r-从netcdf中提取特定lat-long的值

Question

我正在尝试读取一个netCDF文件到R中。netcdfchirps-v2.0.1981.days_p05.nc可从以下位置下载：

ftp://ftp.chg.ucsb.edu/pub/org/chg/products/CHIRPS-2.0/global_daily/netcdf/p05/

该netCDF文件将全球日降雨量描述为经度和纬度的函数，大小为1.1 GB

我还有一套纵向

dat <- structure(list(locatioID = paste0('ID', 1:16), lon = c(73.73, 86, 73.45, 86.41, 85.36, 81.95, 82.57, 75.66, 82.03, 
81.73, 85.66, 85.31, 81.03, 81.70, 87.03, 73.38), 
lat = c(24.59, 20.08, 22.61, 23.33, 23.99, 19.09, 18.85, 15.25, 26.78, 
16.63, 25.98, 23.28, 24.5, 21.23, 25.08, 21.11)), 
row.names = c(1L, 3L, 5L, 8L, 11L, 14L, 17L, 18L, 19L, 21L, 
23L, 26L, 29L, 32L, 33L, 35L), class = "data.frame")
library(ncdf4)  
library(raster)
temp <- nc_open("chirps-v2.0.1981.days_p05.nc")
precip = list()
precip$x = ncvar_get(temp, "longitude")
precip$y = ncvar_get(temp, "latitude")
precip$z = ncvar_get(temp, "precip", start=c(1, 1, 1), count=c(-1, -1, 1))
precip.r = raster(precip)
plot(precip.r)

我有两个问题：

• 有人能向我解释一下启动和计数参数的作用吗？?ncvar_get没有给我直观的感觉。如果我想创建儒略日252的光栅，我需要改变哪个论点？

• 我如何提取dat中每个纬度365天的日降雨量值，以便我有一个16*365天的矩阵/数据帧

Answer 1

您可以使用以下代码从.nc文件中提取数据

dat <- structure(list(locatioID = paste0('ID', 1:16), lon = c(73.73, 86, 73.45, 86.41, 85.36, 81.95, 82.57, 75.66, 82.03, 
81.73, 85.66, 85.31, 81.03, 81.70, 87.03, 73.38), 
lat = c(24.59, 20.08, 22.61, 23.33, 23.99, 19.09, 18.85, 15.25, 26.78, 
16.63, 25.98, 23.28, 24.5, 21.23, 25.08, 21.11)), 
row.names = c(1L, 3L, 5L, 8L, 11L, 14L, 17L, 18L, 19L, 21L, 
23L, 26L, 29L, 32L, 33L, 35L), class = "data.frame")

temp <- brick("chirps-v2.0.1981.days_p05.nc")
xy <- dat[,2:3] #Column 1 is longitude and column 2 is latitude
xy
spts <- SpatialPoints(xy, proj4string=CRS("+proj=longlat +datum=WGS84"))
#Extract data by spatial point
temp2 <- extract(temp, spts)
temp3 <- t(temp2) #transpose raster object
colnames(temp3) <- dat[,1] #It would be better if you have the location names corresponding to the points
head(temp3)
write.csv(temp3, "Rainfall.csv")