OPeNDAP
Encyclopedia
OPeNDAP, an acronym for "Open-source Project for a Network Data Access Protocol", is a data transport architecture and protocol widely used by earth scientists. The protocol is based on HTTP and the current specification is OPeNDAP 2.0 draft. OPeNDAP includes standards for encapsulating structured data, annotating the data with attributes and adding semantics that describe the data. The protocol is maintained by OPeNDAP.org, a publicly-funded non-profit organization that also provides free reference implementations of OPeNDAP servers and clients.
An OPeNDAP client could be an ordinary browser, although this gives limited functionality. Usually, an OPeNDAP client is a graphics program (like GrADS, Ferret
or ncBrowse) or web application (like DChart) linked with an OPeNDAP library.
An OPeNDAP client sends requests to an OPeNDAP server, and receives various types of documents or binary data as a response. One such document is called a DDS (received when a DDS request is sent), that describes the structure of a data set
. A data set, seen from the server side, may be a file, a collection of files or a database. Another document type that may be received is DAS, which gives attribute values on the fields described in the DDS. Binary data is received when the client sends a DODS request.
An OPeNDAP server can serve an arbitrarily large collection of data. Data on the server is often in HDF
or NetCDF
format, but can be in any format including a user-defined format. Compared to ordinary file transfer protocols (e.g. FTP
), a major advantage using OPeNDAP is the ability to retrieve subsets
of files, and also the ability to aggregate data from several files in one transfer operation.
OPeNDAP is widely used by governmental agencies such as NASA
and NOAA to serve satellite, weather and other observed earth science data.
An OPeNDAP client could be an ordinary browser, although this gives limited functionality. Usually, an OPeNDAP client is a graphics program (like GrADS, Ferret
Ferret Data Visualization and Analysis
Ferret is an interactive computer visualization and analysis environment designed to meet the needs of oceanographers and meteorologists analyzing large and complex gridded data sets. Ferret offers a Mathematica-like approach to analysis; new variables may be defined interactively as mathematical...
or ncBrowse) or web application (like DChart) linked with an OPeNDAP library.
An OPeNDAP client sends requests to an OPeNDAP server, and receives various types of documents or binary data as a response. One such document is called a DDS (received when a DDS request is sent), that describes the structure of a data set
Data set
A data set is a collection of data, usually presented in tabular form. Each column represents a particular variable. Each row corresponds to a given member of the data set in question. Its values for each of the variables, such as height and weight of an object or values of random numbers. Each...
. A data set, seen from the server side, may be a file, a collection of files or a database. Another document type that may be received is DAS, which gives attribute values on the fields described in the DDS. Binary data is received when the client sends a DODS request.
An OPeNDAP server can serve an arbitrarily large collection of data. Data on the server is often in HDF
Hierarchical Data Format
Hierarchical Data Format is the name of a set of file formats and libraries designed to store and organize large amounts of numerical data...
or NetCDF
NetCDF
NetCDF is a set of software libraries and self-describing, machine-independent data formats that support the creation, access, and sharing of array-oriented scientific data. The project homepage is hosted by the Unidata program at the University Corporation for Atmospheric Research...
format, but can be in any format including a user-defined format. Compared to ordinary file transfer protocols (e.g. FTP
File Transfer Protocol
File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...
), a major advantage using OPeNDAP is the ability to retrieve subsets
Subsetting
Subsetting is a concept used in research communities to describe the process of retrieving just the parts of large files which are of interest for a specific purpose. This occurs usually in a client—server setting, where the extraction of the parts of interest occurs on the server before the data...
of files, and also the ability to aggregate data from several files in one transfer operation.
OPeNDAP is widely used by governmental agencies such as NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
and NOAA to serve satellite, weather and other observed earth science data.
External links
- OPeNDAP.org
- THREDDS --Thematic Realtime Environmental Distributed Data Services
- dapper - OPeNDAP server for in-situ data
- DChart - Web viewer for NOAA Observing System data (in-situ data)
- GrADS
- ncBrowse - Java viewer for OPeNDAP netCDF files (supports wide range of netCDF conventions)
- NCAR Command Language - analysis and visualization software
- Ferret
- Pydap - client/server implementation written in Python
- ERDDAP - OPeNDAP server for gridded and tabular data; supports a wide range of output file formats
- NASA GCMD OPeNDAP portal Global Change Master Directory (GCMD)Global Change Master Directory (GCMD)The Global Change Master Directory is a directory of Earth Science data sets and related tools/services, many of which are targeted for the use, analysis, and display of the data. The directory holds more than 23,000 data set descriptions, known as DIFs...
- Asia-Pacific Data Research Center -- a textbook example OPenDAP implementation