NetCDF data and datafiles
This is intended as a short introduction to netCDF data.
The complete netCDF documentation can be found at
the UNIDATA web site.
Quick links to
What is netCDF?
NetCDF (network Common Data Form) is an interface for array-orientated
data access and a library that provides an implementation of that
interface. Many groups have adopted netCDF as a standard way to
represent their scientific data. The netCDF software was developed
at the Unidata Program Center in Boulder Colorado USA.
Why use netCDF?
- NetCDF is used extensively in the atmospheric and oceanic
- NetCDF is a portable self-describing binary data format.
- NetCDF is network-transparent, meaning that it can be accessed by computers
that store integers, characters and floating-point numbers in different ways.
- NetCDF provides direct-access: a small subset of a large dataset may be
accessed efficiently, without first reading through all the preceding data.
- NetCDF is appendable: data can be appended to a netCDF dataset along one
dimension without copying the dataset or redefining its structure.
- NetCDF datasets can be read and written in a number of languages,
these include C, C++, FORTRAN, IDL, Python, Perl, and Java.
- The different language implementations
are freely available
from the UNIDATA ftp area
- Several graphics packages support netCDF input, making it
very easy to display and analyse netCDF datasets. For instance
provide both command line and graphical user interfaces for
displaying and analysing gridded data.
- NetCDF is completely and methodically documented in UNIDATA's
- Several groups have
defined conventions for netCDF files, to enable the
exchange of data. BADC has adopted the
Climate and Forecasting (CF) conventions for
NetCDF and CF conventions
The BADC supports and strongly recommends the compliance with the
and Forecast (CF) Metadata Convention.
CF conventions are guidelines and recommendations as to where to put information
within a netCDF file, and they provide advice as to what type of information you
might want to include.
CF conventions allow the creator of the dataset to include information
about the data and the dataset itself (metadata) in a structured way, which makes
it easier for other users to retrieve the information.
describe the general properties and origins of the dataset
while local attributes are used to characterise the recorded variables.
The BADC provides Information on the CF Convention,
illustrated by some examples.
What's in a netCDF file?
We have available a very simple example of a
netCDF file with CF conventions applied. A netCDF dataset (the filename
ends in .nc) is made up of three basic components:
The variables store the actual data, the dimensions give the
relevant dimension information for the variables, and the attributes
provide auxiliary information about the variables or the dataset
itself. You can study an example of a netCDF file
and see more details of the components in a netCDF dataset. The
global attributes can also be added to the netCDF file, following
Reading and writing netCDF
The popularity of netCDF stems from its portability and relative
ease of use. Although it is a binary data format the netCDF distribution
comes complete with the ncdump utility which produces
an ASCII dump of the dataset and thereby provides a quick-look
facility. You can also generate netCDF files from ASCII data files using the
The datasets are commonly written and read using library routines.
The library routines are sensibly named and relatively consistently
named across the different language implementations.
Help and tools
- Guidance and tools provided by the BADC:
- Selected links to available online software packages to write, read and
handle NetCDF files:
for Manipulating or Displaying NetCDF Data
An near-exhaustive list of links to a variety of freely available
and commercial or licensed software packages to create or handle NetCDF
files, provided by
Unidata at UCAR (USA).
Most links below appear in this list.
Available from UCAR (USA), in two versions:
one for Linux, one for Windows.
Provided by the Atmospheric Radiation Measurement (ARM) Program (USA),
the ARM NetCDF Tool Suite (ANTS) is a collection
of netcdf tools and utilities providing various means of
creating and modifying netcdf files.
ANTS is based on NCTOOLS written by Chuck Denham.
Climate Data Analysis Tools available from the
Program for Climate Model Diagnosis
and Intercomparison (PCMDI) (USA).
NetCDF interface to matlab
Available from the Marine and Atmospheric Research Department of the
Commonwealth Scientific and Industrial Research Organisation (Australia).
The interface has options for automatically handling missing values,
scalefactors, and the permutation of hyperslabs.
It also has a simple syntax.
Interactive computer visualization and analysis environment designed
to meet the needs of oceanographers and meteorologists.
It takes NetCDF files as input and runs on most Unix systems, and on
Windows XP/NT/9x using X windows for display.
The FERRET site includes
Initially developed by Dominik Brunner (Switzerland) for HDF files,
the Handy IDL Program for HDF Output Plotting (HIPHOP)
has been expanded to accept NetCDF and GRIB files.
Available from the University of Osaka (Japan), in two versions:
one for MacIntosh, one for Windows.
The BADC also holds routines provided by Jonathan Crosier
(University of Manchester) to assist users of
data in reading the FAAM core data files with the Igor software.
These are freely available from the
- MEXNC, SNCTOOLS and NetCDF
Created by Chuck Denham to read and write NetCDF files.
Provided through SourceForge.
Will cease to be supported (but will still be available) from January
1st, 2007 onwards.
For a more extensive description you are urged to read the
UNIDATA documentation on netCDF.
The UNIDATA documentation also provides the
for netCDF and Best
Practice recommendations on writing netCDF files.
Climate and Forecast (CF) Metadata Convention
defines metadata that provide a definitive description of what the data in each variable represents, and of the spatial and temporal properties of the data.
The CF conventions generalize and extend the
conventions developed under the sponsoring of NOAA.