Recipe: Matlab program as OPeNDAP client: Difference between revisions
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By accessing the collection <code>inst01hr_3d_T_Cv</code> via the OPeNDAP server | By accessing the collection <code>inst01hr_3d_T_Cv</code> via the OPeNDAP server | ||
https://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 and compute its min/max. | 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 and compute its min/max. | ||
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The metadata for the collection <code>inst01hr_3d_T_Cv</code> is available at | The metadata for the collection <code>inst01hr_3d_T_Cv</code> is available at | ||
https://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv.info | |||
==== Code ==== | ==== 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). | 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 computes the max/min of the above data at the surface (level=72). | ||
NOTE: | NOTE: | ||
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<syntaxhighlight lang="matlab" line> | <syntaxhighlight lang="matlab" line> | ||
% opendap url | % opendap url | ||
opdurl = ' | opdurl = 'https://opendap.nccs.nasa.gov:9090/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv' | ||
% bounding box | % bounding box | ||
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</syntaxhighlight> | </syntaxhighlight> | ||
==== | ==== Output ==== | ||
Running this MATLAB script, we get the output | |||
>> g5nr_reader | |||
opdurl = | |||
https://opendap.nccs.nasa.gov:9090/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv | |||
Reading Tsub (subset of T)...done. | |||
max(Tsub): 305.604828 | |||
min(Tsub): 191.695648 | |||
max(Tsub at surface): 305.604828 | |||
min(Tsub at surface): 276.292328 | |||
Reading | |||
== Discussion == | == Discussion == | ||
Latest revision as of 10:25, 9 April 2019
Back to G5NR Data Access Guide.
Problem
By accessing the collection inst01hr_3d_T_Cv via the OPeNDAP server
https://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km
we want to read the surface temperature data inside the box bound by latitudes 25oN, 50oN and longitudes -130oW, -65oW for 2006/Sep/18, 9z and compute its min/max.
Solution
MATLAB version 2012a and later has native openDAP support. The following code has been tested with MATLAB version 2014a.
The metadata for the collection inst01hr_3d_T_Cv is available at
https://opendap.nccs.nasa.gov/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv.info
Code
This code accesses the collection inst01hr_3d_T_Cv 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 computes the max/min of the above 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
instfiles are available starting at 2005/15/15, 2200z. Our desired time, 2006/09/18, 0900z is then the 11772th step.
% opendap url
opdurl = 'https://opendap.nccs.nasa.gov:9090/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv'
% bounding box
% lons = -130:0.5:-65
% lats = 25:0.5:50
imin = round((-130. + 180.)/0.5);
imax = round(( -65. + 180.)/0.5);
jmin = round(( 25 + 90.)/0.5);
jmax = round(( 50 + 90.)/0.5);
% corresponding array sizes
im = imax-imin+1;
jm = jmax-jmin+1;
lm = 72; % read all 72 levels
% read temperature inside the bounding box
% time 11772 corresponds to 2006/09/18, 9z
start = [imin, jmin, 1, 11772];
count = [im, jm, lm, 1];
fprintf('Reading Tsub (subset of T)...');
Tsub = ncread(opdurl, 't', start, count);
fprintf('done.\n')
% compute max/min of Tsub
fprintf('max(Tsub): %f\n', max(Tsub(:)));
fprintf('min(Tsub): %f\n', min(Tsub(:)));
% compute max/min of Tsub at surface
level = 72; % surface
Tsub_surface = Tsub(:,:,72);
fprintf('max(Tsub at surface): %f\n', max(Tsub_surface(:)));
fprintf('min(Tsub at surface): %f\n', min(Tsub_surface(:)));
Output
Running this MATLAB script, we get the output
>> g5nr_reader opdurl = https://opendap.nccs.nasa.gov:9090/dods/OSSE/G5NR/Ganymed/7km/0.5000_deg/inst/inst01hr_3d_T_Cv Reading Tsub (subset of T)...done. max(Tsub): 305.604828 min(Tsub): 191.695648 max(Tsub at surface): 305.604828 min(Tsub at surface): 276.292328
Discussion
See Also
- File Spec: File:G5NR-Ganymed-7km FileSpec-ON6-V1.0.pdf
- Recipe: Fortran program to read data from downloaded file