Wiki Testing/OPeNDAPUserGuide3

The OPeNDAP Client

There are many different data analysis packages in use. Some packages, such as MATLAB and IDL, are commercially available, but many more are written for a specialized need or application. Many of these use one of the widely available sets of scientific data access functions (called an Application Program Interface, or API ) such as NetCDF, JGOFS, or HDF. There is great variety among all these programs, but one feature they share is that they all access data through files containing that data8. That is to say that each program begins by identifying a file containing the data the user wishes to examine or analyze.

An OPeNDAP client is simply a data analysis application linked with the OPeNDAP libraries instead of the standard data access API. Using this program, a user can look at files containing data in the same way as was possible without the OPeNDAP libraries. However, by using these libraries, a user can also use a URL (URL) , instead of a simple file name, to specify data located anywhere on the Internet. Figure 1.1.3 and figure 1.1.3 illustrate the operation of an application program linked with a standard data access API, and the same program linked with the OPeNDAP version of that API.

An OPeNDAP client is then a data analysis application program modified to become a web browser, somewhat like any other web browser (NCSA Mosaic, Netscape Navigator) with which you may be familiar. A web browser can only display the data it receives, however. What makes an OPeNDAP client different from another web browser is that, unlike Netscape, once the data has been received from an OPeNDAP server, the OPeNDAP client application can compute with it.

Like a web browser, an OPeNDAP client accepts a URL from a user, and parses it to come up with a protocol, an address, and a message. (See Section 2.1 for more information about URLs.) The browser then sends a message to the address, directed to the server who can service the desired protocol, asking for the information specified in the remainder of the URL. Unlike a typical web browser, an OPeNDAP client will not know what to do with data returned for a web page containing text and pictures, but an OPeNDAP server will return scientific data that an OPeNDAP client can understand and process.

Here is a simple example, using the ncview program. This program simply prints out the contents of a netCDF formatted data file, specified on the command line, like this:

> ncview fnocl.nc

Using OPeNDAP, this same function may be executed from any computer connected to the Internet by substituting a URL for the filename above:

> dncview http://dods.gso.uri.edu/cgi-bin/nc/data/fnocl.nc

(See figure 2.1 Aside from the fact that the data is remote, and must be specified with a URL, the program will seem to function in the same way it had with the simple netCDF library (albeit somewhat more slowly due to having to make network connections instead of local file operations). You can find dncview (the ncview program linked with the OPeNDAP library) in the

$DODS_ROOT/src/nc-dods/ncview

directory. Running the above command will produce the following output:

netcdf fnocl {
dimensions:
    time_a = 16
    lat = 17 ;
    lon = 21 ;
    time = 16 ;

variables:
    long u(time_a, lat, ion) ; 
        u:units = ``meter per second'' ; 
        u:long_name = ``Vector wind eastward component'' ; 
        u:missing_value = ``-32767'' ; 
        u:scale_factor = ``0.005'' ; 
    long v(time_a, lat, ion) ; 
        v:units = ``meter per second'' ;
        v:long_name = ``Vector wind northward component'' ;
        v:missing_value = ``-32767'' ;
        v:scale_factor = ``0.005'' ; 
    double lat(lat) ;
        lat:units = ``degree North'' ;
    double lon(lon) ;
        lon:units = ``degree East'' ; 
    double time(time) ;
        time:units = ``hours from base_time'' ;

// global attributes: 
        :base_time = ``88- 10-00:00:00'' ; 
        :title = ``FNOC UV wind components 
                           from 1988- 10 to 1988- 13.'' ;
data:
 u =
  -1728, -2449, -3099, -3585, -3254, -2406, -1252,
    662, 2483, 2910, 2819, 2946, 2745, 2734,
  2931, 2601, 2139, 1845, 1754, 1897, 1854, -1686,
...

Although there are packaged OPeNDAP browsing programs that a user can use to look at data, the user can also construct his or her own. Linking an OPeNDAP API with an already existing program allows a user to create a customized web browser that can access data available from any OPeNDAP server connected to the Internet.

The OPeNDAP APIs are designed to accurately mimic the behavior of several different commonly used scientific data APIs. As of this writing (August 25, 2004), the OPeNDAP API set includes:

API	Description	Components
netCDF	Support for gridded data, such as satellite data, interpolated ship station data, or current meter data.	Server and client.
JGOFS	Support for relational data, such as Sequences. Created by the Joint Globar Ocean Flux Study (JGOFS) project for use with oceanographic station data.	Server and client.
HDF	Support for gridded data. Commonly used for astronomical data and model data.	Server only.
DSP	Oceanographic and geophysical satellite data. Provides support for image processing. Developed at the University of Miami/RSMAS. Primarily used for AVHRR and CZCS data.	Server only.
GRIB	Support for gridded binary data. GRIB is the World Meteorological Organization (WMO) format for the storage of weather information and the exchange of weather product messages.	Server only, due in early 1999.
BUFR	The WMO's standard set of codes for the transmission and storage of meteorological data, using a compressed format with each data value occupying the least number of bits necessary to contain its range of values. Suitable for meteorological observations made from a single point or set of points.	Server only, due in early 1999.
FreeForm	On-the-fly conversion of arbitrarily formatted data, including relational data and gridded data. May be used for sequence data, satellite data, model data, or any other data format that can be described in the flexible FreeForm format definition language. This server can be used to serve data stored in almost all home-grown data formats.	Server only; no client required.
native OPeNDAP	The OPeNDAP class library may be used directly by a client program. It supports relational data, array data, gridded data, and a flexible assortment of data types that can be combined to c accommodate most data models.	Client.