On Feb 19, 2013, at 10:48 AM, Taylor Binnington <tbinnington@xxxxxxxxx> 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
Taylor,
I think Benno might have put his finger on the issue over on the opendap-tech
mailing list. That is, instead of passing the fully constrained URL to a read
function, open the URL without the constraints and then use one of the methods
that allows you to specify the start and length (or corner points) of the
segment you want to retrieve. I know they must be there somewhere, because the
C API certainly has it.
>
>
> 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]
> ",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
>
>
>
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> --
> Taylor Binnington
> e. tbinnington@xxxxxxxxx
> c. 647 926 4144
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--
Dr. Christopher Lynnes, NASA/GSFC, ph: 301-614-5185
