#' panel function for maps
#'
#' @description
#' this function produces a \code{\link{panel.polygon}} from a
#' \code{\link{map}} object. This can then be used to visualise
#' the map on top of an \code{\link{spplot}} object.
#' NOTE: currently only tested for unprojected latlon data.
#'
#' @param map.data the map type - see \code{\link{map}} database argument
#' @param col the colour of the fill
#' @param ... additional arguments passed on to \code{\link{panel.polygon}}
#'
#' @return
#' A \code{\link{panel.polygon}} object
#'
#' @author
#' Tim Appelhans
#'
#' @examples
#' library(latticeExtra)
#' rst <- raster(ncols = 360, nrows = 180)
#' rst[] <- rnorm(360 * 180)
#'
#' spplot(rst) + layer(panel.map("world"))
#'
#' @export panel.map
#' @aliases panel.map
panel.map <- function(map.data, col = "transparent", ...) {
mm <- maps::map(map.data, plot = FALSE, fill = TRUE,
col = col)
pp <- panel.polygon(mm$x, mm$y, col = col, ...)
return(pp)
}
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