1.2.2 Ancillary Data

In order for an OPeNDAP client to accept data from an OPeNDAP server, it must be able to allocate the data structures and arrange internal labels to organize the incoming data. The information the client library needs to do this organizing is called the ancillary data⁵. For many APIs, the ancillary data is inherent in the data files themselves, and the OPeNDAP server can glean that information by scanning the data files. For large data archives, where scanning the data files is impractical, and that might not change often, OPeNDAP can cache the ancillary data to speed access times. When a client requests the ancillary data, the OPeNDAP server can check this data cache first before scanning the data files.

This feature is useful in other cases because not all data file formats are self-describing. For example, a data set might contain several files of time vs. temperature data; the header information describing which numbers are temperature and which time may be in a different file or may simply be understood by the user of the local data analysis program equipped to look at this data. As an example, data accessed by OPeNDAP servers using the FreeForm data access API require provider-created ancillary data files.