I have been trying plot the following gridded netcdf file: "air.1999.nc" found at the following website:
http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.html
I have tried the code below based on answers I have found here and elsewhere, but no luck.
library(ncdf);
temp.nc <- open.ncdf("air.1999.nc");
temp <- get.var.ncdf(temp.nc,"air");
temp.nc$dim$lon$vals -> lon
temp.nc$dim$lat$vals -> lat
lat <- rev(lat)
temp <- temp[nrow(temp):1,]
temp[temp==-32767] <- NA
temp <- t(temp)
image(lon,lat,temp)
library(maptools)
data(wrld_simpl)
plot(wrld_simpl, add = TRUE)
This code was from modified from the one found here: The variable from a netcdf file comes out flipped
Does anyone have any ideas or experience with using these type of netcdf files? Thanks
Answer
In the question you linked the whole part from lat <- rev(lat) to temp <- t(temp) was very specific to that particular OP dataset and have absolutely no universal value.
temp.nc <- open.ncdf("~/Downloads/air.1999.nc")
temp.nc
[1] "file ~/Downloads/air.1999.nc has 4 dimensions:"
[1] "lon Size: 144"
[1] "lat Size: 73"
[1] "level Size: 12"
[1] "time Size: 365"
[1] "------------------------"
[1] "file ~/Downloads/air.1999.nc has 2 variables:"
[1] "short air[lon,lat,level,time] Longname:Air temperature Missval:32767"
[1] "short head[level,time] Longname:Missing Missval:NA"
As you can see from these informations, in your case, missing values are represented by the value 32767 so the following should be your first step:
temp <- get.var.ncdf(temp.nc,"air")
temp[temp=="32767"] <- NA
Additionnaly in your case you have 4 dimensions to your data, not just 2, they are longitude, latitude, level (which I'm assuming represent the height) and time.
temp.nc$dim$lon$vals -> lon
temp.nc$dim$lat$vals -> lat
temp.nc$dim$time$vals -> time
temp.nc$dim$level$vals -> lev
If you have a look at lat you see that the values are in reverse (which image will frown upon) so let's reverse them:
lat <- rev(lat)
temp <- temp[, ncol(temp):1, , ] #lat being our dimension number 2
Then the longitude is expressed from 0 to 360 which is not standard, it should be from -180 to 180 so let's change that:
lon <- lon -180
So now let's plot the data for a level of 1000 (i. e. the first one) and the first date:
temp11 <- temp[ , , 1, 1] #Level is the third dimension and time the fourth.
image(lon,lat,temp11)
And then let's superimpose a world map:
library(maptools)
data(wrld_simpl)
plot(wrld_simpl,add=TRUE)
