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Dimensions

A netCDF dimension is a named integer used to specify the shape of one or more of the multi-dimensional variables contained in a netCDF file. A dimension may be used to represent a real physical dimension, for example, time, latitude, longitude, or height. A dimension might also be used to index more abstract quantities, for example, color-table entry number, instrument number, station-time pair, or model-run ID.

Every netCDF dimension has both a name and a size. A dimension name is an arbitrary sequence of alphanumeric characters (as well as the underscore character, `_', and the hyphen character, `-') beginning with a letter. Case is distinguished in netCDF names. A dimension size is an arbitrary positive integer, except that one dimension in a netCDF file can have the size UNLIMITED. Such a dimension is called the unlimited dimension or the record dimension. A variable with an unlimited dimension can grow to any length along that dimension. The unlimited dimension is like a record number in conventional record-oriented files.

CDL dimension declarations may appear on one or more lines following the CDL keyword dimensions. Multiple dimension declarations on the same line may be separated by commas. Each declaration is of the form name = size.

There are four dimensions in the example: lat, lon, level, and time. The first three are assigned fixed sizes; time is assigned the size UNLIMITED, which means it is the unlimited dimension. A netCDF file can have at most one unlimited dimension, but need not have any.

There are several uses for netCDF dimensions:

• Specifying the shapes and sizes of variables.
• Identifying and relating variables that are defined on a common grid.
• Providing a way to define coordinate systems.

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