读取大量 .asc-files，删除行并在 R 中另存为光栅

Question

我需要阅读大量的 .asc-files，删除行并通过 stack() 将它们转换为 raster-stack。 zip-packed 数据的来源在这里：ftp://ftp.dwd.de/pub/CDC/grids_germany/monthly/radiation_global/ 我已经解压了文件。但是现在我写这段代码真的很慢，我的电脑无法完成：

files <- list.files("mydirectory", pattern="\.asc$", 
                    full.names=TRUE, recursive=TRUE)
i <- lapply(files, readLines) #read data
i <- lapply(i, function(x){x[-(1:28)]}) #delete rows
i <- lapply(i, function(x){gsub('-999', ' NA ', x, fixed = TRUE)}) #convert '-999' to NA
i <- lapply(i, function(x){scan(textConnection(x), what = double(), n = 866*654)}) #convert to numeric
i <- lapply(i, function(x){matrix(x, nrow = 866, ncol = 654, byrow = TRUE)}) #convert to matrix
r <- lapply(i, function(x){raster(x)}) #rasterize data
st <- stack(r) #convert to stack-raster

我想知道是否有更好的方法将此数据转换为 raster-files。其他 .asc 文件只有 6 行作为 header ，如下所示： ftp://ftp.dwd.de/pub/CDC/grids_germany/monthly/precipitation/01_Jan/ 。我通过一个更简单的函数读取该数据，该函数仅使用 stack()-function:

loadRaster <- function(directory, output, clipping){
  files <- list.files(path=directory, pattern="\.asc$", 
                     full.names=TRUE, recursive=TRUE)
  stc <- stack(files)
  crs(stc) <- gk3
  stcC <- crop(stc, extent(clipping))
  writeRaster(stcC, filename=output)
} 
#You can ignore the code below "stc <-stack(files)"

Answer 1

最后我通过使用 textConnection() 逐步（再次打开和关闭）得到了它，因为 – 可能是缓慢的原因 – 打开的连接是有限制的。

files <- list.files(path="mydirectory", pattern="\.asc$", 
                    full.names=TRUE, recursive=TRUE)
i <- lapply(files, readLines)
i <- lapply(i, function(x){x[-(1:28)]})
i <- lapply(i, function(x){gsub('-999', ' NA ', x, fixed = TRUE)})
names(i) <- substr(files, 92,97)
i1 <- lapply(i[1:100], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i2 <- lapply(i[101:200], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i3 <- lapply(i[201:300], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i4 <- lapply(i[301:length(i)], function(x){scan(textConnection(x), what = double(), n = 866*654)})
closeAllConnections()
i <- c(i1, i2, i3, i4)
m <- lapply(i, function(x){matrix(x, nrow = 866, ncol = 654, byrow = TRUE)})
r <- lapply(m, function(x){raster(x)})
stc <- stack(r)

读取大量 .asc-files，删除行并在 R 中另存为光栅

Read large number of .asc-files, delete rows and save as raster in R

r

raster

large-files