# fields, Tools for spatial data
# Copyright 2004-2011, Institute for Mathematics Applied Geosciences
# University Corporation for Atmospheric Research
# Licensed under the GPL -- www.gpl.org/licenses/gpl.html
"periodic.plane.3d" <- function(x1, x2, a = 0, b = 365, 
    theta = 1) {
    cov.per1d <- periodic.cov.1d(x1[, 1], x2[, 1], a, b)
    cov.vert <- Exp.cov(x1[, c(2, 3)], x2[, c(2, 3)], theta = theta)
    cov <- cov.per1d * cov.vert
    return(cov)
}