问题
Taking a raster file of monthly temperature data for multiple years which has a name attached accessible via names(object)
in the following format 'Jan.1981', 'Feb.1981' etc (example files for two years that works with code below here - adding all files makes it too big.
Reading in and writing this to NetCDF using the following code:
#Load Packages
library(raster)
library(ncdf4)
#Read in temperature files
r1 <- brick('TavgM_1981.grd')
r2 <- brick('TavgM_1982.grd')
#stack them together
TempStack = stack(r1, r2)
#set the coordinate system (as it was missing)
crs(TempStack) <- ('+proj=lcc +lat_1=53.5 +lat_2=53.5 +lat_0=46.834 +lon_0=5 +x_0=1488375 +y_0=-203375 +datum=WGS84 +to_meter=2500 +no_defs +ellps=WGS84 +towgs84=0,0,0')
#reproject to get in lat/lon instead of meters
TempStack<-projectRaster(TempStack, crs=CRS("+init=epsg:4326"))
#Extract monthly data names to assign to netCDf later
names <- names(TempStack)
#write the raster file to NetCDF
writeRaster(TempStack, "Temp.nc", overwrite=TRUE, format="CDF", varname="Temperature", varunit="degC",
longname="Temperature -- raster stack to netCDF, monthly average", xname="Longitude", yname="Latitude", zname='Time', zunit=names)
When I write this to NetCDF and plot the monthly data it is organised from month 1 to month 24, but I want it to have 'Jan 1981', 'Feb 1981' etc.
I thought by adding the zunit argument in writeRaster would work, but it doesn't, the numbers are all still 1-24 instead of Jan, Feb etc.
回答1:
There are a couple of misconceptions in your example. First, you should realize that the values in a netcdf dimension must be numeric. They are not just labels for layers, they are actual values of that dimension, and cannot therefore take values like "Jan.1980"
, which is a string. One way around this is to save your netcdf file and then add the z dimension values to it as a numeric value. Unfortunately that means we can't use date/time variable types either, but must first convert them to numeric equivalents. Here I use the lubridate
package to do that.
# first we write the netcdf file to disk
writeRaster(TempStack, "Temp.nc", overwrite=TRUE,
format="CDF", varname="Temperature", varunit="degC",
longname="Temperature -- raster stack to netCDF, monthly average",
xname="Longitude", yname="Latitude", zname='Time', zunit='seconds')
# and open a connection to it to make changes.
# note that we use write=TRUE so that we can change it
nc = nc_open('Temp.nc', write = TRUE)
# now convert the strings to numeric values based on their dates
zvals = lubridate::parse_date_time(names, orders = 'm.y', tz = "UTC")
zvals = as.integer(zvals)
# and we can write these numeric dates to the z dimension
ncdf4::ncvar_put(nc, 'Time', zvals)
Haing written the dates to the z dimension like this, we will also need to reverse the process if you want to convert the numeric z values back into raster layer names that look like "Jan.1908" etc. Again, lubridate can help.
ncb = brick('Temp.nc')
zvals = ncvar_get(nc, 'Time')
zvals = as.POSIXct(zvals, origin = lubridate::origin, tz = "UTC")
znames = paste0(lubridate::month(zvals, label=T), '.', lubridate::year(zvals))
names(ncb) = znames
Let's check that worked:
plot(ncb)
来源:https://stackoverflow.com/questions/50120137/keeping-raster-variable-names-when-converting-to-netcdf-using-r