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Next: Errors, Previous: Data Model, Up: Intro
With netCDF-4, the netCDF data model has been extended, in a backwards compatible way.
The new data model, which is known as the “Common Data Model” is part of an effort here at Unidata to find a common engineering language for the development of scientific data solutions. It contains the variables, dimensions, and attributes of the classic data model, but adds:
These features may only be used when working with a netCDF-4/HDF5 file. Files created in classic or 64-bit offset format cannot support groups or user-defined types.
With netCDF-4/HDF5 files, the user may define groups, which may contain variables, dimensions, and attributes. In this way, a group acts as a container for the classic netCDF dataset. But netCDF-4/HDF5 files can have many groups, organized heirarchically.
Each file begins with at least one group, the root group. The user may then add more groups, receiving a new ncid for each group created.
Since each group functions as a complete netCDF classic dataset, it is possible to have variables with the same name in two or more different groups, within the same netCDF-4/HDF5 data file.
Dimensions have a special scope: they may me seen my all variables in their group, and all descendant groups. This allows the user to define dimensions in a top-level group, and use them in many sub-groups.
Since it may be necessary to write code which works with all types of netCDF data files, we also introduce the ability to create netCDF-4/HDF5 files which follow all the rules of the classic netCDF model. That is, these files are in HDF5, but will not support multiple unlimited dimensions, user-defined types, groups, etc. They act just like a classic netCDF file.
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