The OGC Definitions Server makes terminology and concepts published by the OGC accessible to users and systems via the Web.

  • Explains: Terms and their descriptions

  • Connects: Links between terms and uses within and outside OGC

  • Packages: Per-term or as-package options for downloading details singly or in bulk in machine-readable forms.

  • Helps you Discover: Search, browse and cross-referencing to help discover terms.

Using the Definitions Server

In most cases this is a simple as using the Web address (URI) of a term - the Definitions Server will redirect you to the description of that resource. See About: redirection

If a machine-readable form is required, this redirection will be based on the MIME-type requested. See About: content-negotiation

Additional flexibility is provided by alternative "information profiles" available dependending on the type of term. See About: content-negotiation-by-profile

If you need to access a set of terms, such as a code list, a data model or other set the Definitions Server provides these through both links and available profiles suitable for the type of resource.

More information

Up-to-date information about different aspects of the definition server can be accessed as help topics within the Definition Server itself, via the link: http://www.opengis.net/def/about

Contact

The OGC Definitions Server is managed by the OGC Naming Authority. Contact names@opengeospatial.org

Feedback

Feedback is always welcome on how to improve any aspect of the OGC Definitions Server.

If you intend to integrate or replicate aspects of the service please drop us a line so we can ensure future changes do not negatively impact you.

FAIR overview

The FAIR principles (Findable, Accessible, Interoperable, Reusable) have become part of the broader OGC strategic direction and are fully embodied in the Definition Server approach.

Findable

The Definitions Server is implemented using Linked Data principles - so the combination of stable URIs allowing references to be made from outside, and "follow your nose" navigation via links from one term to related *term*s provides enhanced findability.

Currently a limited search capability over Concept definitions is provided, however the contents are indexable by external search engines.

Accessible

The Definitions Server does not make any assumptions about the client software that may be used now or in the future to access definitions, other than the use of HTTP protocols. This enhances accessibility for different environments.

The "Web-friendly" way of using an identifier (i.e. a URL) to get more information is augmented by "content negotiation" - the Definitions Server can deliver both user-friendly Web pages and other forms of resource representations, e.g. JSON-LD or Turtle (TTL).

Figure 1 shows different views of a Concept HY_Feature. The left panel shows an HTML representation, the middle shows the same information using TTL, and the right using JSON. All three representations have the same content, but differ in its serialization/format. This allows both human users to explore the OGC Definition Server, as well as machines to process its content.

representations
Figure 1. Various representations of the same content (fragments, HTML left, TTL middle, JSON right)

Interoperable

The interoperability of these term URI behaviours and the resources they provide access to is a key goal. There are several aspects of this handled using different mechanisms:

  1. Content model: can the client understand how the data is structured?

  2. Encoding: can the client parse the response?

  3. Interaction: how can a client ask for the form it needs?

Content Interoperability

The Definitions Server uses the SKOS vocabulary as its primary data model. This is used in conjunction with the original data model ("native" model) for the type of resource - for example an Application Schema or a Specification, which determines what additional metadata and links are available.

Encoding Interoperability

The Definitions Server currently offers a range of encodings[http://www.opengis.net/def/about/encodings] for all terms: 1. HTML 2. JSON (using JSON-LD augmentations to specify URLs ) 3. RDF (as XML,TTL or JSON-LD) 4. Plain text

Use of JSON-LD is suggested for most applications because its simply JSON with extra annotations to turn relevant element names and values into unambiguous URIs. It’s an encoding of RDF, but easily parsed by browsers.

Where applicable certain types of resources are also available in the original or additional formats. For example Application Schemas will be made available as XML schema (XSD) and UML (XMI) forms.

Interaction (API) Interoperability

The Definitions Server maximises interoperability through use of HTTP to access resources.

Flexibility is managed through three key mechanisms based on W3C recommendations:

  1. separation of the concepts of "things" and "information resources representing things" [https://www.w3.org/2001/tag/issues.html#httpRange-14]

  2. Content-negotiation (asking for a specific encoding type in a HTTP GET response) [https://www.w3.org/Protocols/rfc2616/rfc2616-sec12.html]

  3. Content-negotiation-by-application-profile (CNAP) (asking for a particular view of available information) [https://www.w3.org/TR/dx-prof-conneg/]

NB. CNAP is a recommendation undergoing final steps towards Recommendation at W3C with input from OGC Definitions Server design. The full implementation of this specification is underway including formalisation of the different view profiles needed.

As needs are identified new profiles can be specified and arbitrary "graphs" created by custom API endpoints to deliver different views of the available semantic resources. Interoperability is supported by publishing these profiles as definition resources for reuse.

Reusable

The OGC Naming Authority manages the Definitions Server to ensure all term URIs are stable with transparent governance. These identifiers can thus be safely used in external context. All content is freely available for re-use. Re-use is envisaged largely through the machine-readable versions