R 如何在地图上覆盖透明光栅？

我正试图找到一种方法来添加

alpha

函数，以便在

ggmap

上覆盖

光栅层

。有一些建议，但我想使用

光栅文件

。有没有办法添加

alpha

并使

光栅

层

透明

library(raster)
library(ggmap)
location <- get_map(location = c(lon =22, lat =40), zoom = 6, maptype = "hybrid")
# get the raster layer
tmin <- raster::getData('worldclim', var='tmin', res=0.5, lon=22, lat=40)[[1]]
ggmap(location)+inset_raster(as.raster(tmin), xmin = tmin@extent[1], xmax = tmin@extent[2],ymin =tmin@extent[3], ymax = tmin@extent[4])+ coord_cartesian()

库（光栅）
图书馆（ggmap）
location不久前，我修改了
rasterVis:：gplot
函数，以使用经典的{ggplot2}函数（如
geom_tile
）检索要绘制的数据。函数是my中的
gplot\u数据
。如果您不想获得完整的软件包，可以直接使用以下功能：

#' Transform raster as data.frame to be later used with ggplot
#' Modified from rasterVis::gplot
#'
#' @param x A Raster* object
#' @param maxpixels Maximum number of pixels to use
#'
#' @details rasterVis::gplot is nice to plot a raster in a ggplot but
#' if you want to plot different rasters on the same plot, you are stuck.
#' If you want to add other information or transform your raster as a
#' category raster, you can not do it. With `SDMSelect::gplot_data`, you retrieve your
#' raster as a data.frame that can be modified as wanted using `dplyr` and
#' then plot in `ggplot` using `geom_tile`.
#' If Raster has levels, they will be joined to the final tibble.
#'
#' @export

gplot_data <- function(x, maxpixels = 50000)  {
  x <- raster::sampleRegular(x, maxpixels, asRaster = TRUE)
  coords <- raster::xyFromCell(x, seq_len(raster::ncell(x)))
  ## Extract values
  dat <- utils::stack(as.data.frame(raster::getValues(x)))
  names(dat) <- c('value', 'variable')

  dat <- dplyr::as.tbl(data.frame(coords, dat))

  if (!is.null(levels(x))) {
    dat <- dplyr::left_join(dat, levels(x)[[1]],
                            by = c("value" = "ID"))
  }
  dat
}

不久前，我修改了
rasterVis:：gplot
函数，以检索使用经典的{ggplot2}函数（如
geom\u tile
绘制的数据。函数是my中的
gplot\u数据
。如果您不想获得完整的软件包，可以直接使用以下功能：

#' Transform raster as data.frame to be later used with ggplot
#' Modified from rasterVis::gplot
#'
#' @param x A Raster* object
#' @param maxpixels Maximum number of pixels to use
#'
#' @details rasterVis::gplot is nice to plot a raster in a ggplot but
#' if you want to plot different rasters on the same plot, you are stuck.
#' If you want to add other information or transform your raster as a
#' category raster, you can not do it. With `SDMSelect::gplot_data`, you retrieve your
#' raster as a data.frame that can be modified as wanted using `dplyr` and
#' then plot in `ggplot` using `geom_tile`.
#' If Raster has levels, they will be joined to the final tibble.
#'
#' @export

gplot_data <- function(x, maxpixels = 50000)  {
  x <- raster::sampleRegular(x, maxpixels, asRaster = TRUE)
  coords <- raster::xyFromCell(x, seq_len(raster::ncell(x)))
  ## Extract values
  dat <- utils::stack(as.data.frame(raster::getValues(x)))
  names(dat) <- c('value', 'variable')

  dat <- dplyr::as.tbl(data.frame(coords, dat))

  if (!is.null(levels(x))) {
    dat <- dplyr::left_join(dat, levels(x)[[1]],
                            by = c("value" = "ID"))
  }
  dat
}