[gradsusr] how to read a hdf5 format file by GrADS?

[James E Johnson]

On Wednesday 09 July 2014 17:38:52 chunchun wrote:
> Hi All,
> 
>     Is there anyone has any idea about reading hdf5 file by GrADS?
>     I followed http://www.iges.org/grads/gadoc/gradcomdxdfopen.html,and
>  tried ncdump on the file,but it didn't work.
> 
>    One file is attached here.
> 
>   Thanks a lot!
> 
> Chun
> 

You should get the newer ncdump with built-in HDF5/netCDF4 support. Then it 
will work dumping HDF5 or netCDF4 (which are actually special case HDF5 
files), unless its using some of the lower level obscure HDF5 object types.

If you dump it you'll see the file structure is:
netcdf NDVI_APR_30N070E {
dimensions:
        phony_dim_0 = 1201 ;
variables:
        float \30-second_ndvi_data(phony_dim_0, phony_dim_0) ;
data:

 \30-second_ndvi_data =
  0.4392157, 0.4431373, 0.4431373, 0.4431373, 0.4392157, 0.4392157,
    0.4313726, 0.4313726, 0.427451, 0.4313726, 0.427451, 0.4156863,
    .
    :
    0.3960785, 0.3960785, 0.3960785, 0.3960785, 0.3960785, 0.3960785,
    0.3960785 ;
}

This is a very poorly designed file which isn't taking advantage of HDF5's 
self-describing features, i.e. no variable level attributes to describe what 
the data represent, and no file level attributes to describe the product or 
file. It has no dimension/coordinates defined, but the variable is a square 
array of size 1201 x 1201, however one has no idea what the 1st 
dimension/coordinate represents and no idea what the 2nd dimension/coordinate 
represents. Its possible they are not coordinates at all, it could be a square 
matrix for all one knows. One is forced to have to rely on some external 
document to interpret what and how to use this type of data file. The HDF5 
file is so poorly designed you have no idea what the data are, other than rely 
on the variable name to know its some NDVI at 30-second whatever.

As for GrADS, you can construct a control file (.ctl) to read the single 
variable data array. But because this file FAILS to self-describe itself fully 
(other that it is 32-bit float array size 1201 x 1201), its unclear what if 
any lat/lon boundary the array has. Its up to you to locate the external 
documentation about what this data product file really is supposed to 
represent. Where did you get this file from?

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