For the data obtained from observation or processing to be usable, sometimes after a long time, by teams which were not directly involved in the initial acquisition, descriptions must be associated with the observations. These descriptions are known as metadata (i.e. data about data) and, for observations of the Earth (or sea!), include: the geographical location (latitude, longitude, depth), the observation data and time, the instrument (or type) used, the scientist(s) who supervised the measurements, the context of the measurement (project, cruise or dive, contract, etc.) which also provides information on the objectives of the initial observation and its degree of accuracy.

Two categories of metadata can be distinguished:

Discovery metadata

This information informs users of the existence of an observation (or datum, map, etc.) and determines its utility in relation to their own needs.

Under the European Directive INSPIRE, as well as other European directives related to environmental data, it is mandatory to provide a description of available datasets and to generate discovery metadata for observations and data generated using public funding.
Discovery metadata must be standardised, to make the descriptions homogeneous and facilitate their use. The family of ISO 19115 standards is mandatory (the Darwin Core standard may constitute an alternative for biodiversity data). These metadata are made available to users via online web portals (via the "ACCESS DATA" section of this portal for instance, or the GeoCatalogue section of the French national GéoPortail).

Usage metadata

Once the useful data have been identified, usage metadata allow the data to be used, analysed, compared, etc.

These metadata are more detailed and intrinsically linked to the data they describe (in general in the same file) and include, in addition to discovery metadata: precise descriptions of the parameters (or variables) measured, units and methods used, quality indicators or confidence intervals, data from other sensors required for processing (e.g. ship navigation data for the cruise). The formalism of usage metadata is variable according to the type of data. It is often established through collaboration or international programmes and is described in the data format.