p
r.report map=inputmap units=c null=* nsteps=1

To create random vector point locations within some, but not all, non-zero categories of the input raster map layer, the user must first create a reclassified raster map layer of the original raster map layer (e.g., using the GRASS program r.reclass) that contains only the desired categories, and then use the reclassed raster map layer as input to r.random.

EXAMPLES

Random vector elevation points sampled from elevation map in the Spearfish region, result stored in 2D vector map:

g.region raster=elevation.10m -p
r.random elevation.10m vector=elevrand n=100
v.db.select elevrand
v.univar elevrand col=value type=point

Random vector elevation points sampled from elevation map in the Spearfish region with collocated values sampled from landuse map, result stored in 3D vector map:

g.region raster=elevation.10m -p
r.random -d elevation.10m cover=landcover.30m vector=luserand3d n=100

# data output (value: elevation, covervalue: landuse class):
v.db.select luserand3d
cat|value|covervalue
1|1151.406616|81
2|1172.121216|71
3|1183.219604|71
...