## 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