2.4 Attributes

NetCDF attributes are used to store data about the data (ancillary data or metadata), similar in many ways to the information stored in data dictionaries and schema in conventional database systems. Most attributes provide information about a specific variable. These are identified by the name (or ID) of that variable, together with the name of the attribute.

Some attributes provide information about the dataset as a whole and are called global attributes. These are identified by the attribute name together with a blank variable name (in CDL) or a special null "global variable" ID (in C or Fortran).

In netCDF-4 file, attributes can also be added at the group level.

An attribute has an associated variable (the null "global variable" for a global or group-level attribute), a name, a data type, a length, and a value. The current version treats all attributes as vectors; scalar values are treated as single-element vectors.

Conventional attribute names should be used where applicable. New names should be as meaningful as possible.

The external type of an attribute is specified when it is created. The types permitted for attributes are the same as the netCDF external data types for variables. Attributes with the same name for different variables should sometimes be of different types. For example, the attribute valid_max specifying the maximum valid data value for a variable of type int should be of type int, whereas the attribute valid_max for a variable of type double should instead be of type double.

Attributes are more dynamic than variables or dimensions; they can be deleted and have their type, length, and values changed after they are created, whereas the netCDF interface provides no way to delete a variable or to change its type or shape.

The CDL notation for defining an attribute is

         variable_name:attribute_name = list_of_values;

for a variable attribute, or

         :attribute_name = list_of_values;

for a global attribute. For a group level attribute (netCDF-4 files only):

         :group_name/subgroup_name/attribute_name = list_of_values;

Groups will be created as needed to store the attributes.

The type and length of each attribute are not explicitly declared in CDL; they are derived from the values assigned to the attribute. All values of an attribute must be of the same type. The notation used for constant values of the various netCDF types is discussed later (see CDL Notation for Data Constants).

In the netCDF example (see The NetCDF Data Model), units is an attribute for the variable lat that has a 13-character array value 'degrees_north'. And valid_range is an attribute for the variable rh that has length 2 and values '0.0' and '1.0'.

One global attribute, called “source”, is defined for the example netCDF dataset. This is a character array intended for documenting the data. Actual netCDF datasets might have more global attributes to document the origin, history, conventions, and other characteristics of the dataset as a whole.

Most generic applications that process netCDF datasets assume standard attribute conventions and it is strongly recommended that these be followed unless there are good reasons for not doing so. For information about units, long_name, valid_min, valid_max, valid_range, scale_factor, add_offset, _FillValue, and other conventional attributes, see Attribute Conventions.

Attributes may be added to a netCDF dataset long after it is first defined, so you don't have to anticipate all potentially useful attributes. However adding new attributes to an existing classic or 64-bit offset format dataset can incur the same expense as copying the dataset. For a more extensive discussion see File Structure and Performance.