trend.spatial {geoR} | R Documentation |
Builds the Trend Matrix
Description
Builds the trend matrix in accordance to a specification of the mean provided by the user.
Usage
trend.spatial(trend, geodata, add.to.trend)
Arguments
trend |
specifies the mean part of the model.
See |
geodata |
optional. An object of the class |
add.to.trend |
optional. Specifies aditional terms to the mean part of the model. See details below. |
Details
The implicity model assumes that there is an underlying process
with mean , where
denotes the coordinates
of a spatial location.
The argument
trend
defines the form of the mean and the
following options are allowed:
"cte"
the mean is assumed to be constant over the region, in which case
. This is the default option.
"1st"
the mean is assumed to be a first order polynomial on the coordinates:
"2nd"
the mean is assumed to be a second order polynomial on the coordinates:
~ model
a model specification. See
formula
for further details on how to specify a model in R using formulas. Notice that the model term before the~
is not necessary. Typically used to include covariates (external trend) in the model.
Denote by and
the spatial coordinates.
The following specifications are equivalent:
-
trend = "1st"
andtrend = ~ x1 + x2
-
trend = "2nd"
andtrend = ~ x1 + x2 + I(x1^2) + I(x2^2) + I(x1*x2)
Search path for covariates
Typically, functions in the package geoR which calls
trend.spatial
will have the arguments geodata
,
coords
and data
.
When the trend is specifed as trend = ~ model
the terms included in the model will be searched for in the following
path sequence (modified in version 1.7-6, no longer attach objects):
in the object
geodata
(coerced to data-frame)in the users/session Global environment
in the session search path
The argument add.to.trend
adds terms to what is specified in
the argument trend
. This seems redundant but allow
specifications of the type: trend="2nd", add.trend=~other.covariates
.
Value
An object of the class trend.spatial
which is an trend
matrix, where
is the number of spatial
locations and
is the number of mean parameters in the model.
Note
This is an auxiliary function typically called by other geoR functions.
Author(s)
Paulo J. Ribeiro Jr. paulojus@leg.ufpr.br,
Peter J. Diggle p.diggle@lancaster.ac.uk.
References
Further information on the package geoR can be found at:
http://www.leg.ufpr.br/geoR/.
See Also
The section DETAILS
in the documentation for
likfit
for more about the underlying model.
Examples
# a first order polynomial trend
trend.spatial("1st", sic.100)[1:5,]
# a second order polynomial trend
trend.spatial("2nd", sic.100)[1:5,]
# a trend with a covariate
trend.spatial(~altitude, sic.100)[1:5,]
# a first degree trend plus a covariate
trend.spatial(~coords+altitude, sic.100)[1:5,]
# with produces the same as
trend.spatial("1st", sic.100, add=~altitude)[1:5,]
# and yet another exemple
trend.spatial("2nd", sic.100, add=~altitude)[1:5,]