The OPeNDAP Framework
OPeNDAP (Open-source Project for a Network Data Access Protocol) is a framework that simplifies all aspects of scientific data networking.
OPeNDAP makes remote, scientific data accessible over the internet, through
familiar data analysis and visualisation packages. OPeNDAP has evolved from
the Distributed Oceanographic Data
System (DODS). The terms OPeNDAP and DODS are currently interchangeable.
The OPeNDAP framework uses a client/server model. The client, the user’s
application program, sends requests for data over the network to an OPeNDAP
server. The server takes the request for data and delivers the data directly
to the client program in the format needed by that client. The client and server
communicate using Uniform Resource Locator (URL) addresses. Each data set on
an OPeNDAP server has a unique URL. Any data in the self-describing formats
commonly used by oceanographers such as netCDF can easily be served to client
applications via an OPeNDAP server.
OPeNDAP URLs
With OPeNDAP, you access data using a URL, just like a URL you
would use to access a web page. Using the URL, you can subsample the data you
are looking at. That is, you can request an entire data file, or just a small
piece of it. The OPeNDAP
Quick Start Guide describes how to form an OPeNDAP URL. It will also
familiarise you with the OPeNDAP dataset descriptors and how to use the DODS Dataset
Query Form to constrain your data request and subsample the data.
Briefly, OPeNDAP data URLs end in .nc (for netCDF files, or .hdf for HDF formatted files). For example, http://dods.acecrc.org.au/cgi-bin/nph-dods/woce3/sat_sst/avhrr/data/10_deg/sst10d19950107.nc
To access most of the features of OPeNDAP servers, append one of the following
suffixes to the URL: .ascii, .das, .dds, .dods., .info, .ver or .help. Using
these suffixes, you can ask the server for:
| asc or ascii |
ASCII representation of the data |
| das |
attribute object |
| dds |
data type object |
| dods |
data object, encoded as a Multipurpose Internet Mail Extensions (MIME)
document |
| info |
info object (attributes, types and other information) |
| html |
html form for the dataset (DODS Data Access Form) |
| ver |
return the version number of the server |
| help |
help information |
For example, to request the DODS Data Access Form for the OPeNDAP
URL listed above, you would append '.html' to the URL: http://dods.acecrc.org.au/cgi-bin/nph-dods/woce3/sat_sst/avhrr/data/10_deg/sst10d19950107.nc.html
See the OPeNDAP
Quick Start Guide for more information on how to use the DODS Data Access
Form.
Configuring analysis programs to work with OPeNDAP
Free OPeNDAP software transforms common analysis programs into to OPeNDAP clients
by re-linking the programs to OPeNDAP-enabled code libraries. This process does
not interfere with the programs’ other capabilities. Setting up an analysis
application to work as an OPeNDAP client is a fairly simple process, but may
require assistance from local IT support. The OPeNDAP software can be downloaded
from the OPeNDAP
Client Software page. Documentation on compiling OPeNDAP clients can be found
on the OPeNDAP web site.
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