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I ran ncdump using your url and with fetch display enabled. It is making 24 requests to the server each of size one float [Reading a float at a time is a function of the way ncdump works, not the dap code]. Can you please do the following command and send me the output: ncdump '[log][show=fetch]http://goldsmr2.sci.gsfc.nasa.gov/opendap/hyrax/MERRA/MAT1NXSLV.5.2.0/1990/01/MERRA100.prod\ .assim.tavg1_2d_slv_Nx.19900101.hdf?U50M_EOSGRID_Data_Fields[0:23][282:282][441\ :441]' =Dennis Heimbigner Unidata Taylor Binnington wrote:
Thank you Dennis. I'm using version 4.2.1, netcdf-cxx4 by Lynton Appel, on openSUSE 12.1, so I think it's fairly up-to-date. The problem is not that I am unable to open the file (I, too, can print out the 24 values), but that doing so apparently uses ~19 mb of bandwidth, according to the iftop monitoring utility. This seems quite excessive. Please let me know if there is specific information that I should provide here. Much appreciated, Taylor On Mon, Feb 18, 2013 at 5:57 PM, Dennis Heimbigner <dmh@xxxxxxxxxxxxxxxx>wrote:The opendap code in the netcdf-C libraries (used by the C++ library) will in fact properly handle the constraint you specified. I just ran it and as expected, it is only pulling in the 24 values you requested (took some 5 seconds to obtain and print out the data). One possibility is that you are using a rather old version of the netcdf-C library. =Dennis Heimbigner Unidata Taylor Binnington wrote:Hello, I have an 'understanding' type of question. I have omitted details of my OS, NetCDF installation, and a minimal example since I do not think they are important for my question, but I can provide these details if they are important. When I attempt to open a MERRA file using the NetCDF C++ libraries, with a line like the following, in my code: NcFile datafile(" http://goldsmr2.sci.gsfc.nasa.**gov/opendap/hyrax/MERRA/** MAT1NXSLV.5.2.0/1990/01/**MERRA100.prod.assim.tavg1_2d_** slv_Nx.19900101.hdf?U50M_**EOSGRID_Data_Fields[0:23][282:**282][441:441]<http://goldsmr2.sci.gsfc.nasa.gov/opendap/hyrax/MERRA/MAT1NXSLV.5.2.0/1990/01/MERRA100.prod.assim.tavg1_2d_slv_Nx.19900101.hdf?U50M_EOSGRID_Data_Fields[0:23][282:282][441:441]> ",NcFile::read); ...I notice that quite a lot (nearly 20 MB) is being transferred from Hyrax to my PC. However, all that I am interested in are the actual array values (24 separate 32-bit real numbers) of the U50M_EOSGRID_Data_Fields. It has been suggested that the entire array (of size 24*361*540 32-bit reals) is being transferred by NetCDF before any subsetting takes place, is this true? If so, is it then not possible to use OPeNDAP URLs to subset on-the-fly through the NetCDF libraries? Thank you for your help! Taylor ------------------------------**------------------------------** ------------ ______________________________**_________________ netcdfgroup mailing list netcdfgroup@xxxxxxxxxxxxxxxx For list information or to unsubscribe, visit: http://www.unidata.ucar.edu/**mailing_lists/<http://www.unidata.ucar.edu/mailing_lists/>
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