Defining concepts with semantic relations
A GACS concept is formally defined by its "semantic neighborhood", a web of associative and hierarchical relations with other concepts in GACS. The concept alfalfa, for example, is defined by relations such as:
- fodder plants
- fodder plants
- fodder plants
These relations are reflected in the GACS definition of alfalfa as: "A valuable leguminous crop for forage or hay used in livestock feeding". However, less than 20% of the concepts in GACS have definitions, and crafting definitions for the other 80% would be expensive, especially across translations in multiple languages. Moreover, narrative definitions are less useful than semantic relations for automated processing because relations between concepts can only roughly be computed from label strings, sentence parsing, and the co-occurrence of words.
Explicit semantic relations provide a basis for browsing broader, narrower, or related concepts and for expanding queries. They can be used to disambiguate meanings and to check concept hierarchies for consistency. Because they cluster related examples, they are less vulnerable than definitions to misinterpretation by translators. In GACS, definitions are "nice to have", but semantic relations are essential. For maintainability, the set of semantic relations used in GACS has been kept as simple and obvious as possible.
Broader, narrower, and related concepts
GACS concepts are linked among themselves with related, broader, and narrower relations as defined in SKOS, which are themselves based on standard thesaurus relations (RT, BT, and NT). Broader and narrower do not formally distinguish between "is a" and "part of" relations. In GACS, broader and narrower are generally intended to imply general-to-specific "is a" relations, though not in a formally strict ontological sense, and there are plenty of exceptions (for example, Maryland, which is "part of" the North Eastern States (USA)).
All concepts in GACS are grouped under three concepts that have no broader concepts (top-level concepts):
- Objects: material things that can be seen, touched, visited (locations), including conceptual objects such as ideas and models.
- Events and actions: things that happen, such as processes or phenomena.
- Properties: attributes, characteristics, or qualities of things.
In thesaurus practice, top concepts are typically intended for use in faceted browsing or in creating microthesauri. In GACS, the top concepts can be used as entry points for browsing GACS by thematic group in the Hierarchy tab of the Skosmos browser. The top concepts are too abstract to be used for indexing. Concepts further down in the hierarchy are more concrete and more useful both for indexing and for clarifying meanings (disambiguation). While a certain amount of polyhierarchy (where concepts have more than one broader concept) may be inevitable, even desirable, it seems unrealistic to expect that a complex polyhierarchy could be enforced and maintained in a principled way and at scale. For GACS, the goal is to to keep the hierarchy simple, and as close to the ideal of cascading "is a" relations, as pragmatically possible.
Differentiating between types of concept can help clarify the meaning of concepts across the hierarchy. While top concepts, with their hierarchies, might in principle be used to group concepts by the type of thing they represent, such as "organisms", the GACS hierarchy does not follow the principle of general-to-specific (hyponymy) strictly enough to ensure that "is a" ("type of") relations always hold; the hierarchy also contains "part of" (meronym) relations. The GACS Working Group opted to explore the usefulness of concept types by creating types of SKOS concept for just four particularly obvious and clearly defined types of things: Chemical, Geographical, Organism, and Product, with Topic as a fifth "catch-all" type for all concepts not otherwise covered (and with the expectation that Topic might be further differentiated over time).
Custom (non-SKOS) relations between concepts
Some thesauri, such as AGROVOC and CAB Thesaurus, use semantic relations beyond standard SKOS properties to express more specific relations between concepts. However, efforts to "ontologize" thesauri with such additional relations struggle to ensure that the properties would be applied consistently, comprehensively, and maintainably. The GACS Working Group decided that custom relations should only be created for use cases important enough to justify the effort. Two properties qualified: hasProduct and productOf, for relating fish as a product to fish as an organism.
Mapping to concepts in external vocabularies
GACS was created at the intersection of three existing thesauri, and each concept in GACS has a SKOS "exact match" relation back to the concept or concepts in AGROVOC, CABT, and NALT from which it was created.
In addition, the 540 concepts of type Geographical have SKOS "close match" relations to entities in Wikidata. These mappings facilitate access from GACS to Wikidata information about geographic places (such as coordinates, population, language, and religions); to Wikidata mappings out to other geo-oriented vocabularies such as GeoNames and OpenStreetMap; and to mappings out to library-oriented authority files such as the Library of Congress Name Authority File, GND (German union authority), BNF, VIAF and ISNI. As an example, see the Wikidata entry for Alberta) and the corresponding GACS concept.
This exercise suggests a model by which GACS might, in the future, devolve or delegate responsibility for specific types of GACS concepts to external authorities. Geographic places are an obvious candidate for delegation because they are not specific to agriculture and because they have been given URIs in so many other sources. Once mapped to a external source, a given set of GACS concepts could be reduced to a set of URIs clearly marked as dependent on the external URIs, and the GACS community could promote the use of the external URIs. GACS concepts cannot simply be deleted because their URIs must be credibly persistent, in accordance with explicit policies, if GACS itself is to be credibly deployable. On the other hand, the GACS URIs also provide a fallback option should the external authority ever cease operation, a contingency that would be prudent to anticipate by policy.