For Earth observation data discovery, it is essential to know about various used catalog protocols that define standardized methods and formats for organizing, describing, and accessing Earth observation data catalogs.
Some commonly used protocols shared between diffent platforms include OpenSearch, OGC CSW, OGC API – Records, STAC, OData and more.
OpenSearch
The OpenSearch specification was launched in 2005 by A9.com, an Amazon subsidiary, as a means for sharing search queries and search results in a standardized format. The specification was intended to allow syndication of search results that could then be aggregated by one large index. OpenSearch provides a simple to use description of the search interface, which is called an OpenSearch Description document (OSDD). A client (e.g., a browser) can use this description to check which response formats are supported and how a query/filter can be formulated. The OpenSearch based REST services are usually offered by existing EO data platforms for compatibility reasons as the protocol itself is stable and not extended anymore. The data models of most catalogs build on top of XML or GeoJSON and allow filtering on a set of simple but not standardized properties. The protocol supports both textual and geospatial search and filtering capabilities, making it suitable for a wide range of applications, including web search engines and geospatial data catalogs.
OGC CSW
Catalogue Service for the Web (CSW) standardized by Open Geospatial Consortium (OGC), offers a framework for publishing, discovering, and accessing metadata records, allowing users to effectively search and retrieve geospatial data and related information. The catalogue is made up of records that describe geospatial data, linked geospatial services and related resources. CSW enables metadata query using metadata core (mandatory) elements. Catalogue services support the use of one of several identified query languages to find and return results using well-known content models (metadata schemas) and encodings.
OGC API – Features
OGC API – Features is a modern and flexible geospatial data access protocol developed by the OGC. It provides a standardized and RESTful approach for querying, retrieving, and manipulating geospatial feature data over the web. By leveraging the power of web technologies such as HTTP, JSON, and GeoJSON, OGC API – Features simplifies the process of accessing and working with geospatial data. It allows users to retrieve specific features based on spatial and attribute filters, perform spatial and attribute queries, and even modify feature data through standard HTTP methods.
OGC API – Records
OGC API – Records is a multi-part draft specification (built on top of OGC API – Features) that offers the capability to create, modify, and query metadata on the Web. The draft specification enables the discovery of geospatial resources by standardizing the way collections of descriptive information about the resources (metadata) are exposed. The specification also enables the discovery and sharing of related resources that may be referenced from geospatial resources or their metadata by standardizing the way all kinds of records are exposed and managed.
ODAta
OData (Open Data Protocol) specifies a variety of best practices for creating and using REST APIs that can be handled by a large set of client tools like common web browsers, download-managers. The OData protocol can be used for building URI for performing search queries and product downloads for example on the Copernicus Dataspace.
STAC
STAC stands for SpatioTemporal Asset Catalog. It is a community specification that provides a common way for describing and cataloging assets that have a connection to space and time, usually but not necessarily on the Earth. The STAC specification focuses on organizing and sharing geospatial data in a way that is accessible, interoperable, and scalable. The STAC Specification consists of 4 semi-independent specifications (Catalog, Collection, Item and API) which can work independently or be used together. All of them can be and are enriched by a variety of extensions. It is a relatively new specification but increasingly integrated by various data providers and seen as future of EO Data cataloguing and discovery. The data model in the dataspace is still evolving to comply fully with all standardized properties. Because of that, more attention is provided to STAC than other catalogue protocols in this tutorial.