Unidata

• Groups are like directories in a file system, except they are all within a file.
• When using groups, the "ncid" becomes the ID of the file and the group within that file. That is, different groups in the same file will have different ncids.
• Every file contains at least the root group. The ncid of the root groups is returned on an open or create.
• Groups (other than the root group) are part of the enhanced model. Using groups means that no existing netCDF software (except ncdump) will be able to read the file.
• Dimensions are scoped such that they can be seen in all child groups.
• A global attribute is now global to the group, not the file.
• To read enhanced model netCDF-4 files in a general way, recursion is required.
• Use of Groups is optional, with backward compatibility maintained by putting everything in the top-level unnamed Group.
• Unlike HDF5, netCDF-4 requires that Groups form a strict hierarchy.

2009 Unidata NetCDF Workshop for Developers and Data Providers > Using Groups and the New NetCDF-4 Atomic Types