# r.random.surface.1grass - Man Page

Generates random surface(s) with spatial dependence.

## Keywords

raster, surface, random

## Synopsis

`r.random.surfacer.random.surface --helpr.random.surface [-u] output=string[,string,...]  [distance=float]   [exponent=float]   [flat=float]   [seed=integer]   [high=integer]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]`

### Flags

-u

Uniformly distributed cell values

--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

output=string[,string,...]Â [required]

Name for output raster map(s)

distance=float

Maximum distance of spatial correlation (value >= 0.0)
Default: 0.0

exponent=float

Distance decay exponent (value > 0.0)
Default: 1.0

flat=float

Distance filter remains flat before beginning exponent
Default: 0.0

seed=integer

Random seed, default [random]

high=integer

Maximum cell value of distribution
Default: 255

## Description

r.random.surface generates a spatially dependent random surface. The random surface is composed of values representing the deviation from the mean of the initial random values driving the algorithm. The initial random values are independent Gaussian random deviates with a mean of 0 and standard deviation of 1. The initial values are spread over each output map using filter(s) of diameter distance.  The influence of each random value on nearby cells is determined by a distance decay function based on exponent. If multiple filters are passed over the output maps, each filter is given a weight based on the weight inputs.  The resulting random surface can have any mean and variance, but the theoretical mean of an infinitely large map is 0.0 and a variance of 1.0. Description of the algorithm is in the Notes section.

The random surface generated are composed of floating point numbers, and saved in the category description files of the output map(s).  Cell values are uniformly or normally distributed between 1 and high values inclusive (determined by whether the -u flag is used). The category names indicate the average floating point value and the range of floating point values that each cell value represents.

r.random.surface’s original goal is to generate random fields for spatial error modeling. A procedure to use r.random.surface in spatial error modeling is given in the Notes section.

### Detailed parameter description

output

Random surface(s). The cell values are a random distribution between the low and high values inclusive.  The category values of the output map(s) are in the form #.# #.# to #.# where each #.# is a floating point number. The first number is the average of the random values the cell value represents. The other two numbers are the range of random values for that cell value. The average mean value of generated output map(s) is 0. The average variance of map(s) generated is 1. The random values represent the standard deviation from the mean of that random surface.

distance

Distance determines the spatial dependence of the output map(s). The distance value indicates the minimum distance at which two map cells have no relationship to each other. A distance value of 0.0 indicates that there is no spatial dependence (i.e., adjacent cell values have no relationship to each other). As the distance value increases, adjacent cell values will have values closer to each other. But the range and distribution of cell values over the output map(s) will remain the same.  Visually, the clumps of lower and higher values gets larger as distance increases. If multiple values are given, each output map will have multiple filters, one for each set of distance, exponent, and weight values.

exponent

Exponent determines the distance decay exponent for a particular filter. The exponent value(s) have the property of determining the texture of the random surface. Texture will decrease as the exponent value(s) get closer to 1.0. Normally, exponent will be 1.0 or less. If there are no exponent values given, each filter will be given an exponent value of 1.0. If there is at least one exponent value given, there must be one exponent value for each distance value.

flat

Flat determines the distance at which the filter.

weight

Weight determines the relative importance of each filter. For example, if there were two filters driving the algorithm and weight=1.0, 2.0 was given in the command line: The second filter would be twice as important as the first filter. If no weight values are given, each filter will be just as important as the other filters defining the random field. If weight values exist, there must be a weight value for each filter of the random field.

high

Specifies the high end of the range of cell values in the output map(s). Specifying a very large high value will minimize the errors caused by the random surface’s discretization. The word errors is in quotes because errors in discretization are often going to cancel each other out and the spatial statistics are far more sensitive to the initial independent random deviates than any potential discretization errors.

seed

Specifies the random seed(s), one for each map, that r.random.surface will use to generate the initial set of random values that the resulting map is based on. If the random seed is not given, r.random.surface will get a seed from the process ID number.

## See Also

r.random, r.random.cells, r.mapcalc, r.surf.random

## Authors

Charles Ehlschlaeger, Michael Goodchild, and Chih-chang Lin; National Center for Geographic Information and Analysis, University of California, Santa Barbara.

## Source Code

Available at: r.random.surface source code (history)

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Â© 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual

## Info

GRASS 7.8.5 GRASS GIS User's Manual