NetCDF Format

SENTINEL-3 Altimetry netCDF Format

The netCDF format is extremely flexible and self-describing. It has been adopted as a de facto standard for many operational oceanography systems. The SRAL/MWR Level-2 product files also follow the climate and forecast netCDF conventions CF-1.4.

A netCDF file contains dimensions, variables and attributes.

A dimension may be used to represent a real physical dimension (e.g. time, latitude, longitude, or height). A dimension might also be used to index other quantities (e.g. waveforms index).

Variables are used to store the bulk of the data in a netCDF file. A variable represents an array of values of the same type. A variable has a name, a data type and a shape described by its list of dimensions, specified when the variable is created. A variable may also have associated attributes, which may be added, deleted or changed after the variable is created.

NetCDF attributes are used to store information about the data (ancillary data or metadata). Most attributes provide information about a specific variable. These are identified by the name of the variable, together with the name of the attribute. Examples of attributes are units, scale factor, offset to be added and reference time calendar.

An example of a netCDF file:

netcdf example {

  • dimensions:                                                             // dimensions name are declared first
    • time = 2680;
    double time(time);                                                   // variable [type] [name]([dimension])

    • time:long_name = "time";                               // variable attributes
      time:units = "seconds since 2000-01-01 00:00:00.0";

The Common Data Language (CDL) is used to describe the content of a data set. The CDL is textual notation that describes the netCDF object and it is readable by a human. The netCDF utility 'ncdump' converts netCDF objects binary to CDL text. The netCDF utility 'ncgen' creates netCDF binary files from CDL text files.

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