Vocabulary Service Standard

Call for Participation

Executive Summary

In today’s increasingly digital world, sharing and understanding data consistently across organizations and borders has become essential. A fundamental requirement for effective data sharing is using clear and universally understood terminologies, known as “vocabularies.” Currently, there is no widely accepted international standard or unified approach for managing and distributing these vocabularies via online services. This lack of standardization creates confusion, redundancy, and inefficiency as organizations struggle to combine, manage, and interpret data from multiple sources. To emphasize a critical point: While we aim to develop a universally accepted standard for vocabulary services (i.e., the digital services that provide access to vocabularies), we are explicitly not pursuing a single universal set of vocabularies—in fact, the goal is to support diversity and foster reuse, which ultimately enhances interoperability.

To address these challenges, the Open Geospatial Consortium (OGC) proposes developing a new international standard—a “Vocabulary Service Standard”—that will simplify how vocabularies are shared, managed, and used across diverse systems. This standard establishes consistent methods for accessing vocabulary information, tracking changes, and documenting sources and updates. By standardizing vocabulary services, the OGC seeks to enhance interoperability, meaning different systems and organizations can seamlessly understand and use each other’s data, while significantly reducing the complexity and cost of managing data and terminologies.

The proposed Vocabulary Service standard will benefit technical experts, policymakers, and organizations by enabling clearer communication, improving the governance of data resources, and fostering greater collaboration and innovation across sectors and international boundaries.

Introduction

A vocabulary service is a digital service that provides access to structured sets of terms—called vocabularies—used to describe, categorize, or annotate data consistently across systems. These vocabularies can include controlled vocabularies, taxonomies, thesauri, ontologies, and code lists.

A vocabulary service typically allows users and systems to:

• Discover vocabularies relevant to a domain or task
  
  
• Access vocabulary content in standardized formats (e.g., SKOS, JSON-LD, RDF)
  
  
• Search and browse terms, definitions, relationships, and metadata
  
  
• Resolve identifiers (e.g., URIs) for specific terms to retrieve human- and machine-readable descriptions
  
  
• Reuse and reference terms in their own data or metadata
  
  
• Version and maintain vocabularies over time

A vocabulary service aims to promote semantic interoperability—the ability of systems to exchange data with unambiguous, shared meaning—by making vocabularies discoverable, accessible, and usable across different domains and applications. Despite this importance, no modern standard currently exists for sharing vocabularies via APIs, and no international standards organization has a working group addressing this gap. Existing community initiatives reflect a fragmented landscape, with lots of different vocabulary reuse patterns in use. The following figure illustrates the situation. Pattern 1 integrates all elements found elsewhere in a copy-paste-reuse approach. Pattern 2 illustrates a reuse-through-matching approach, which maps elements in one repository to the elements in another. Explicit filters are used in pattern 3, while pattern 4 shows the classic mix of partially agreed vocabularies, partial copies of existing vocabularies, and finally selected options that no longer have any connection to their original sources.

The variety of patterns is a significant obstacle to interoperability. Every copy of a vocabulary carries the risk that changes to the original will not be noticed or adjustments will be made in the copies that cannot be traced back to the original. What’s required are canonical forms of provenance metadata and robust synchronization mechanisms to support federation and subsetting across distributed systems. A standard for federated vocabulary services must address access, synchronization, and subset identification consistently. Moreover, governance and provenance frameworks are essential to ensure the integrity and traceability of vocabularies. Given the geographic dimensions of many vocabularies, federation becomes particularly relevant to OGC, and aligning these services with other OGC APIs would strengthen the interoperability of the broader OGC ecosystem. To address these needs, a proposal is set forth to establish an OGC Standards Working Group to develop and test vocabulary service models and collaborate with external standards groups. This would ensure compatibility with OGC’s RAINBOW infrastructure and support the evolution of federated, provenance-aware vocabulary services as part of a globally coherent open data environment.

The following list shows the logic behind the planned effort:

• Sharing data means sharing the understanding of terminology
  
  
• Vocabulary services provide access to vocabularies
  
  
• There is no accepted modern service standard for sharing vocabularies via API
  
  
• There is no international standards organisation with working groups working on a vocabulary service standard
  
  
• Several initiatives are happening within communities of practice
  
  
• Vocabulary sharing often requires aggregation from multiple sources
  
  
• Large, aggregated vocabularies often require the declaration of “relevant subsets” when used in more specific contexts.
  
  
• Federation and subsetting require canonical forms of provenance metadata and synchronisation
  
  
• Federated vocabularies require standardization of access, synchronization, and identification of subsets of larger vocabularies.
  
  
• Governance and provenance arrangements need to be established
  
  
• Federation is intrinsically a geographic concern in many cases; hence, OGC has a specific interest
  
  
• Federation Vocabulary services, compatible with other OGC APIs, extend the power of OGC-enabled ecosystems.
  
  
• This proposal is made to initiate an OGC Working Group that can test ideas and seek out liaisons with other relevant standards groups.

This proposal aims to raise international awareness of the problem of shared and reused vocabularies and to create a solution in the form of a standardized vocabulary service.

Background

In an increasingly interconnected digital ecosystem, using common terminology, accessible in a standardised way through vocabulary services, plays a pivotal role in ensuring that diverse systems can communicate, integrate, and interpret information consistently. As data is exchanged across organizational, national, and disciplinary boundaries, the meaning behind terms must remain clear, precise, and interoperable. Vocabulary services provide the mechanisms to manage, publish, and maintain controlled vocabularies, taxonomies, ontologies, and code lists that underpin semantic interoperability. Without them, efforts to harmonize data sets, develop reusable services, or enable federated knowledge systems will be fragmented, error-prone, and unsustainable. Each data user suffers the burden of discovery and interpretation, without the means to resolve any ambiguities. Effective vocabulary services allow different actors to reference common concepts, track provenance and evolution of terms, and support machine-actionable understanding of data, thereby forming a foundational element of modern open data infrastructures, FAIR (Findable, Accessible, Interoperable, Reusable) data principles, and semantic web technologies.

The challenge, however, is that the state of the art for vocabulary services today is fragmented and insufficient. There is currently no standardized vocabulary service widely accepted or implemented in practice. Several efforts tied to particular software platforms exist and can provide insights into what a truly interoperable solution might look like.

While well-known vocabulary data models include SKOS (Simple Knowledge Organization System), OWL (Web Ontology Language), and RDFS (Resource Description Framework Schema), these only define the structure and relationships within vocabularies. They do not define how a service offering vocabularies should behave or operate.

One major issue is that the “Web of Data”, which was envisioned to rely heavily on shared vocabularies, is challenging to realize partly due to this lack of a common vocabulary service model. As a result, interoperability, a cornerstone principle, is heavily compromised when dealing with vocabularies across different systems.

Requirements

Analyzing the current situation, we need to distinguish between standards for vocabularies and standards for vocabulary services. The W3C standards, such as SKOS, are solid and implementable for vocabulary structures and can be used as starting points for discussions (starting points because they lack some essential features, particularly when it comes to using subsets of existing vocabularies). On the other hand, various ISO Standards exist. These ISO guidelines are helpful for designing processes, but do not translate into service APIs or operational behavior.

The goal is to define a service approach independent of any specific software platform. Although many European institutions use the same platform, relying on a single backend technology is not the answer. Instead, a standard API and behavior definition, regardless of implementation technology, is needed to enable true interoperability.

Several critical capabilities are involved in vocabulary service functionality. A vocabulary service must be able to serve individual terms and manage sets and subsets of terms. It should advertise and describe different subsets of vocabularies and explain relationships between them, including overlaps and extensions.

Moreover, the service must enable consumers to retrieve not just the content but also the provenance information, indicating the origin and management history of the vocabularies and their subsets. Today’s absence of such capabilities forces users to manually copy vocabularies, fragmenting knowledge and preventing efficient reuse. The ideal vocabulary service must support these complex relationships, dynamic updates, and provenance, enabling genuine distributed and federated use of vocabularies across organizations.

Other initiatives

Other organizations also consider provisioning vocabularies and vocabulary services.

A workshop to consider vocabulary federation is planned (submission pending) at SciDataCon 2025 (13-16 October 2025): Overview · Indico.

Participants should include the OntoPortal Alliance | OntoPortal, a federation based on a specific technology with likely highly relevant use cases and APIs. However, an external standards organization will need to host things.

Draft APIs for Vocabulary services have been discussed in previous testbeds. Still, they have not been formally reviewed against the requirements discussed here or considered seriously for adoption as an SWG deliverable.

Proposal

It is proposed that an OGC Standards Working Group (SWG) be established to develop a Vocabulary Service API with two conformance classes:

1. Vocabulary Access
2. Vocabulary Management

This SWG should also define how vocabulary services, particularly unions and subsets of vocabularies, should be referenced in service and dataset metadata standards such as OGC API Records, STAC, ISO 19115, etc.

These should be implemented as OGC APIs, using the Building Block testing approach to ensure that any solutions are tested to be compatible with other OGC API extensions.

This SWG should be supported by formally resourced activities from OGC Research to ensure that the inputs and outputs take into account existing research results and implementations such as OGC RAINBOW. At the same time, this initiative shall support OGC RAINBOW’s evolution into a compatible infrastructure. 

Activities should actively seek collaborations with external activities to the greatest extent possible. The proposed SWG may operate under the auspices of existing OGC working groups, such as “Architecture” or “Geo-Semantics DWG.” It may co-publish through the OGC/W3C Spatial Data on the Web WG and be actively promoted through the W3C community as a joint activity at all stages.

Comments or Questions?
Please email info@ogc.org with any comments or questions about this call for participation.