OPeNDAP Data Access System

Open-source Project for a Network Data Access Protocol (OPeNDAP) is a data transport and protocol, based on HTTP, used by Earth Scientists. OPeNDAP specifies how to request a subset of a large dataset. It also specifies the over-the-network format for the response. The simplicity, flexibility, and efficiency of the protocol have led to its wide usage in the oceanographic and other scientific communities. There are many DAP compatible servers such as Hyrax, THREDDS, PyDAP,  and ERDDAP. OPeNDAP servers manage large amounts of remotely sensed and model data, typically in grid formats such as NetCDF, GRIB, and HDF.

OPeNDAP can be used to:

–Make netCDF data files available over the Internet
–Adapt existing software that use the netCDF API to read data served by an OPeNDAP data server
In general, any program that uses netCDF can become a client in the OPeNDAP client-sever system.

OPeNDAP Response Types

OPeNDAP provides three basic data types, and Request to Client OPeNDAP will result in following three different requests for data to OPeNDAP Server:

  • Request  for the Data Descriptor Structure (DDS):  is a data structure to describe datasets and subsets of those datasets – Shape of Data
    • Syntactic Metadata - Rigid
  • Request for the Data Attribute Structure (DAS):  is a set of name-value pairs used to describe the data in a particular dataset
    • Semantic Metadata - Flexible
  • Request for actual Data (DODS):  is data structure used by the DODS software to describe datasets

OPeNDAP Client-Server Architecture

OPeNDAP Servers:

•Tables (JGOFS)

OPeNDAP Clients:

•Ferret and GrADS (netCDF C)
•IDV, VisAD, and ncBrowse (netCDF Java)

OPeNDAP Services at GSFC

Currently, the GES DISC offers the datasets listed at through OPeNDAP.
For example, AIRS data are made available through

OPeNDAP Support of IDV, GrADS, and Panoply

There are a variety of tools and file formats used with Earth science data, including IDV, GrADS, and Panoply. In general, IDV and Panoply have the easiest learning curves while GrADS is somewhat harder.OPeNDAP allows for remote access of data at a sub-file level, making the data easier to use.

IDV Example
Carbon Monoxide Plume from California wildfires, seen in AIRS Daily Level 3 (AIRX3STD) on 29 Aug 2009:
Panopoly Example:

Tropospheric NO2 from OMI for July 7, 2010:

Other Examples: 

Additional examples for Panoply and IDV, as well as examples for GrADS can be found at: