OpenDAP

OPeNDAP is an open software project and data access protocol designed to provide remote access to distributed, array-oriented scientific data. It enables clients to retrieve subsets of large multi-dimensional datasets without transferring entire files. Data are commonly stored in NetCDF, HDF, or other scientific formats and exposed through OPeNDAP servers over standard HTTP or HTTPS.

The core of OPeNDAP is the Data Access Protocol (DAP), which allows a client to request a dataset or a subset by attaching a constraint expression to the URL. The constraint expression specifies variables and ranges along dimensions. The server responds with a dataset descriptor that describes the data (and its metadata) and, if requested, the corresponding data payload. This mechanism supports efficient subsetting, selection, and aggregation while preserving metadata and data types. The protocol has evolved through versions such as DAP2 and DAP4, and remains aligned with modern web and data-portal practices.

OPeNDAP servers form the backbone of many data portals and are often used in conjunction with THREDDS Data Server and other discovery catalogs. Clients range from scripting environments and analysis tools to specialized desktop applications, with multiple libraries available to facilitate access to OPeNDAP endpoints (for example, Python and MATLAB ecosystems).

Originating in the late 1990s at UCAR for weather, climate, and oceanographic communities, OPeNDAP has grown into a standards-based, interoperable framework for sharing scientific data across institutions. It remains a widely adopted approach for programmatic data access, enabling efficient data retrieval and integration in research workflows.