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NAME
r.random - Creates randomly placed raster cells or vector points
Creates a raster map and vector point map containing randomly located cells and points.
KEYWORDS
raster, sampling, vector, random, level1
SYNOPSIS
r.random
r.random --help
r.random [-snzb] input=name [cover=name] npoints=number[%] [raster=name]
[vector=name] [seed=integer] [--overwrite] [--help] [--verbose] [--quiet] [--ui]
Flags:
-s
Generate random seed (result is non-deterministic)
-n
Generate points also for NULL cells
-z
Generate vector points as 3D points
Input raster values will be used for Z coordinates
-b
Do not build topology
Do not build topology for vector 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 raster map
cover=name
Name of cover raster map
npoints=number[%] [required]
The number of points (or cells) to generate
The number of vector points or raster cells to generate, possibly as a percentage of
number of cells
raster=name
Name for output raster map
vector=name
Name for output vector map
seed=integer
Seed for rand() function
DESCRIPTION
The module r.random creates a raster map with values in random places. Alternatively, it
creates random vector points at these places. Number of random cells or points can be a
fixed number or a percentage of cells from the input. By default, generated cells or
points will be subset of non-NULL cells of the input. Resulting raster map consists of
original cell values at the selected random locations and NULL (no data) values elsewhere.
Placement of cells and points
The module allows the user to create a raster map and/or a vector points map containing
coordinates of points whose locations have been randomly determined. The module places
these randomly generated vector points within the current computational region and raster
mask (if any), on non-NULL raster cells in a user-specified raster map. If the user sets
the -n flag, points will be randomly generated across all cells (even those with NULL
values). Cells in the resulting raster overlap with the cells of the input raster based
on the current computational region. Points in the resulting vector map are placed in
cell centers of these cells.
Number of cells and points
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’s cells (e.g., 10%, or 3.05%).
The number of cells considered for the percentage reflects whether or not the -n flag was
given. Options are 0-100; fractions of percent may be stated as decimals (e.g., 66.67%, or
0.05%).
Values
The cell values and corresponding category names (if present) associated with the random
point locations in the input map are assigned to the newly generated cells in the raster
map. If the -n 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.
If a cover raster map is specified, values are taken from the cover raster map instead of
the input raster map. If a cover raster map is specified and the cover map contains NULL
(no data) values, these points are suppressed in the resulting vector or raster map.
Vector output
The vector 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 -n 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.
NOTES
To decide on the number of points r.random will create, use r.univar, g.region, or
r.report. r.univar is the fastest way to obtain number of non-NULL cells and NULL cells
in a raster map given the current computational region and raster mask:
r.univar map=inputmap
The text output contains total number of null and non-null cells (called cells in the
machine-readable shell script style output), total null cells (null_cells), and number of
non-null cells (n). Alternatively, you can use the following to examine the computational
region and the raster map:
g.region -p
r.report map=inputmap units=c null="*" nsteps=1
To create random vector point locations within some, but not all, categories of a integer
input raster map (aka CELL raster map), the user must first create a reclassified raster
map of the original raster map (e.g., using the GRASS module r.reclass) that contains only
the desired categories, and then use the reclassed raster map as input to r.random.
EXAMPLES
Random 2D vector elevation points
Random vector elevation points sampled from elevation map in the North Carolina sample
dataset region, result stored in 2D vector map:
g.region raster=elevation -p
r.random elevation vector=elevrand n=100
v.db.select elevrand
v.univar elevrand col=value type=point
Random 3D vector elevation points
Random vector elevation points sampled from elevation map in the North Carolina sample
dataset region with collocated values sampled from landuse map, result stored in 3D vector
map:
g.region raster=elevation -p
r.random -z elevation cover=landclass96 vector=luserand3d n=100
# data output (value: elevation, covervalue: landuse class):
v.db.select luserand3d
cat|value|covervalue
1|111.229591|5
2|71.093758|1
3|122.51075|5
4|146.17395|4
...
SEE ALSO
• g.region for setting the computational region and examining the total number of
cells,
• r.reclass for working only with subset of values in the raster map,
• v.random for generating vector points without any involvement of raster data,
• r.random.cells for generating random cells with with spatial dependence (minimal
distance),
• r.surf.random as an option for generating random cell values,
• v.surf.rst as an option for creating a surface from sampled points.
AUTHORS
Dr. James Hinthorne, GIS Laboratory, Central Washington University
Modified for GRASS 5.0 by Eric G. Miller
Cover map support by Markus Neteler, 2007
SOURCE CODE
Available at: r.random source code (history)
Accessed: Mon Jun 13 15:09:34 2022
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