Re: [netcdfgroup] NetCDF external links

On Jul 13, 2016, at 6:04 PM, Roy Mendelssohn - NOAA Federal 
<roy.mendelssohn@xxxxxxxx<mailto:roy.mendelssohn@xxxxxxxx>> wrote:

Since Aleksander has kindly answered my first question, this raises a second 
question on HDF virtual files - is each file in the virtual file a "valid" file 
in and of itself, that is combined by whoever is the "base" file of the virtual 
file, or does it depend on the "base" file.

Letâs first inter-calibrate the HDF and netCDF vocabularies to avoid confusion:


  *   HDF5 dataset = netCDF variable
  *   HDF5 file = netCDF dataset.

About HDF5 virtual datasets:


  *   There are no HDF5 virtual files, only HDF5 virtual datasets inside a 
normal HDF5 file.
  *   The data for a HDF5 virtual dataset comes from source HDF5 datasets 
located in other HDF5 files.
  *   HDF5 virtual datasets can have âlocalâ data in addition to the data 
coming from source datasets.
  *   Every HDF5 file with virtual datasets is a valid HDF5 file. Source 
datasets are just ordinary HDF5 datasets so files with them are valid HDF5 
files, too.
  *   How the data from a source dataset is presented in its virtual dataset is 
controlled by a mapping between the two dataset's HDF5 dataspaces (start, 
stride, count, block).
  *   These mappings are set on HDF5 dataset creation and cannot be changed.
  *   There are HDF5 dataset access property settings that define how to deal 
with missing source dataset data. (See my previous email.) These can be changed 
each time a virtual dataset is opened for access.
  *   The fill value is used for any missing data in a virtual dataset.

Userâs guide for the virtual dataset feature is being prepared; in the 
meantime: https://www.hdfgroup.org/HDF5/Tutor/vds.html.

-Aleksandar