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Foreword

Unidata is a National Science Foundation-sponsored program empowering U.S. universities, through innovative applications of computers and networks, to make the best use of atmospheric and related data for enhancing education and research. The Unidata Program Center currently offers universities several (supported) software packages for analyzing and displaying such data, some developed by Unidata and some by other organizations, including NASA, the University of Wisconsin, Purdue University, and MacDonald Dettwiler. Underlying these is a Unidata- developed system for acquiring and managing data in real time, making practical the Unidata principle that each university should acquire and manage its own data holdings as local requirements dictate. (It is significant that the Unidata program has no data center -- the management of data is a "distributed" function.)

The Network Common Data Form (netCDF) software described in this guide was originally intended to provide a common data access method for the various Unidata applications. These deal with a variety of data types that encompass single-point observations, time series, regularly-spaced grids, and satellite or radar images.

The netCDF software functions as an I/O library, callable from C or FORTRAN, which stores and retrieves scientific data structures in self- describing, machine-independent files. Each netCDF file can contain an unlimited number of multi-dimensional, named variables (with differing types that include integers, reals, characters, bytes, etc.), and each variable may be accompanied by ancillary data, such as units of measure or descriptive text. The interface includes a method for appending data to existing netCDF files in prescribed ways, functionality that is not unlike a (fixed length) record structure. However, the netCDF library also allows direct-access storage and retrieval of data by variable name and index and therefore is useful only for disk-resident (or memory- resident) files.

NetCDF access has been implemented in about half of Unidata's software, so far, and it is planned that such commonality will extend across all Unidata applications in order to:

Facilitate the use of common data files by distinct applications.
Permit data files to be transported between or shared by dissimilar computers transparently, i.e., without translation.
Reduce the programming effort usually spent interpreting formats in a way that is equally effective for FORTRAN and C programmers.
Reduce errors arising from misinterpreting data and ancillary data.
Facilitate using output from one application as input to another.
Establish an interface standard which simplifies the inclusion of new software into the Unidata system.

A measure of success has been achieved. The netCDF is now in use on computing platforms that range from CRAYs to Personal Computers and include many (UNIX- and VMS-based) workstations. It can be used to create a complex data set on one computer (say in FORTRAN) and retrieve that same (self-describing) data set on another computer (say in C) without intermediate translations -- netCDF files can be transferred across a network, or they can be accessed remotely using a suitable network file system.

Because we believe that the use of netCDF access in non-Unidata software will benefit Unidata's primary constituency -- such use may result in more options for analyzing and displaying Unidata information -- the netCDF library is distributed without licensing or other significant restrictions, and current versions can be obtained via anonymous FTP. Apparently the software has been well received by a wide range of institutions beyond the atmospheric science community, and a substantial number of public domain and commercial data analysis systems can now accept netCDF files as input.

Several organizations have adopted the netCDF as a data access standard, and there is an effort underway at the National Center for Supercomputer Applications (NCSA, which is associated with the University of Illinois at Urbana-Champaign) to support the netCDF programming interfaces as a means to store and retrieve data in "HDF files," i.e., in the format used by the popular NCSA tools. We have encouraged and cooperated with these efforts.

Questions occasionally arise about the level of support provided for the netCDF software. Unidata's formal position, stated in the copyright notice which accompanies the netCDF library, is that the software is provided "as is" ... In practice, the software is updated from time to time, and Unidata intends to continue making improvements for the foreseeable future. Because Unidata's mission is to serve atmospheric scientists at U.S. universities, problems reported by that community necessarily receive the greatest attention.

We hope the reader will find the software useful and will give us feedback on its application as well as suggestions for its improvement.

David Fulker
Unidata Program Center Director University Corporation for Atmospheric Research

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