View source for Recipe: R program as OPeNDAP client

Back to [[G5NR Data Access Guide]].

== Problem ==

By accessing the collection <code>inst01hr_3d_T_Cv</code> via the OPeNDAP server
 http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km

we want to read the surface temperature data inside the box bound by latitudes 25<sup>o</sup>N, 50<sup>o</sup>N and longitudes -130<sup>o</sup>W, -65<sup>o</sup>W for 2006/Sep/18, 9z, compute its min/max and plot the data at the surface.

== Solution ==
The following code has been tested with R version 3.1.2.

The metadata for the collection <code>inst01hr_3d_T_Cv</code> is available at
 http://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv.info

==== Code ====
This code accesses the collection <code>inst01hr_3d_T_Cv</code> from the OPeNDAP server and reads a subset of the temperature data (all levels inside the bounding box specified above) and computes its max/min. It then plots the data at the surface (level=72).

NOTE:
# Instead of reading a downloaded NetCDF-4 file, we read an OPeNDAP URL.
# While in the downloaded file, the temperature variable appears in the uppercase (T), on the OPeNDAP server, this variable is in lowercase.
# Via the OPeNDAP URL, we now have access to all times for which data exists. The hourly <code>inst</code> files are available starting at 2005/15/15, 2200z. Our desired time, 2006/09/18, 0900z is then the 11772<sup>th</sup> step.

<syntaxhighlight lang="rsplus">
# Load Libraries
library(ncdf4)
library(rworldmap)
# Url
url <- "http://opendap.nccs.nasa.gov:80/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv"
nc <- nc_open(url)
# Bounding Box
imin <- round((-130.+180.)/0.5)-1
imax <- round(( -64.+180.)/0.5)-1
jmin <- round((  25 +  90.)/0.5) - 1
jmax <- round((  50 +  90.)/0.5) - 1
im <- imax-imin+1
jm <- jmax-jmin+1
lm <- 72
# Start and Count vectors
offset <- c(imin, jmin, 1, 11771)
count <- c(im, jm, lm, 1)
t <- ncvar_get(nc,"t",start=offset,count=count)
# Statistics
print(summary(t))
# Plot Data
mapGriddedData(t[1:im,1:jm,71],xlim=c(-130.,-65.),ylim=c(25.,50.))
</syntaxhighlight>


==== Output ====
Running this R script
> source("g5nr.R")

we get the text output

   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
  190.7   221.2   248.1   248.7   275.3   305.8 

and the plot

<!-- [[Image:SurfaceAirTempSubsetIDL.png|500px]] -->

== Discussion ==

== See Also ==

# File Spec: [[File:G5NR-Ganymed-7km FileSpec-ON6-V1.0.pdf]]
# [[Recipe: Fortran program to read data from downloaded file]]

== No Warranty ==

== Copyright ==