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#
# fields is a package for analysis of spatial data written for
# the R software environment.
# Copyright (C) 2024 Colorado School of Mines
# 1500 Illinois St., Golden, CO 80401
# Contact: Douglas Nychka, douglasnychka@gmail.com,
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with the R software environment if not, write to the Free Software
# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
# or see http://www.r-project.org/Licenses/GPL-2
##END HEADER
Exp.simple.cov <- function(x1, x2=NULL, aRange = 1, C = NA,
marginal = FALSE, theta=NULL) {
# this is a simple exponential covariance function
# with the calling format and behaviour used in fields.
#
# different locations are the different rows of x1 and x2.
# this function can return three different results
# depending on the values of C and marginal.
# The three cases:
# 1) cross covaraince matrix
# 2) cross covariance matrix times a vector (C)
# 3) the diagonal elements of covariance matrix at locations x1.
#
# theta argument has been depreciated.
if( !is.null( theta)){
aRange<- theta
}
if( !is.null(x2)){
x2<- x1
}
# CASE 1:
if (is.na(C[1]) & !marginal) {
# rdist finds the cross distance matrix between the
# locations at x1, x2.
#
return(exp(-rdist(x1, x2)/aRange))
}
# CASE 2:
# or return multiplication of cov( x2,x1) with vector C
if (!is.na(C[1])) {
return(exp(-rdist(x1, x2)/aRange) %*% C)
#
# if the rows of X1 are large
# this line could be replaced by a call to C or FORTRAN
# to make the multiply use less memory.
#
# there are also other algorithms for fast multiplies when
# X2 is on a grid.
#
}
# CASE 3
# return marginal variance (in this case it is trivial a constant vector
# with 1.0)
if (marginal) {
return(rep(1, nrow(x1)))
}
}
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