Note: This document is for an older version of GRASS GIS that is outdated. You should upgrade, and read the current manual page.
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.
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.
Random vector elevation points sampled from elevation map in the Spearfish region, result stored in 2D vector map:
g.region rast=elevation.10m -p r.random elevation.10m vector_output=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 rast=elevation.10m -p r.random -d elevation.10m cover=landcover.30m vector_output=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 ...
Modified for GRASS 5.0 by Eric G. Miller
Cover map support by Markus Neteler, 2007
Last changed: $Date: 2009-04-02 02:06:06 -0800 (Thu, 02 Apr 2009) $
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