RandomFields (version 3.1.36)

RFsimulate: Simulation of Random Fields

Description

This function simulates unconditional random fields:

• univariate and multivariat, spatial and spatio-temporal Gaussian random fields
• fields based on Gaussian fields such as Chi2 fields or Binary fields, see RP.
• stationary Poisson fields
• stationary max-stable random fields.

It also simulates conditional random fields for

• univariate and multivariat, spatial and spatio-temporal Gaussian random fields

Here, only the simulation of Gaussian random fields is described. For other kind of random fields (binary, max-stable, etc.) or more sophisticated approches see RFsimulateAdvanced.

Usage

RFsimulate(model, x, y=NULL, z=NULL, T=NULL, grid=NULL, distances, dim, data, given=NULL, err.model, n=1, ...)

Arguments

model

object of class RMmodel, RFformula or formula; specifies the model to be simulated; the best is to consider the examples below, first.

• if of class RMmodel, model specifies a covariance or variogram model of a Gaussian random field; type RFgetModelNames(type="variogram") for a list of available models; see also RMmodel
• if of class RFformula or formula , submodel specifies a linear mixed model where random effects can be modelled by Gaussian random fields; see RFformula for details on model specification.
• for (many) more options see RFsimulateAdvanced.

x

vector of x coordinates, or object of class GridTopology or raster; For more options see RFsimulateAdvanced.

y

optional vector of y coordinates

z

optional vector of z coordinates

T

optional vector of time coordinates, T must always be an equidistant vector. Instead of T=seq(from=From, by=By, len=Len) one may also write T=c(From, By, Len).

grid

logical; RandomFields can find itself the correct value in nearly all cases, so that usually grid need not be given. See also RFsimulateAdvanced.

distances

another alternative to pass the (relative) coordinates, see RFsimulateAdvanced.

dim

Only used if distances are given.

data

For conditional simulation and random imputing only. If data is missing, unconditional simulation is performed. Matrix, data.frame or object of class RFsp; coordinates and response values of measurements in case that conditional simulation is to be performed; If given is not given and data is a matrix or data is a data.frame, then the first columns are interpreted as coordinate vectors, and the last column(s) as (multiple) measurement(s) of the field; if the argument x is missing, data may contain NAs, which are then replaced by conditionally simulated values (random imputing); for details on matching of variable names see Details; if of class RFsp

given

optional, matrix or list. If given matrix then the coordinates can be given separately, namely by given where, in each row, a single location is given. If given is a list, it may consist of x, y, z, T, grid. If given is provided, data must be a matrix or an array containing the data only.

err.model

For conditional simulation and random imputing only. Usually err.model=RMnugget(var=var), or not given at all (error-free measurements).

n

number of realizations to generate. For a very advanced feature, see the notes in RFsimulateAdvanced.

...

for advanced use: further options and control arguments for the simulation that are passed to and processed by RFoptions

Value

an object of the virtual class RFsp; result is of class RMmodel.

• RFspatialGridDataFrame if the space-time dimension is greater than 1 and the coordinates are on a grid,
• RFgridDataFrame if the space-time dimension equals 1 and the coordinates are on a grid,
• RFspatialPointsDataFrame if the space-time dimension is greater than 1 and the coordinates are not on a grid,
• RFpointsDataFrame if the space-time dimension equals 1 and the coordinates are not on a grid.

In case of a multivariateIf n > 1 the repetitions make the last dimension.See RFsimulateAdvanced for additional options.

Details

By default, all Gaussian random fields have zero mean. Simulating with trend can be done by including RMtrend in the model, see the examples below.

If data is passed, conditional simulation based on simple kriging is performed:

• if of class RFsp, ncol(data@coords) must equal the dimension of the index space. If data@data contains only a single variable, variable names are optional. If data@data contains more than one variable, variables must be named and model must be given in the tilde notation resp ~ ... (see RFformula) and "resp" must be contained in names(data@data).
• If data is a matrix or a data.frame, either ncol(data) equals $(dimension of index space + 1)$ and the order of the columns is (x, y, z, T, response) or, if data contains more than one response variable (i.e. ncol(data) > (dimension of index space + 1)), colnames(data) must contain colnames(x) or those of "x", "y", "z", "T" that are not missing. The response variable name is matched with model, which must be given in the tilde notation. If "x", "y", "z", "T" are missing and data contains NAs, colnames(data) must contain an element which starts with ‘data’; the corresponding column and those behind it are interpreted as the given data and those before the corresponding column are interpreted as the coordinates.
• if x is missing, RFsimulate searches for NAs in the data and performs a conditional simulation for them.

Specification of err.model: In geostatistics we have two different interpretations of a nugget effect: small scale variability and measurement error. The result of conditional simulation usually does not include the measurement error. Hence the measurement error err.model must be given separately. For sake of generality, any model (and not only the nugget effect) is allowed. Consequently, err.model is ignored when unconditional simulation is performed.

References

Gneiting, T. and Schlather, M. (2013) Statistical modeling with covariance functions. In preparation.

Lantuejoul, Ch. (2002) Geostatistical simulation. New York: Springer. Schlather, M. (1999) An introduction to positive definite functions and to unconditional simulation of random fields. Technical report ST 99-10, Dept. of Maths and Statistics, Lancaster University.

See RFsimulateAdvanced for more specific literature.

See Also

RFempiricalvariogram, RFfit, RFgetModelInfo, RFgui, RMmodel, RFoptions, RFsimulateAdvanced, RFsimulate.more.examples

Examples

RFoptions(seed=0) ## *ANY* simulation will have the random seed 0; set
##                   RFoptions(seed=NA) to make them all random again


#############################################################
## ##
## ONLY TWO VERY BASIC EXAMPLES ARE GIVEN HERE ##
## see ##
## ?RMsimulate.more.examples ##
## and ##
## ?RFsimulateAdvanced ##
## for more examples ##
## ##
#############################################################

#############################################################
## ##
## Unconditional simulation ## 
## ##
#############################################################

## first let us look at the list of implemented models
RFgetModelNames(type="positive definite", domain="single variable",
                iso="isotropic") 

## our choice is the exponential model;
## the model includes nugget effect and the mean:
model <- RMexp(var=5, scale=10) + # with variance 4 and scale 10
 RMnugget(var=1) + # nugget
 RMtrend(mean=0.5) # and mean
 
## define the locations:
from <- 0
to <- 20
x.seq <- seq(from, to, length=200) 
y.seq <- seq(from, to, length=200)

simu <- RFsimulate(model, x=x.seq, y=y.seq)
plot(simu)



#############################################################
## ##
## Conditional simulation ## 
## ##
#############################################################

# first we simulate some random values at a
# 100 random locations:
n <- 100
x <- runif(n=n, min=-1, max=1)
y <- runif(n=n, min=-1, max=1)
data <- RFsimulate(model = RMexp(), x=x, y=y, grid=FALSE)
plot(data)

# let simulate a field conditional on the above data
x.seq.cond <- y.seq.cond <- seq(-1.5, 1.5, length=n)
model <- RMexp()
cond <- RFsimulate(model, x=x.seq.cond, y=y.seq.cond, data=data)
plot(cond, data)